Help: Glossary

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Background Information	Table of Contents
	Glossary

[ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z ]

A

aa index: Geomagnetic antipodal activity index
See indices background page.
Ablation: The erosion of a solid body by a high-temperature gas stream moving with high velocity, e.g. a reentry vehicle's heat shield which melts or chars under the effects of air friction.
Absolute zero: The temperature at which all heat action ceases, -273.16 °C, 0 K.
Absorbed dose: The dose which is transferred through ionization and excitation.
Absorption line: In spectroscopy, and in particular the Solar Fraunhofer spectrum, a characteristic wavelength of emitted radiation that is partially absorbed by the medium between the source and the observer. (See H alpha)
Acceleration: A change in velocity, including changes of direction and decreases as well as increases in speed.
Accelerometer: A device that senses changes in speed along its axis.
Active: A descriptive word specifically meaning (1) a probability of >= 50% for an M-class x-ray flare (see x-ray flare class) in a sunspot region; (2) disturbed geomagnetic levels such that 16 <= A_k index < 30.
Active dark filament (ADF): A filament displaying motion or changes in shape, location, or absorption characteristics.
Active longitude: The approximate centre of a range of heliographic longitudes in either the northern or southern Solar hemisphere (seldom both at the same time) containing one or more large and complex active regions formed by the frequent, localized emergence of new magnetic flux. Individual sunspot groups within the complex can have relatively short lifetimes (a week or two); the complex may persist for several Solar rotations because additional spot groups form as earlier ones decay.
Active prominence: A prominence moving and changing in appearance over a few minutes of time.
Active prominence region: A portion of the Solar limb displaying active prominences; typically associated with an active region.
Active region (AR): A localized, transient volume of the Solar atmosphere in which plages, sunspots, facula, flares, etc., may be observed. Active regions are the result of enhanced magnetic fields; they are at least bipolar and may be complex if the region contains two or more bipolar groups.
Active Sun: The Sun during times of frequent Solar activity such as sunspots, flares, and associated phenomena.
Actuator: A device which transforms an electric signal into a measured motion using hydraulic, pneumatic or pyrotechnic (explosive) action.
Adiabatic invariant: Quantities associated with approximately periodic motions which almost do not change, and which are useful in calculating the way ions and electrons move in a magnetic field.
Aerodynamic heating: The heating of a body due to the passage of air or other gases over the body; caused by friction and compression processes.
Aeronautics: The science of building and operating vehicles for flight.
Aileron: A hinged surface on the wing of an aircraft or spacecraft used to adjust the craft's angle of flight.
Aimpoint: The point in the planet's plane that spacecraft aim for to either fly by or enter orbit.
Air lock: A compartment separating areas with a different environment, especially different air pressures, that is used for entry to and departure from a spacecraft.
Airglow: The visible light that appears at night in the upper atmosphere.
Albedo: Reflectivity; the ratio of reflected light to incident light. The fraction of the sunlight which is reflected off a planet.
Alfven wave: A transverse wave in magnetized plasma characterized by a change of direction of the magnetic field (rather than a change of intensity).
Altimeter: A device that measures altitude above the surface of a planet or moon. Spacecraft altimeters work by timing the round trip of radio signals bounced off the surface.
am index: Geomagnetic subauroral index
See indices background page.
Ambient: Environmental conditions, such as pressure or temperature.
Analog computer: A computing machine that works on the principle of measuring, as distinct from counting, in which the measurements obtained (as voltages, resistances, etc.) are translated into desired data.
Angle of attack: The angle between the velocity vector and the longitudinal axis of a missile or rocket.
Angstrom: A unit for the measurement of wavelength. Equals one hundred millionth of a centimeter.
Annular: Pertaining to, or having the form of a ring.
Anomaly: The angular distance between the position of a planet and its last perihelion, or between that of a satellite and its last perigee. Also, an unexpected response of a spacecraft (See satellite anomaly).
Anticyclonic: Description of the sense of rotation of an atmospheric spot or storm; anticyclonic motion is clockwise in the northern hemisphere and counterclockwise in the southern hemisphere; it suggests a high pressure region (See cyclonic).
Antimatter: Matter consisting of antiparticles such as antiprotons, antielectrons (positrons), and antineutrons.
Antipodal: Pertaining to, or located on, the opposite side of the Earth.
A_p index: Planetary geomagnetic activity index
See indices background page.
Aperture: The diameter of an opening; the diameter of the primary lens or mirror of a telescope.
Apex: The point towards which a body is moving.
Aphelion: The point in orbit where a body is farthest from the Sun.
Apoapsis: The farthest point of an orbit from the body being orbited.
Apogee: The point in orbit where a body is farthest from Earth.
Apojove: Apoapsis in Jupiter orbit.
Apolune: Apoapsis in lunar orbit.
Apselene: Apoapsis in lunar orbit.
Arcade: A series of magnetic loops, overlying a Solar inversion line.
Arch filament system (AFS): A system of small, arched linear-absorption features connecting bright, compact plage of opposite polarity. An AFS is a sign of emerging bipolar magnetic flux and possibly rapid or continued growth in an active region.
Argument: Angular distance.
Argument of periapsis: The argument (angular distance) of periapsis from the ascending node.
Artificial gravity: Use of centrifugal force to simulate weight reaction in a condition of free-fall. May be achieved by spinning the vehicle to make the centrifugal force of the outer periphery or bodies within the vehicle to replace the weight reaction experienced at the Earth's surface.
Ascending node: The point at which an orbit crosses the equator plane going north.
Asteroid: Small bodies composed of rock and metal in orbit about the Sun.
Asteroid Belt: A 1/2 AU wide region between the orbits of Mars and Jupiter where most asteroids are found.
Astronaut: A person who flies in space, whether as a crew member or passenger.
Astronautics: The science and technology of space flight.
Astronomical unit (AU): Average Earth-Sun distance equal to 1.4959787 10¹¹ m or 214.94 Solar radii.
Astrophysics: The branch of the astronomy dealing with the physics of astronomical objects and phenomena.
Atmosphere: The layer of gas surrounding the Earth or other planets. The upper atmosphere is the region of Earth's atmosphere above the troposphere (which extends to about 20 km). Regions of the upper atmosphere are the stratosphere, mesosphere and thermosphere.
Atmospheric pressure: The weight of air on surfaces within the Earth's atmosphere, about 101325 Pa at sea level. Such pressure is also supplied artificially in spacecraft and spacesuits.
Atmospherics: Also known as "sferics," transient radio waves produced by naturally occurring electric discharges (e.g., lightning) in the Earth's atmosphere.
Atom: The smallest unit of a chemical element that can still retain the properties of that element. Atoms combine to form molecules, and they themselves contain several kinds of smaller particles. An atom has a dense central core (the nucleus) consisting of positively charged particles (protons) and uncharged particles (neutrons). Negatively charged particles (electrons) are scattered in a relatively large space around this nucleus and move about it in orbital patterns at extremely high speeds. An atom contains the same number of protons as electrons and thus is electrically neutral and stable under most conditions.
Attenuation: The decrease of a propagating physical quantity, such as a radio signal, with increasing distance from the source, or from some obstruction.
Attitude: The direction in which the satellite is oriented in space.
Attitude control: The system that turns and maintains a spacecraft in the required direction as indicated by its sensors.
atto-: SI prefix indicating 10^-18.
Aurora: A glow in the sky, seen often in a ring-shaped region around the magnetic poles ("auroral zone") and occasionally further equatorward. The name comes from an older one, "Aurora Borealis", Latin for "northern dawn," given because an aurora near the northern horizon (its usual location when seen in most of Europe) looks like the glow of the sky preceding sunrise. Also known as "northern lights", although it occurs both north and south (Aurora Australis) of the equator. The aurora is generally caused by fast electrons from space, guided earthward by magnetic field lines, and its light comes from collisions between such electrons and the atoms of the upper atmosphere, typically 100 km above ground.
Auroral electrojet index (AE): Auroral zone magnetic activity index
See indices background page.
Auroral oval: The region in which aurora can be seen at any single time, as observed (for instance) by satellite cameras. It resembles a circle centered a few hundred kilometers nightward of the magnetic pole, and its size varies with magnetic activity. During large magnetic storms it expands greatly, making auroras visible at regions far from the pole, where they are rarely seen.
Autopilot: A system or device that controls a vehicle's flight at a preset course and altitude.
Autumnal equinox: The equinox that occurs in September (see also vernal equinox).
Azimuth: Motion around a vertical axis; for instance, a measure of position around the horizon.

B

Back-up: An item kept available to replace an item which fails to perform satisfactorily.
Backscattering: Reflecting light back in the direction of the source.
Ballistics: The science that deals with the motion, behavior, appearance or modification of missiles acted upon by propellants, rifling, wind, gravity, temperature or other modifying conditions of force.
Ballute: An aerodynamic braking device to protect spacecraft from the severe pressure and heating environment experienced during high speed flight through the atmosphere.
Balmer line: A series of spectral lines present in atomic hydrogen, due to the absorption and emission of energy at particular frequencies corresponding to the energy levels inside the atom.
B-angle: As viewed from the Earth, the heliographic latitude of the centre of the Solar disk. The centre of the Solar disk usually does not coincide with the heliographic equator, due to a tilt of the Solar axis with respect to the ecliptic (See also Solar coordinates).
Bar: Unit of pressure, equal to the sea-level pressure of Earth's atmosphere (101325 Pa).
Bartels' rotation number: Index counting 27-day intervals, starting from 8 February 1832
See indices background page.
Barycenter: The common centre of mass about which two or more bodies revolve.
Basin: A large, >200 km, circular depression from the explosive impact of an asteroid or similar sized body on a planet surface, usually rimmed by mountains.
Big Bang Theory: The theory that the universe was once clustered and at the 'beginning' it exploded out, as shown by the fact that objects are still moving out from the centre.
Binary star: Two stars revolving around a common centre of gravity.
Bipolar magnetic region (BMR): A region of the Solar photosphere containing at least two areas of enhanced magnetic fields of opposing polarity.
Bi-propellant: A rocket propellant consisting of two unmixed or uncombined chemicals (fuel and oxidizer) fed separately into the combustion chamber (e.g. 1,1-dimethylhydrazine as fuel and nitrogen tetroxide as oxidizer).
Birefringent filter: An optical device that passes a narrow range of wavelengths near a selected optical wavelength. Used especially in Solar telescopes to pass selected lines (usually H alpha) in the Fraunhofer spectrum.
Birkeland currents: Electric currents linking the Earth's ionosphere with more distant regions, flowing along magnetic field lines. Named for Kristian Birkeland, a pioneer of auroral research who first proposed such currents around 1900, these currents are often associated with the polar aurora.
Bit: A basic unit of computer information; abbreviation of binary digit.
Black hole: An object whose gravity is so strong that the escape velocity exceeds the speed of light.
Blackout (physiological): A temporary loss of vision and/or consciousness when a person is subjected to high accelerations.
Blackout (radio): A temporary loss of radio communications which occurs between a spacecraft reentering the atmosphere and ground stations due to an ionized sheath of plasma which develops around the vehicle.
Blue moon: The occasional blue color seen in the moon due to the Earth's atmosphere.
Boilerplate: A metal replica of the flight model (e.g. of a spacecraft) but usually heavier and cruder for test purposes.
Boost: The extra power given to a rocket or space vehicle during liftoff, climb or flight, as with a booster rocket.
Booster: The first stage of a missile or rocket.
Bow shock: A sharp front formed in the Solar wind ahead of the magnetosphere, marked with a sudden slowing-down (from supersonic to subsonic speeds) of the flow near Earth. It is quite similar to the shock forming ahead of the wing of a supersonic airplace. After passing near Earth, the slowed-down flow gains speed again, to the same value as the surrounding Solar wind.
Bremsstrahlung: High-energy electromagnetic radiation emitted by charged particles slowing down by scattering off atomic nuclei. The primary particle might ultimately be absorbed while the bremsstrahlung can be highly penetrating. In space, the most common source of bremsstrahlung is electron scattering.
Bright point: A short-lived brightening of flare intensity, less than ten millionths of the Solar hemisphere in area.
Bright surge on the disk (BSD): A bright (high temperature) stream of gas (surge) seen against the Solar disk. BSDs are often flare related and commonly fan out from the flare site. See also bright surge on the limb, and dark surge on the disk.
Bright surge on the limb (BSL): A bright stream of gas (surge) emanating from the chromosphere that moves outward more than 0.15 Solar radii above the limb. It may decelerate and return to the Sun. Most BSLs assume a linear radial shape but can be inclined and/or fan shaped because they apparently follow magnetic lines of force.
Brightness temperature: The equivalent blackbody temperature at a specified wavelength of a uniform source filling the resolution element of the telescope.
Burn: Combustion action in rockets. Propulsion in space is achieved through a sequence of burns.
Burst: A transient enhancement of the Solar radio emission, usually associated with an active region or flare.
Butterfly diagram: A plot of observed Solar active region latitudes versus time. This diagram, which resembles a butterfly, shows that the average latitude of active region formation drifts from high to low latitudes during a sunspot cycle.

C

C-band: A range of microwave radio frequencies in the neighborhood of 4 to 8 GHz, used for spacecraft communications on Mercury and Gemini flights.
Calibration: Setting an measuring instrument before measuring for accurate results.
Carrier: The main frequency of a radio signal generated by a transmitter prior to application of modulation.
Carrington rotation number: The number of rotations of the Sun (as seen from the Earth) since 9 November 1853.
See indices background page.
Celestial equator: The projection of Earth's geographic equator onto the celestial sphere.
Celestial sphere: The apparent sphere of sky that surrounds the Earth; used as a convention for specifying the location of a celestial object.
centi-: SI prefix indicating 10^-2.
Centimeter burst: A Solar radio burst in the centimeter wavelength range (0.01 to 0.1 m), or 30 000 to 3000 MHz in the frequency range.
Central meridian passage (CMP): The rotation of an active region or other feature across the longitude meridian that passes through the apparent centre of the Solar disk.
Centrifugal force: The outward force felt by a body rotating about an axis.
Centripetal acceleration: The inward acceleration of a body revolving around another body.
Charge Coupled Device: A solid-state imaging detector.
Charon: The only known satellite of Pluto.
Chromosphere: A reddish layer in the Sun's atmosphere, the transition between the photosphere and the corona. The chromosphere is the source of the strongest lines in the Solar spectrum, including the Balmer alpha line of hydrogen and the H and K lines of calcium, and is the source of the red (chromium) colour often seen around the rim of the moon at total Solar eclipses.
C index: Character figure.
See indices background page.
Coherent: Two-way communications mode wherein the spacecraft generates its downlink frequency based upon the frequency of the uplink it receives.
Coma: The cloud of diffuse material surrounding the nucleus of a comet.
Comet: A small body of ice and dust in orbit around the Sun. While passing near the Sun, a comet's vaporized ice becomes its "tail".
Comprehensive flare index (CFI): See indices background page.
Conjugate points: Two points on the Earth's surface at opposite ends of a geomagnetic field line.
Constellation: A group of stars that make a shape, often named after mythological characters, people, animals, and things.
Continuum: Optical radiation arising from broadband emission from the photosphere.
Continuum storm: General term for Solar noise lasting for hours and sometimes days, in which the intensity varies smoothly with frequency over a wide range in the meter and decimeter wavelengths.
Convection: Rising warm air and falling cool air. The transfer of energy by moving currents of fluid or gas containing energy.
Coordinated Universal Time (UTC): By international agreement, the local time at the prime meridian, which passes through Greenwich, England. It was formerly known as Greenwich Mean Time, or sometimes simply Universal Time. There are 24 time zones around the world, labeled alphabetically. The time zone centered at Greenwich has the double designation of A and Z. Especially in the military community, Coordinated Universal Time is often referenced as Z or Zulu Time.
Core: The innermost layer of a planet or star.
Corona: The outermost layer of the Sun's atmosphere, visible to the eye during a total Solar eclipse; it can also be observed through special filters and best of all, by X-ray cameras aboard satellites. The corona is very hot, up to 1-1.5 million degrees centigrade, and is the source of the Solar wind.
Coronagraph: An optical device that makes it possible to observe the corona at times other than during an eclipse. A simple lens focuses the Sun onto an occulting disk that prevents the light from the Solar disk from proceeding farther along the optical path, effectively providing an artificial eclipse.
Coronal hole: An extended region of the corona, exceptionally low in density and associated with unipolar photospheric regions having "open" magnetic field topology. Coronal holes are largest and most stable at or near the Solar poles, and are a source of high-speed Solar wind. Coronal holes are visible in several wavelengths, most notably Solar x-rays.
Coronal loops: A typical structure of enhanced corona observed in EUV lines and soft x-rays. They are sometimes related to H alpha loops. Coronal loops represent "closed" magnetic topology.
Coronal mass ejection (CME): A huge cloud of hot plasma, expelled sometimes from the Sun. It may accelerate ions and electrons, and may travel through interplanetary space as far as the Earth's orbit and beyond it, often preceded by a shock front. When the shock reaches Earth, a magnetic storm may result.
Coronal rain: Material condensing in the corona and appearing to rain down into the chromosphere as observed in H alpha at the Solar limb above strong sunspots.
Coronal streamer: A large-scale structure in the white-light corona often overlying a principal inversion line in the Solar photospheric magnetic fields. (See helmet streamer).
Coronal transients: A general term for short-time-scale changes in the corona.
Co-rotating: Sharing the rotation of a planetary magnetic field, as in a co-rotating plasma.
Corrected geomagnetic coordinates: A nonspherical coordinate system based on a magnetic dipole axis that is offset from the Earth's centre.
Cosmic noise: The broad spectrum of radio noise arriving at the Earth from sources outside the Solar system.
Cosmic ray: An extremely energetic (relativistic) charged particle primarily originating outside the Earth's magnetosphere.
Cosmic year: The time it takes the Sun to revolve around the centre of the galaxy, approximately 225 million years.
Crater: A round impression left in a planet or satellite from a meteoroid.
Critical frequency: In ionospheric radio propagation, that frequency capable of penetration just to the layer of maximum ionization with vertical propagation. Radiowaves of lower frequencies are refracted back to the ground; higher frequencies pass through.
Crochet: A sudden deviation in the sunlit geomagnetic field H component associated with extraordinary Solar flare x-ray emission. The effect can be as much as 50 nT and last up to 30 minutes. The event is also known as an SFE (Solar flare effect).
Crust: The outer layer of Earth and other terrestrial planets.
Current: This term is often used in discussion of radiation transport to refer to the rate of transport of particles through a boundary. In contrast to flux, current is dependent on the direction in which the particle crosses the boundary (it is a vector integral). An isotropic omnidirectional flux, f, incident on a plane gives rise to a current of f/4 normally in each direction across the plane.
Cusp: Two regions of weak magnetic field, on the sunward boundary of the magnetosphere, one on each side of the equator. They separate magnetic field lines closing on the front from those swept into the Earth's magnetotail. The term cusp implies conical symmetry around the axis of the bundle of converging (northern hemisphere) or diverging (southern hemisphere) field lines. In practice, "cusp" and "cleft" are often used interchangeably. However, "cleft" implies greater extension in longitude (local time) and hence a wedge-shaped structure.
Cyclonic: Description of the sense of rotation of an atmospheric spot or storm; cyclonic motion is counter clockwise in the northern hemisphere and clockwise in the southern hemisphere (See also anticyclonic).

D

Dark matter

A form of matter which has not been directly observed but whose existence has been deduced by it's gravitational effects.

Dark surge on the disk (DSD)

Dark gaseous ejections on the Sun visible in H alpha. They usually originate from small subflare-like brightenings. Material is usually seen to be ejected, to decelerate at a gravitational rate, and to flow back to the point of origin. DSDs can occur intermittently for days from an active region.

Debye length

The maximum distance at which Coulomb fields of charged particles in a plasma may be expected to interact.

Decibel (dB)

A unit used to express the ratio between two levels of power. By definition dB = 10 log (P₂/P₁). (Doubling the power is approx an increase of 3 dB).

Declination

The measure of a celestial body's apparent height above or below the celestial equator. In geomagnetic applications, the angle between true north and the horizontal component of the local geomagnetic field.

Density

The mass to volume ratio.

Density Wave

A kind of wave induced in a flat plane of a resisting medium (such as the rings of Saturn) by gravitational forces, often assuming the form of a tightly wound spiral.

Descending node

The point at which an orbit crosses the ecliptic plane going south.

Differential charging

The charging of different areas of a spacecraft or satellite to different potentials in response to sunlight, the charged particle environment, and the design and composition of the structural materials themselves. Discharge may occur through arcing and generally is detrimental.

Differential particle flux

The differential particle directional flux j(E,w) denotes the number of particles of energy E per unit energy interval, per unit area, per unit time, per unit solid angle of observation, passing through an area perpendicular to the viewing direction; the angle w is the angle between the viewing direction and the local magnetic field. It is approximately obtained from the count rate of a physical detector measuring the flux of particles between energy E and E + dE, geometric factor G, and solid angle of view dW through the relationship

j(E,w) = C/(G * dE * dW * dt)

where C is the number of detector counts in time dt.

Differential rotation

The change in Solar rotation rate with latitude. Low latitudes rotate at a faster angular rate (approx. 14 degrees/day) than do high latitudes (approx. 12 degrees/day).

Diffuse aurora

A spread-out glow often covering much of the auroral oval. It is not seen by the eye but can be observed well by satellite cameras.

Dip

The geomagnetic inclination angle.

Dip equator

An irregular, imaginary line around the Earth where the geomagnetic inclination angle is measured to be zero. It lies near the geographic equator.

Dipole

A compact source of magnetic force, with two magnetic poles. A bar magnet, coil or current loop, if their size is small, create a dipole field. The Earth's field, as a first approximation, also resembles that of a dipole.

Disappearing Solar filament (DSF)

A Solar filament (prominence) that disappears suddenly (on a time scale of minutes to hours). The prominence material is often seen to ascend but is also seen to fall into the Sun or just fade. (Historically, DSFs have been called disparitions brusques because they were first studied by French astronomers.) DSFs are a possible indicator of coronal mass ejections.

Discrete aurora

The typical ribbon-like structures of aurora observed from the ground. From space they may appear as brighter spots in the diffuse aurora.

Disk

The visible surface of the Sun (or any heavenly body) projected against the sky.

Disparition brusque (DB)

See disappearing Solar filament.

Doppler shift

A change in the perceived frequency of a radiated signal caused by motion of the source relative to the observer.

Dose

A quantity of radiation delivered at a position. In its broadest sense it can include the flux of particles, but in the context of space energetic particle radiation effects, it usually refers to the energy absorbed locally per unit mass as a result of radiation exposure.

Dose equivalent

A dose normally applied to biological effects and including scaling factors to account for the more severe effects of certain kinds of radiation.

Downlink

A radio connection intact wherein a spacecraft can communicate to Earth.

D region

A daytime region of the Earth's ionosphere beginning at approximately 40 km, to 90 km altitude. Radiowave absorption in layers in this region can be significantly increased in response to increased ionization associated with Solar activity.

Drift

A magnetically trapped ion or electron moves as if it were attached to a magnetic field line. Drift is one of the features of such motion, namely its slow shift from one magnetic longitude to another. In the Earth's magnetic field, such drifts gradually move particles all the way around Earth. Viewed from far above the north magnetic pole, ions drift around the Earth clockwise, electrons counter-clockwise, resulting in an electric current circling the Earth, the ring current.

Dst index

Disturbance storm-time index.
See indices background page.

Dust

Particulates which have a direct relation to a specific Solar system body and which are usually found close to the surface of this body (e.g. Lunar, Martian or Cometary dust).

Dynamo process

The generation of an electric currents by the flow of an electrically conducting fluid through a magnetic field. For instance, the magnetic field originating inside the Earth is believed to come from a dynamo process involving the flow of molten iron in the Earth's hot core. The energy required by the current is obtained from the motion of the flow.

Dyne

A unit of force equal to the force required to accelerate a 1 g mass 1 cm s^-2.

E

Earth: Third planet from the Sun, a terrestrial planet.
Earth infrared: The Earth-emitted thermal radiation. It is also called Outgoing Longwave Radiation.
Earth radius: The average radius of the Earth, a convenient unit of distance in describing phenomena and orbits in the Earth's neighborhood in space. 1 R_E = 6371.2 km approximately.
Eccentric: Noncircular; elliptical (applied to an orbit).
Eccentricity: The distance between the foci of an ellipse divided by the major axis.
Eclipse: The cutting off of light from one celestial body by another. (1) A Solar Eclipse occurs when the moon comes between the Earth and the Sun. In a total eclipse, the Solar disk is completely obscured; in a partial eclipse the Solar disk is only partly obscured. An annular eclipse occurs when the moon is near its apogee and the apparent diameter of the moon is less than that of the Sun so that the Sun is never completely obscured. "First and last contacts" are defined as the times of tangency of the Solar and lunar disks. A central eclipse (which can be total or annular) has two additional times of tangency: "second contact," when maximum eclipse begins, and "third contact," when it ends. The last glimpses of the Sun through the lunar valleys, just before second contact, are known as Baily's beads. (2) A lunar eclipse occurs when the moon enters the shadow cast by the Earth. (3) Spacecraft in the Earth's shadow are said to be in eclipse.
Ecliptic: The plane in which the Earth orbits the Sun.
Eddy: (1) Meteorology: A whirl or circular current of air running contrary to the general flow. (2) Fluid mechanics: A deviation in the steady flow of a fluid, causing a vortex.
Electrojet: (1) Auroral: A current that flows in the ionosphere in the auroral zone. (2) Equatorial: A thin electric current layer in the ionosphere over the dip equator at about 100 to 115 km altitude.
Electromagnetic: Relating to the interplay between electric and magnetic fields. In particular, light (as well as x-rays, microwaves, gamma rays, etc.) is electromagnetic radiation, consisting of alternating electric and magnetic fields which move through space.
Electron: A stable elementary particle that is a primary constituent of ordinary matter, contained in the atoms of all elements and described as having a ngative charge of 1.602 10^-19 C, a rest mass of 9.11 10^-31 kg and a spin of 1/2. Electrons flowing in a conductor constitute an electric current.
Electrostatic discharge (ESD): An abrupt equalization of electric potentials. In space, ESD can occur between objects or portions of a single object (see differential charging); ESD may occur locally within a dielectric or cable. The consequences may include material damage, a spacecraft anomaly, phantom commands, disrupted telemetry, and contaminated data.
Elevation: Motion around a horizontal axis; opposite to azimuth.
Ellipse: A closed plane curve generated in such a way that the sums of its distances from the two fixed points (the foci) is constant.
Emerging flux region: An area on the Sun where new magnetic flux is erupting. An EFR is a bipolar magnetic region that first produces a small bipolar plage visible in the chromosphere, which may develop an arch filament system and the initial spots of a sunspot group. An EFR may be isolated from other Solar activity or may occur within an active region.
Emission line: In spectroscopy, a particular wavelength of emitted radiation, more intense than the background continuum.
Emission measure: The integral of the square of the electron density over volume; the units are inverse volume (m^-3).
Energetic particle: Particles that can penetrate outer surfaces of spacecraft. For electrons, this is typically above 100 keV, while for protons and other ions this is above 1 Mev. Neutrons, gamma-rays and X-rays are also considered energetic particles in this context.
Ephemeris: An astronomical almanac listing Solar coordinates and the positions of the Sun and other heavenly bodies at regular intervals in time.
Ephemeris time: A measurement of time defined by orbital motions. Equates to Mean Solar Time corrected for irregularities in Earth's motions.
Equator: An imaginary circle around a body which is everywhere equidistant from the poles, defining the boundary between the northern and southern hemispheres.
Equinox: One of the two points of intersection of the celestial equator and the ecliptic. The Sun passes through the vernal equinox on about 21 March and through the autumnal equinox on about 22 September.
Equivalent Fluence: A quantity which attempts to represent the damage at different energies and from different species. Damage coefficients are used to scale the effect caused by particles to the damage to the damage caused by a standard particle and energy. For example, for Solar cell degradation it is often taken that one 10 MeV proton is "equivalent" to 3000 1 MeV electrons. This concept also occurs in consideration of non-ionizing energy loss effects (NIEL).
E region: A daytime region of the Earth's ionosphere roughly between the altitudes of 90 and 160 km. E region characteristics (electron density, height, etc.) depend on the Solar zenith angle and Solar activity. The ionization in the E layer is caused mainly by x-rays in the range 0.8 to 10.4 nm (See also sporadic E).
Eruptive: With regard to Solar flare predictions, a probability of >50% that an active region will produce C class x-ray flares.
Eruptive prominence on limb (EPL): A Solar prominence that becomes activated and is seen to ascend from the Sun; sometimes associated with a coronal mass ejection (See also disappearing Solar filament).
Evershed effect: Horizontal motion of the Solar atmosphere near a sunspot, having velocities of a few km s^-1. In the photosphere, matter streams away from the umbra. In the chromosphere, the direction of flow is toward the umbra.
EUV: A portion of the electromagnetic spectrum from approximately 100 to 1000 angstroms.
Exosphere: The part of the Earth atmosphere above the thermosphere which extends into space. H and He atoms can attain escape velocities at the outer rim of the exosphere.
Extraordinary mode: One of the two modes of propagation of electromagnetic waves in a magnetic plasma. For propagation along the direction of the magnetic field, it is the mode in which the electric vector rotates in the same sense that an electron gyrates freely about the field. For propagation perpendicular to the magnetic field, the electric vector oscillates perpendicular to the primary magnetic field. (See also ordinary mode.)
Extremely high frequency (EHF): That portion of the radio frequency spectrum from 30 to 300 GHz.
extremely low frequency (ELF): That portion of the radio frequency spectrum from 30 to 3000 Hz.

F

F_10.7 flux

The Sun emits radio energy with slowly varying intensity. This radio flux, which originates from atmospheric layers high in the Sun's chromosphere and low in its corona, changes gradually from day to day in response to the number of spot groups on the disk. Solar flux from the entire Solar disk at a frequency of 2800 MHz (10.7 cm) has been recorded routinely by a radio telescope near Ottawa since 14 February 1947. The observed values are adjusted for the changing Sun-Earth distance (adjusted values) and for uncertainties in antenna gain (absolute values). Fluxes are given in units of 10^-22 J s^-1 m^-2 Hz^-1 or 10⁴ JANSKY. Often used names for this index are F_10.7, Covington index or Ottawa radio flux. Empirical formulas have been established for the relationship between F_10.7 and Solar sunspot number. It is measured daily at 17:00 UT. The values range from a theoretical minimum of approximately 67 to actually-observed numbers greater than 300. Low Solar flux numbers dominate during the lower portions of the 11-year sunspot cycle, rising as the cycle proceeds with the average Solar flux a fairly reliable indicator of the cycle's long-term behavior.

Facula

White-light plage; a bright region of the photosphere seen in white light, seldom visible except near the Solar limb. Corresponds with concentrated magnetic fields that may presage sunspot formation.

Fault

A crack or break in the crust of a planet along which slippage or movement can take place.

Fault protection

A program built into a spacecraft which detects errors and faults in the spacecraft before they become monstrous and fixes them or reroutes to avoid them.

F corona

Of the white-light corona (the corona seen by the eye at a total Solar eclipse ), that portion which is caused by sunlight scattered or reflected by solid particles (dust) in interplanetary space. The same phenomenon produces zodiacal light.

femto

SI prefix indicating 10^-15.

Fibril

A linear feature in the H alpha chromosphere of the Sun, occurring near strong sunspots and plage or in filament channels. Fibrils parallel strong magnetic fields, as if mapping the field direction.

Filament

A mass of gas suspended over the chromosphere by magnetic fields and seen as dark ribbons threaded over the Solar disk. A filament on the limb of the Sun seen in emission against the dark sky is called a prominence. Filaments occur directly over magnetic-polarity inversion lines, unless they are active.

Filament channel

A broad pattern of fibrils in the chromosphere, marking a portion of a magnetic polarity inversion line where a filament may soon form or where a filament recently disappeared. Filament channels interconnect separate filaments and active regions on a common inversion line.

Fission

The splitting of heavier atomic nuclei into lighter ones. In the case of heavy atoms (e.g., uranium, plutonium), this will release energy. Fission is how nuclear power plants produce energy (See also fusion).

Flare

A sudden eruption of energy in the Solar atmosphere lasting minutes to hours, from which radiation and particles are emitted. Flares are classified on the basis of area at the time of maximum brightness in H alpha.

Importance 0 (Subflare): <= 2.0 hemispheric square degrees
Importance 1: 2.1-5.1 square degrees
Importance 2: 5.2-12.4 square degrees
Importance 3: 12.5-24.7 square degrees
Importance 4: >= 24.8 square degrees

One square degree is equal to (1.214 x 10⁴ km)² = 48.5 millionths of the visible Solar hemisphere. A brightness qualifier F, N, or B is generally appended to the importance character to indicate faint, normal, or brilliant (for example, 2B).

Fluence

A time-integration of the flux.

Fluorescence

The phenomenon of emitting light upon absorbing radiation of an invisible wavelength.

Flux

The amount of radiation crossing a surface per unit of time, often expressed in "integral form" as particles per unit area per unit time (e.g. electrons cm^-2 s^-1 above a certain treshold energy. The directional flux is differential with respect to solid angle (e.g. particles cm^-2 sr^-1 s^-1) while the "differential" flux is differential with respect to energy (e.g. particles cm^-2 MeV^-1 s^-1). In some cases fluxes are also treated as differential with respect to Linear Energy Transfer.

fmin

The lowest frequency at which echo traces are observed on an ionogram. It increases with increasing D region absorption.

foEs

The maximum ordinary mode radiowave frequency capable of vertical reflection from the sporadic E layer of the ionosphere.

foF2

The maximum ordinary mode radiowave frequency capable of vertical reflection from the F2 layer of the ionosphere. (See F region)

Follower spot (f-spot)

In a magnetically bipolar or multipolar sunspot group, the main spot in that portion of the group east of the principal inversion line is called the follower or f-spot. Leader and follower describe the positions of spots with respect to apparent motion due to Solar rotation. (Compare leader spot)

Forbush decrease

An abrupt decrease, of at least 10%, of the background galactic cosmic ray intensity as observed by neutron monitors. It is associated with major plasma and magnetic field enhancements in the Solar wind at or beyond the Earth.

Forward Scattering

Reflecting light approximately away from the source.

Fraunhofer spectrum

The system of dark lines superposed on the continuous Solar spectrum formed by the absorption of photons by atoms and molecules in the Solar and terrestrial atmospheres.

F region

The upper region of the ionosphere, above approximately 160 km altitude. F region electron densities are highly variable, depending on the local time, Solar activity, season, and geomagnetic activity. The F region contains the F1 and F2 layers. The F2 layer is more dense and peaks at altitudes between 200 and 600 km. The F1 layer, which forms at lower altitudes in the daytime, has a smaller peak in electron density.

Fusion

The combining of lighter elements into heavier ones. For lighter elements (e.g., hydrogen, helium) this processes releases energy. Fusion is how stars produce energy, and is being researched as a way to produce power on Earth.

G

Gal

Unit of gravity field measurement corresponding to a gravitational acceleration of 1 cm s^-2.

Galaxy

A large group of stars, nebulae, and dust; The galaxy of which the nearby stars and Sun are members.

Galilean

The four large satellites of Jupiter so named because Galileo discovered them when he turned his telescope toward Jupiter: Io, Europa, Ganymede, and Callisto.

Gamma

A unit of magnetic field intensity equal to 1 x 10^-05 Gauss, also equal to 1 nT.

Gamma ray

High energy radiation (energies in excess of 100 keV) observed during large, extremely energetic Solar flares, in the neighborhood of 100 fm wavelength.

Geocentric

Earth centered.

Geocorona

The outer region of the Earth's atmosphere lying above the thermosphere and composed mostly of hydrogen.

Geomagnetic activity

Natural variations in the geomagnetic field classified quantitatively into quiet, unsettled, active, and geomagnetic storm levels according to the observed geomagnetic index.

Geomagnetic elements

The components of the geomagnetic field at the surface of the Earth. These elements are usually denoted thus in the literature:

X: the geographic northward component;
Y: the geographic eastward component;
Z: the vertical component, reckoned positive downward;
H: the horizontal intensity, of magnitude sqrt(X² + Y²)
F: the total intensity sqrt(H² + Z²)
I: the inclination (or dip) angle, arctan(Z/H)
D: the declination angle, measured from the geographic north direction to the H component direction, positive in an eastward direction. D = arctan (Y/X)

However, care must be taken, as not everyone uses the same definitions. e.g. in the definitions by The Solar Terrestrial Physics program at the National Geophysical Data Center the geomagnetic northward and geomagnetic eastward components are called the H and D components. The H axis direction is defined by the mean direction of the horizontal component of the field; the D component is expressed in nT and is related to the direction of the horizontal component relative to geomagnetic north by using the small-angle approximation. Thus the D component = H (the horizontal intensity) multiplied by delta D (the declination angle relative to geomagnetic north, expressed in radians).

Geomagnetic field

The magnetic field in and around the Earth. The intensity of the magnetic field at the Earth's surface is approximately 32,000 nT at the equator and 62,000 nT at the north pole (the place where a compass needle points vertically downward). The geomagnetic field is dynamic and undergoes continual slow secular changes as well as short-term disturbances (see geomagnetic activity). The geomagnetic field can be approximated by a centered dipole field, with the axis of the dipole inclined to the Earth's rotational axis by about 11.5 degrees. Geomagnetic dipole north is near geographic coordinate 78.3 N 69 W (Thule, Greenland), and dipole south is near 79 S 110 E (near Vostok, Antarctica). The observed or dip poles, where the magnetic field is vertical to the Earth's surface, are near 76 N 101 W, and 66 S 141 E. The adopted origin of geomagnetic longitude is the meridian passing through the geomagnetic poles (dipole model) and the geographic south pole (See also corrected geomagnetic coordinates).

Geomagnetic storm

A worldwide disturbance of the Earth's magnetic field, distinct from regular diurnal variations.A storm is precisely defined as occurring when the daily A_p index exceeds 29.

Initial Phase: Of a geomagnetic storm, that period when there may be an increase of the middle-latitude horizontal intensity (H) (see geomagnetic elements) at the surface of the Earth. The initial phase can last for hours (up to a day), but some storms proceed directly into the main phase without showing an initial phase.
Main Phase: Of a geomagnetic storm, that period when the horizontal magnetic field at middle latitudes is generally decreasing, owing to the effects of an increasing westward-flowing magnetospheric ring current. The northward component can be depressed as much as several hundred nT in intense storms. The main phase can last for hours, but typically lasts less than 1 day.
Recovery Phase: Of a geomagnetic storm, that period when the depressed northward field component returns to normal levels. Recovery is typically complete in one to two days, but can take longer.

Geospace

Also called the Solar-terrestrial environment, geospace is the domain of Sun-Earth interactions. It consists of the particles, fields, and radiation environment from the Sun to Earth's space plasma environment and upper atmosphere. Geospace is considered to be the fourth physical geosphere (after solid Earth, oceans, and atmosphere).

Geostationary

A geosynchronous orbit in which the spacecraft is constrained to a constant latitude.

Geosynchronous

A direct, circular, low inclination orbit about the Earth having a period of 23 hours 56 minutes 4 seconds. A satellite in geosynchronous orbit above the Earth's equator will stay over the same point on Earth at all times. Communications satellites are often put in geosynchronous orbits so that satellite dishes on Earth can remain pointed at the same point in the sky at all times.

Giga

SI prefix indicating 10⁹.

Global Positioning System (GPS)

A network of Earth-orbiting satellites used for precise position-finding in surveying and navigation.

Gradual commencement

The commencement of a geomagnetic storm that has no well-defined onset (See also sudden commencement).

Granulation

Cellular structure of the photosphere visible at high spatial resolution. Individual granules, which represent the tops of small convection cells, are 200 to 2000 km in diameter and have lifetimes of 8 to 10 minutes.

Gravitational waves

Einsteinian distortions of the space-time medium predicted by general relativity theory (not yet detected as of November 1995).

Gravity

A mutual physical force attracting all bodies in the universe.

Gravity waves

Certain atmospheric waves within a planet's atmosphere.

Great circle

An imaginary circle on the surface of a sphere whose centre is at the centre of the sphere.

Great Red Spot

A huge anticyclonically rotating meteorological eddy on Jupiter.

Green line

A coronal emission line at 530.3 nm from Fe XIV (an iron atom from which 13 electrons have been stripped). The green line is one of the strongest (and first-recognized) visible coronal lines. It identifies moderate-temperature regions of the corona; it is enhanced in coronal streamers above inversion lines, and diminished in coronal holes.

Greenwich Mean Time

Similar to UTC but not updated with leap seconds (See Coordinated Universal Time).

Ground-level event (GLE)

A sharp increase in ground-level cosmic ray count to at least 10% above background, associated with Solar protons of energies greater than 500 MeV. GLEs are relatively rare, occurring only a few times each Solar cycle. When they occur, GLEs begin a few minutes after flare maximum and last for a few tens of minutes to hours. Intense particle fluxes at lower energies can be expected to follow this initial burst of relativistic particles. GLEs are detected by neutron monitors, e.g., the monitor at Thule, Greenland.

Guiding centre

The centre point of the gyration of ion and electrons in a magnetic field. The guiding centre of radiation belt particles bounces back and forth between mirror points and also undergoes a slow drift.

Gyration

The circular motion of ions and electrons around magnetic field lines.

Gyroradius

A charged particle moving in a magnetic field will orbit around the magnetic field lines. The radius of this orbit is called the gyroradius (also known as the Larmor radius). The gyroradius is larger for faster or more massive particles and smaller for stronger magnetic fields.

H

Hale boundary: A large-scale magnetic inversion line of a particular magnetic orientation in the Solar photosphere or across a sector boundary in the Solar wind. If the polarity of the western (leading) side of the boundary is the same as that of the nearer Solar pole at the start of a sunspot cycle, the boundary is said to be "Hale." If the polarity is opposite, the boundary is "anti-Hale." At the beginning of Cycle 22 (1987), the northern Solar pole was negative; therefore in the northern hemisphere a Hale boundary separates a leading negative polarity region from a following positive one. The boundary between the leader spot and follower spot of a typical sunspot group in either hemisphere is a Hale boundary.
Halley's comet: A bright comet, visible to the naked eye, whose path we can predict.
H alpha: The first atomic transition in the hydrogen Balmer series; wavelength = 656.3 nm. This absorption line of neutral hydrogen falls in the red part of the visible spectrum and is convenient for Solar observations. The H alpha line is universally used for patrol observations of Solar flares, filaments, prominences, and the fine structure of active regions.
Helio-: Prefix referring to the Sun.
Heliocentric: Centered on the Sun.
Heliopause: The boundary theorized to be roughly circular or teardrop-shaped, marking the edge of the Sun's influence, perhaps 100 AU from the Sun (i.e. the boundary surface between the Solar wind an the external galactic medium).
Heliosphere: The cavity within the boundary of the heliopause, containing the Sun and Solar system.
Helmet streamer: A feature of the white light corona (seen in eclipse or with a coronagraph) that looks like a ray extending away from the Sun out to about 1 Solar radius, having an arch-like base containing a cavity usually occupied by a prominence.
Heterosphere: The Earth atmosphere above 105 km altitude where species-wise concentration profiles establish due to diffusive equilibrium, with N₂ dominance below 200 km, O dominance from 200 to 600 km, and He dominance as of 600 km altitude.
High frequency (HF): That portion of the radio frequency spectrum between 3 and 30 MHz.
High latitude: With reference to zones of geomagnetic activity, 50 degrees to 80 degrees geomagnetic latitude. The other zones are equatorial, polar, and middle latitude.
High-speed stream: A feature of the Solar wind having velocities exceeding approximately 600 km s^-1 (about double average Solar wind values). High-speed streams that originate in coronal holes are less dense than those originating in the average Solar wind.
Homologous flares: Solar flares that occur repetitively in an active region, with essentially the same position and with a common pattern of development.
Homosphere: The Earth atmosphere below 105 km altitude where complete vertical mixing yields a near-homogeneous composition of about 78.1% N₂, 20.9% O₂, 0.9% Ar, and 0.1% CO₂ and trace constituents. The homopause (or turbopause) marks the ceiling of the homosphere. The homosphere can be broadly divided into three distinct regimes: the troposphere (0 to 12 km), the stratosphere (12 to 50 km) and the mesosphere (50 to 90 km).
Horizon: The line marking the apparent junction of Earth and sky.
Hour Angle: The angular distance of a celestial object measured westward along the celestial equator from the zenith crossing.
Hyder flare: A filament-associated two-ribbon flare, often occurring in spotless regions. The flare is generally slow (30-60 minutes rise time in H alpha and x-ray) and follows the disappearance of a quiescent filament. The flare presumably results from the impact on the chromosphere of infalling filament material. The Hyder flare is named for Dr. C. Hyder, who published studies of such flares in 1967.
Hydrosphere: The water on or around the surface of a planet.
Hyperbolic: A trajectory path to a planet shaped like a hyperbola.

I

Inclination: Of an orbit, the angle between the orbital plane of a revolving body and some fundamental plane, usually the plane of the celestial equator.
Inclination of the geomagnetic field: The angle between the local geomagnetic field direction and the horizon (See geomagnetic elements).
Inferior planet: Planet which orbits closer to the Sun than the Earth's orbit.
Inferior conjunction: Alignment of Earth, Sun, and an inferior planet on the same side of the Sun.
Infrared: Electromagnetic radiation, whose wavelength is longer than the longest visible red but shorter than radio waves, in the neighborhood of 100 µm wavelength.
Integral particle flux: The integral directional particle flux J(E,w) is literally the mathematical integral, with respect to the energy E, of the differential particle flux j(E,w). It denotes the number of particles of energy equal to or greater than E, per unit area, per unit solid angle, per unit time, passing through an area perpendicular to the viewing direction.
Interactive Data Language (IDL): graphics package developed by Exelis Visual Information Solutions
Interferometery: The location and examination of sources of electromagnetic radiation through the simultaneous use of two or more separated telescopes. Interferometers produce overlapping wave patterns from the radiation. These patterns are studied to determine the brightness and angular structure of the emitting source.
Interplanetary magnetic field (IMF): The weak magnetic field filling interplanetary space, with field lines usually connected to the Sun. The IMF is kept out of the Earth's magnetosphere, but the interaction of the two plays a major role in the flow of energy from the Solar wind to the Earth's environment.
Interplanetary sector: a region of interplanetary space in which all magnetic field lines point either away from the Sun ("away sector") or towards the Sun ("towards sector"). Typically the interplanetary magnetic field consists of 4 sectors, but 2 or 6 are not unusual.
Interplanetary shock: the abrupt boundary formed at the front of a plasma cloud (e.g. from a coronal mass ejection) moving much faster than the rest of the Solar wind, as it pushes its way through interplanetary space.
Invariant magnetic latitude: The geomagnetic latitude at which a particular line of force of the geomagnetic field, intersects the Earth.
Inversion line: The locus of points on the Solar surface where the radial magnetic field vanishes. Inversion lines separate regions of opposing polarity and are often superposed by thin, dark filaments, which can be used as tracers. Inside active regions, the areas close to and along inversion lines are preferred places of flare occurrence. Filament channels, plage corridors, arch-filament systems, and fibril patterns surrounding active regions can be used to infer the positions of inversion lines.
Ion: An atom or molecular fragment that has a positive electrical charge (cation) due to the loss of one or more electrons or a negative electrical charge (anion) due to the gain of one or more electrons. The simplest ion is the hydrogen nucleus, a single proton.
Ion-acoustic waves: Longitudinal waves in a plasma similar to sound waves in a neutral gas. Amplitudes of electron and ion oscillations are not quite the same, and the resulting Coulomb repulsion provides the potential energy to drive the waves.
Ionogram: A plot (record) of the group path height of reflection of ionospherically returned (echoed) radio waves as a function of frequency.
Ionosphere: The ionised plasma at the top of the Earth atmosphere, produced by the dissociation of atmospheric atoms, mainly by sunlight. It is generally divided into layers D (50-90 km), E (85-140 km) and F1 at low altitudes and F2 at higher altitudes (300 km). For most space applications, the electron density is the most important characteristic. The ionosphere influences radiowave propagation of frequencies less than about 300 MHz.
Ionospheric storm: A disturbance in the F region of the ionosphere, which occurs in connection with geomagnetic activity. In general, there are two phases of an ionospheric storm, an initial increase in electron density (the positive phase) lasting a few hours, followed by a decrease lasting a few days. At low latitudes only the positive phase is usually seen. Individual storms can vary, and their behavior depends on geomagnetic latitude, season, and local time. The phases of an ionospheric storm are not related to the initial and main phases of a geomagnetic storm.
Isotropic: A property of a distribution of particles where the flux is constant over all directions.

J

Jansky: Unit used to express flux. 1 Jansky = 10^-26 W m^-2 Hz^-1 Bandwidth.
Jet Propulsion: Propulsion generated by reaction to the momentum of air of gaseous material being forced out of the back of the engine.
Jovian Planet: Any of the four biggest planets: Jupiter, Saturn, Uranus, and Neptune.
Jupiter: Fifth planet from the Sun, a gas giant or Jovian planet.

K

K-band: A range of microwave radio frequencies in the neighborhood of 12 to 40 GHz.
K corona: Of the white-light corona (that is, the corona seen by the eye at a total Solar eclipse), that portion which is caused by sunlight scattered by electrons in the hot outer atmosphere of the Sun. This is the "true" corona. Corona graphs are specifically constructed to separate the K corona from the F corona.
Kelvin (K): A unit of absolute temperature. One kelvin is equal to 1 degree C, but zero on the Kelvin scale corresponds to absolute zero (-273.16 degrees C).
Kelvin-Helmholtz instability: A mechanism often invoked to explain phenomena at the magnetopause (and sometimes the plasmapause), especially the observed magnetic pulsations.
Kilo: SI prefix indicating 10³.
K_p index: 3-Hourly range index.
See indices background page.

L

L-band: A range of microwave radio frequencies in the neighborhood of 1 to 2 GHz.
Langrangian point: In a system dominated by two attracting bodies (such as Sun and Earth), a point at which a third, much smaller body (such as a satellite) keeps the same position relative to the other two. Theoretically, the Sun-Earth system has 5 Lagrangian points, but only two are important: L1 (L-one), on the sunward side of Earth, about 4 times the distance of the Moon, and L2 at approximately the same distance on the midnight side. The only two lagrangian stable points, L4 and L5, lie in the orbit of the primary body, leading and trailing it by a 60-degree arc. Jupiter's trojan asteroids can be found orbiting around the Jupiter-Sun L4 and L5 points.
Latchup: With reference to the effect of energetic particles on spacecraft microcircuits, a serious type of single event upset in which the microcircuit is either permanently stuck or cannot be reset without being turned off and on.
Latitude: Circles in parallel planes to that of the equator defining north-south measurements, also called parallels.
Leader spot: In a magnetically bipolar or multipolar sunspot group, the main spot in that portion of the group west of the principal inversion line; also called the preceding or p-spot. Leader and follower describe the positions of spots with respect to apparent motion due to Solar rotation (Compare follower spot).
Leading side: For a satellite that keeps the same face toward the planet, the hemisphere that faces forward, into the direction of motion.
Leap year: Every fourth year, in which a 366th day is added since the Earth's revolution takes 365 days 5 hr 49 min.
Light: Electromagnetic radiation that is visible to the eye, in the neighbourhood of 1 nanometer wavelength.
Light bridge: Observed in white light, a bright tongue or streaks penetrating or crossing sunspot umbrae. Light bridges typically develop slowly and have lifetimes of several days. The appearance of a light bridge is frequently a sign of impending active region division or dissolution. The more brilliant forms occur with overlying bright plage and often occur during the most active phase of the sunspot group.
Light curve: A plot of intensity in a particular wavelength or band of wavelengths against time, especially with reference to a Solar flare; for example, the time history of the x-ray output of a flare.
Light speed: 299792 km s^-1, the constant c.
Light time: The amount of time it takes light or radio signals to travel a certain distance at light speed.
Light year: The distance light travels in a vacuum in one year.
Limb: The edge of the Solar disk, corresponding to the level at which the Solar atmosphere becomes transparent to visible light.
Limb darkening: For certain Solar spectral lines, a lessening of the intensity of the line from the centre of the Solar disk to the limb, caused by the existence of a temperature gradient in the Sun and the line-of-sight through the Solar atmosphere.
Limb flare: A flare at the edge (limb) of the Solar disk; the elevated portions of the flare are seen with particular clarity against the dark sky background.
Linear Energy Transfer: The rate of energy deposit from a slowing energetic particle with distance travelled in matter, the energy being imparted to the material. The greater the LET, the more damaging the particle.
Lithosphere: The crust of a planet.
Lobes: In the magnetotail, the two regions (north and south) separated by the neutral sheet.
Local time: Time adjusted for location around the Earth or other planets in time zones.
Long duration (or decay) event (LDE): With reference to x-ray events, those events that are not impulsive in appearance. The exact time threshold separating impulsive from long-duration events is not well defined, but operationally, any event requiring hours (1 or more) to return to background levels would probably be regarded as an LDE. It has been shown that the likelihood of a coronal mass ejection increases with the duration of an x-ray event, and becomes virtually certain for durations of 6 hours or more.
Longitude: Great circles that pass through both the north and south poles, also called meridians.
Longitudinal component: That component of the magnetic field vector parallel to the direction of view, radial from the Solar surface at disk centre.
Loop prominence system (LPS): A system of prominences in the form of loops associated with major flares, bridging the magnetic inversion line. The lifetime of an LPS is a few hours. Loop prominences observed in H alpha are distinctly brighter than other prominences, and material typically flows downward along both legs from condensation "knots" near the top of the loop. LPSs show a high correlation with proton flares.
Low frequency (LF): That portion of the radio frequency spectrum from 30 to 300 kHz.
Lowest usable frequency (LUF): The lowest frequency that allows reliable long-range HF radio communication by ionospheric refraction.
Lunar: Referring to the moon.

M

M(3000)

The ratio of the maximum frequency reflected once from an ionospheric layer over a 3000-km range to the critical frequency of the layer.

Magnetic bay

A relatively smooth excursion of the H (horizontal) component (see geomagnetic elements) of the geomagnetic field away from and returning to quiet levels. Bays are "positive" if H increases and "negative" if H decreases.

Magnetic cloud

In general, any identifiable parcel of Solar wind. More specifically, a region of about 0.25 AU in radial dimension in which the magnetic field strength is high and the direction of one component of the magnetic field changes appreciably by means of a rotation nearly parallel to a plane. Magnetic clouds may be one manifestation of coronal mass ejections in the interplanetary medium.

Magnetic field

A region of space near a magnetized body where magnetic forces can be detected.

Magnetic field line

Lines everywhere pointing in the direction of the magnetic force, used as a device to help visualize magnetic fields. In a plasma, magnetic field lines also guide the motion of ions and electrons, and direct the flow of some electric currents.

Magnetic local time (MLT)

On Earth, analogous to geographic local time; MLT at a given location is determined by the angle subtended at the geomagnetic axis between the geomagnetic midnight meridian and the meridian that passes through the location. 15 degrees = 1 h. The geomagnetic meridian containing the sub-Solar point defines geomagnetic local noon, and the opposite meridian defines geomagnetic midnight.

Magnetic pole

Two meanings: (1) the points on Earth towards which the compass needle points. (Several slightly different definitions exist, because the field is not exactly that of a dipole.) (2) A concentrated source of magnetic force, e.g. a bar magnet has two magnetic poles near its end.

Magnetic reconnection

In a plasma, the process by which plasma particles riding along two different field lines find themselves sharing the same field line. For instance, Solar wind particles on an interplanetary field line, and magnetospheric ones on a field line attached to Earth, finding themselves united on an "open" field line, which has one end anchored on Earth and the other in distant space. Magnetic reconnection can occur when plasma flows through a neutral point or a neutral line where the intensity of the magnetic field is zero and its direction is not defined. It is an important concept in the theories of energy transfer from the Solar wind to the magnetosphere, and of energy release in substorms.

Magnetic Storms and Substorms

A series of terrestrial disturbances -- namely, the precipitation of auroras and rapid changes in Earth's magnetic field -- caused by high-speed blasts of the Solar wind. Magnetic storms have measurable effects worldwide, such as radio communication blackouts and power grid failures. Magnetic storms are far less frequent than magnetic substorms which are initiated by processes in Earth's magnetotail and are restricted to the auroral ovals.

Magnetogram

A plot showing the amplitude of one or more vector components of a magnetic field versus space or time. Solar magnetograms are a graphic representation of Solar magnetic field strengths and polarity.

Magnetohydrodynamics (MHD)

Just as hydrodynamics is the study of the motion and dynamics of fluids such as water, magnetohydrodynamics is the study of plasma motion and dynamics in the presence of a magnetic field.

Magnetometer

Instrument for measuring magnetic fields. Spacecraft often carry fluxgate magnetometers, which measure components of the magnetic field (3 of them are combined to give its strength and direction) but need to be calibrated. Rubidium-vapor and similar instruments measure only the strength, but their reading is absolute, related to atomic constants.

Magnetopause

The boundary of the magnetosphere, lying inside the bow shock. The location in space where Earth's magnetic field balances the pressure of the Solar wind. It is located about 63,000 km from Earth in the direction of the Sun, or about 1/6th the distance to the moon's orbit.

Magnetopause current sheet

An electric current sheet that more or less coincides with the magnetopause.

Magnetosheath

The region between the bow shock and the magnetopause, characterized by very turbulent plasma. For the Earth, along the Sun-Earth axis, the magnetosheath is about 2 R_E thick.

Magnetosphere

The area above the ionosphere dominated by the Earth's magnetic field and bounded by the magnetopause. This can be usefully divided into the cold low altitude regime, or plasmasphere and the hot high altitude regime. Like the plasmapause, the magnetopause is a dynamic boundary. Its location is controlled by the balance between the ram pressure of the flowing Solar wind and the magnetic pressure of the terrestrial magnetic field.

Magnetotail

The region of the magnetosphere containing field lines stretched away from the Sun. It starts about 8 R_E nightward of the Earth and has been observed to distances of at least 22 R_E.

Major axis

The maximum diameter of an ellipse.

Mantle

Middle layer of the Earth; between the crust and the core.

Mars

Fourth planet from the Sun, a terrestrial planet.

Mass

The measure of the amount of matter in an object. The mass of an object is different from but proportional to its weight (weight is the gravitational force applied to a mass).

Maunder minimum

An approximately 70-year period, centered near 1670, during which practically no sunspots were observed, named after Solar astronomer E. W. Maunder.

Maximum usable frequency (MUF)

The highest frequency that allows reliable HF radio communication over a given ground range by ionospheric refraction. Frequencies higher than the MUF penetrate the ionosphere and become useful for extraterrestrial communications.

Mean

Synonym for average.

Mean Solar time

Time based on an average of the variations caused by Earth's non-circular orbit.

Medium frequency (MF)

That portion of the radio frequency spectrum from 0.3 to 3 MHz.

Mega-

SI prefix indicating 10⁶.

Mercury

First planet from the Sun, a terrestrial planet.

Meridian

Great circle that passes through both the north and south poles, also called line of longitude.

Mesosphere

Part of the Earth homosphere between 50 and 90 km, where the temperature decreases with altitude from a maximum of 280 K at the stratopause to a minimum of 150 K at the mesopause.

Meteor

The luminous phenomenon seen when a meteoroid enters the atmosphere, commonly known as a shooting star.

Meteorite

A part of a meteoroid that survives through the Earth's atmosphere.

Meteoroid

Particles in space which are of natural origin. Nearly all meteoroids originate from asteroids or comets.

micro- (µ)

SI prefix indicating 10^-6.

Microwave

Generically, any radio frequency of 500 MHz or more.

Microwave burst

A radiowave signal associated with optical and/or x-ray flares. Microwave bursts occur mostly at centimeter wavelengths (6 cm = 4995 MHz) but are generally broadband, often extending into the millimeter and decimeter domains (See also U-burst).

Middle latitude

With reference to zones of geomagnetic activity, 20 degrees to 50 degrees geomagnetic latitude. Other zones are equatorial, polar, and high latitude.

Milky Way

The galaxy which includes the Sun and Earth.

milli- (m)

SI prefix indicating 10^-3.

Mirror point

For a particle spiraling around a field line of the magnetosphere, the point where the stronger field (of the Earth, usually) causes the spiral to flatten and then unwind again in the opposite direction. In the inner magnetosphere, ions and electrons are trapped between two mirror points, one north of the equator and one south of it; because of the "mirroring", ions and electrons are turned back before they reach the atmosphere, where they might have got lost.

Modulation

The process of modifying a radio frequency by shifting its phase, frequency, or amplitude to carry information.

Moon

A small natural body which orbits a larger one; a natural satellite.

Moreton wave

A wave disturbance (also known as a flare blast wave) generated by large flares, which is seen to propagate horizontally across the disk of the Sun at a typical velocity of about 1000 km s^-1. Its presence is more visible in wings of the H alpha line. It can cause filaments to erupt as the wave apparently disturbs supporting magnetic fields.

Mound prominence (MDP)

H alpha structure at the Solar limb that is the elevated top of numerous small surges and/or a dense, low-lying prominence.

Mount Wilson magnetic classification

Classification of the magnetic character of sunspots according to rules set forth by the Mount Wilson Observatory in California.

alpha: A unipolar sunspot group.
beta: A sunspot group having both positive and negative magnetic polarities (bipolar), with a simple and distinct division between the polarities.
gamma: A complex active region in which the positive and negative polarities are so irregularly distributed as to prevent classification as a bipolar group.
beta-gamma: A sunspot group that is bipolar but which is sufficiently complex that no single, continuous line can be drawn between spots of opposite polarities.
delta: A qualifier to magnetic class (see below) indicating that umbrae separated by less than 2 degrees within one penumbra have opposite polarity.
beta-delta: A sunspot group of general beta magnetic classification but containing one (or more) delta spot(s).
beta-gamma-delta: A sunspot group of beta-gamma magnetic classification but containing one (or more) delta spot(s).
gamma-delta: A sunspot group of gamma magnetic classification but containing one (or more) delta spot(s).

Multipath

Describing a degraded condition of radio propagation in which the radio wave splits and arrives at the receiver via different paths. Because each path will generally have different lengths, arrival times, and phases, the signal received will suffer fading.

N

Nadir: The direction from a spacecraft directly down toward the centre of a planet. Opposite the zenith.
nano- (n): SI prefix indicating 10^-9.
Nautical mile: The distance spanned by one minute of arc in latitude, 1.852 km.
Neptune: Eighth planet from the Sun, a gas giant or Jovian planet.
Network: (1) Chromospheric: a large-scale brightness pattern in chromospheric and transition region spectral lines, which is located at the borders of the photospheric supergranulation and coincides with regions of local magnetic enhancement. These cellular patterns are typically 3 x 10⁴ km across. (2) Photospheric: a bright pattern that appears in spectroheliograms in certain Fraunhofer spectrum lines. It coincides in gross outline with the chromospheric network.
Neutral point or neutral line: A point or line along which the magnetic intensity is zero. Plays an important role in magnetic reconnection. Neutral lines are, more properly, inversion lines.
Neutron monitor: A ground-based detector that counts secondary neutrons generated by processes originating with the impact of atmospheric molecules and atoms by very energetic particles (galactic or Solar cosmic rays).
Newton: SI unit of force;a body of one kilogram under it effects accelerates one meter per square second.
Node: Point where an orbit crosses a plane.
Noise storm: A transient enhancement of Solar radio emission, particularly at 245 MHz, consisting of an elevated background emission (radiation) and Type I radio bursts.
Non-coherent: Communications mode wherein a spacecraft generates its downlink frequency independent of any uplink frequency.
Non-great-circle propagation: Describing a degraded condition of radio propagation caused by horizontal gradients in the ionospheric electron density. The radio wave is refracted away from its normal great-circle path, which is the shortest distance between two points on the Earth. Strong horizontal gradients are associated with the equatorward boundary of the auroral oval (especially in the night sector) and the sunrise terminator.
Non-Ionizing Energy Loss (NIEL): The portion of energy absorption which results in damage to the lattice structure of solids through displacement of atoms.
Nucleus: The central body of a comet.

O

Omnidirectional: An omnidirectional flux is defined as the scalar integral of the flux over all directions. This implies that no consideration is taken of the directional distribution of the particles which can be non-isotropic. The flux at a point is the number of particles crossing a sphere of unit cross-sectional surface area (i.e. of radius pi^-1/2). An omnidirectional flux should not be confused with an isotropic flux.
One-way: Communications mode consisting only of downlink received from a spacecraft.
Oort cloud: A large number of comets theorized to orbit the Sun in the neighbourhood of 50,000 AU.
Orbit: The path of an object that is moving around a second object or point.
Ordinary mode: One of the two modes of propagation of electromagnetic waves in a magnetic plasma. For propagation along the direction of the magnetic field, it is the mode in which the electric vector rotates opposite to the direction of an electron gyrating freely about the field. For propagation perpendicular to the magnetic field, the electric vector oscillates parallel to the primary magnetic field (See also extraordinary mode).

P

Parallel

Circle in parallel planes to that of the equator defining north-south measurements, also called line of latitude.

Particle flux unit (p.f.u.)

1 particle cm^-2 s^-1 sr^-1.

Penumbra

The sunspot area that may surround the darker umbra or umbrae. In its mature form it consists of linear bright and dark elements radial from the sunspot umbra.

Periapsis

Point of an orbit closest to the body being orbited.

Perigee

Point on an orbit nearest to the Earth.

Perihelion

Periapsis in Solar orbit.

Perijove

Periapsis in Jupiter orbit.

Perilune

Periapsis in lunar orbit.

Periselene

Periapsis in lunar orbit.

Period of revolution

Time of one complete cycle in orbital motion - referred to as a year when applied to Earth.

Period of rotation

Time of one complete cycle - referred to as a day when applied to Earth.

Persistence

Continuation of existing conditions. When a physical parameter varies slowly, the best prediction is often persistence.

Perturbation

A small disturbing effect.

Peta-

SI prefix indicating 10¹⁵.

Phantom command

An apparent (but unintended) spacecraft command caused by the natural environment (See single event upset or electrostatic discharge).

Phase

(1) The angular distance between peaks or troughs of two waveforms of similar frequency. (2) The particular appearance of a body's state of illumination, such as the full phase of the moon.

Phase angle

The angle in which waves come to a body.

Photometry

The measurement of light intensities.

Photosphere

The layer of the Sun from which all visible light reaches us. The Sun is too hot to have a solid surface and the photosphere consists of a plasma at about 6000 °C. Above the photosphere are the chromosphere, and the corona. Sunspots and faculae are observed in the photosphere.

Photovoltaic

Materials that convert light into electric current.

pico- (p)

SI prefix indicating 10^-12.

Pitch angle

In a plasma, the angle between the velocity vector of a charged particle and the direction of the ambient magnetic field.

Plage

On the Sun, an extended emission feature of an active region that is seen from the time of emergence of the first magnetic flux until the widely scattered remnant magnetic fields merge with the background. Magnetic fields are more intense in plage, and temperatures are higher than in surrounding, quiescent regions.

Plage corridor

A low-intensity division in chromospheric plage coinciding with a polarity inversion line and marked by narrow filament segments and/or fibrils spanning the corridor.

Plasma

A partly or wholly ionised gas whose particles exhibit collective response to magnetic and electric fields. The collective motion is brought about by the electrostatic Coulomb force between charged particles. This causes the particles to rearrange themselves to counteract electric fields within a distance of the order of the Debye length. On spatial scales larger then the Debye length, plasmas are electrically neutral. Electrically conductive fourth state of matter from solid, liquid, and gas, consisting of ions and electrons.

Plasma frequency

The characteristic frequency of free plasma oscillations, determined by the balance between electron kinetic energy and ion Coulomb attraction.

Plasmapause

The outer surface of the plasmasphere.

Plasmasheet

In the magnetosphere, the core of the magnetotail in which the plasma is hotter and denser than in the tail lobes north and south of it. The plasmasheet is thought to be separated from the tail lobes by the sheet of the "last closed field lines" and it typically lies beyond geosynchronous orbit.

Plasmasphere

The region of the atmosphere consisting of cold dense plasma originating in the ionosphere and trapped by the Earth's magnetic field. The outer edge of the plasmasphere, the plasmapause, lies at an L-shell of 3 to 6, with a bulge in the dusk region of magnetic local time.

Plasma wave

An oscillation or wave in a plasma that falls in the audio range of frequency.

Pluto

Ninth planet from the Sun, sometimes classified as a small terrestrial planet.

Polar cap absorption (PCA)

An anomalous condition of the polar ionosphere whereby HF and VHF (3-300 MHz) radiowaves are absorbed, and LF and VLF (3-300 kHz) radiowaves are reflected at lower altitudes than normal. PCAs generally originate with major Solar flares, beginning within a few hours of the event and maximizing within a day or two of onset. As measured by a riometer, the PCA event threshold is 2 dB of absorption at 30 MHz for daytime and 0.5 dB at night. In practice, the absorption is inferred from the proton flux at energies greater than 10 MeV, so that PCAs and proton events are simultaneous. However, the transpolar radio paths may still be disturbed for days, up to weeks, following the end of a proton event, and there is some ambiguity about the operational use of the term PCA.

Polar crown

A nearly continuous ring of filaments occasionally encircling either polar region of the Sun (latitudes higher than 50 degrees).

Polar plumes

Fine, ray-like structures of the Solar corona, best observed in the Solar polar regions during Solar minimum.

Polar rain

In the Earth's upper atmosphere, a weak, structureless, near-isotropic flux of electrons precipitating into the polar caps.

Pore

A feature in the photosphere, 1 to 3 arc seconds in extent, usually not much darker than the dark spaces between photospheric granules. It is distinguished from a sunspot by its short lifetime, 10 to 100 minutes.

Post-flare loops

A loop prominence system often seen after a major two-ribbon flare, which bridges the ribbons. Lifetimes are several hours.

Preheating

A slow brightening of an active region, both optically and in x-rays, that sometimes precedes moderate and larger Solar flare events by some tens of minutes.

Primitive

Used in a chemical sense, indicating an unmodified material representative of the original composition of the Solar nebula.

Prominence

A term identifying cloud-like features in the Solar atmosphere. The features appear as bright structures in the corona above the Solar limb and as dark filaments when seen projected against the Solar disk. Prominences are further classified by their shape (for example, mound prominence, coronal rain) and activity. They are most clearly and most often observed in H alpha.

Proton

A proton is a subatomic particle found in the nucleus of every atom. The particle has a positive electrical charge equal and opposite to that of the electron. The mass of a proton is 1.673 10^-27 kg.

Proton event

The measurement of proton flux reaching and sustaining >= 10 p.f.u. for at least 15 min at energies > 10 MeV by the primary Space Environment Services Center geosynchronous satellite. The start time of the event is defined as the earliest time at which event thresholds have been reached. There are two event thresholds, namely p10 and p100 (p10, a proton event reaching 10 p.f.u. at > 10 MeV and p100 reaching 100 p.f.u. at > 100 MeV).

Proton flare

Any flare producing significant counts of protons with energies exceeding 10 MeV in the vicinity of the Earth.

Pulsation

A rapid fluctuation of the geomagnetic field having periods from a fraction of a second to tens of minutes and lasting from minutes to hours. There are two main patterns: Pc (a continuous, almost sinusoidal pattern), and Pi (an irregular pattern). Pulsations occur at magnetically quiet as well as disturbed times. Pc's are grouped, according to their physical and morphological properties, into five categories:

Pc1: periods 0.2-5 s. May occur in bursts ("pearls"), or in consecutive groups of pulsations with sharply decreasing frequency.
Pc2: periods 5-10 s. Do not seem to be physically related to Pc1 or Pc3.
Pc3: periods 10-45 s. Are observed over a wide range of latitudes.
Pc4: periods 45-150 s. Are also known as Pc II or Pc.
Pc5: periods 150-600 s. Are sometimes called giant micropulsations.

Q

Quiescent prominence: A long, sheet-like prominence nearly vertical to the Solar surface. Except in an occasional activated phase, shows little large-scale motion, develops very slowly, and has a lifetime of several Solar rotations. Quiescent prominences form within the remnants of decayed active regions, in quiet areas of the Sun between active regions, or at high Solar latitudes where active regions seldom or never form (See filament).
Quiet: A descriptive word specifically meaning (1) a probability of less than 50% for a C-class flare (see x-ray flare class) in a sunspot region; (2) geomagnetic activity levels such that A_k < 8.
Quiet day curve (QDC): Especially in connection with the components of the geomagnetic field, the trace expected in the absence of activity. The K index and Q index are measured from deviations relative to a QDC. Riometer and neutron monitor deviations are also measured relative to a QDC.

R

R_E

Unit of distance equal to the radius of the Earth, 6371.2 km.

R_J

Unit of distance equal to the radius of Jupiter, 71398 km.

R_S

Unit of distance equal to the radius of Saturn, 60300 km.

Radar aurora

Radar returns from electron density irregularities in auroral regions. The strength of radar auroral returns is aspect dependent.

Radian

Unit of angular measurement equal to the angle at the centre of a circle subtended by an arc equal in length to the radius. Equals about 57.296 degrees.

Radiation

The transfer of energy by means of a particle (including photons). Depending on the context it includes a.o. UV, visible, thermal, microwave and radio-wave radiation.

Radiation belt

The region of high-energy particles trapped in the Earth's magnetic field. There are two belts. The inner belt is part of the plasmasphere and corotates with the Earth; its maximum proton density lies near 5000 km. Inner belt protons are mostly high energy (MeV range) and originate from the decay of secondary neutrons created during collisions between cosmic rays and upper atmospheric particles. The outer belt extends on to the magnetopause on the sunward side (10 R_E under normal quiet conditions) and to about 6 R_E on the nightside. The altitude of maximum proton density is near 16,000-20,000 km. Outer belt protons are lower energy (about 200 eV to 1 MeV) and come from the Solar wind. The outer belt is also characterized by highly variable fluxes of energetic electrons. The radiation belts are often called the "Van Allen radiation belts" because they were discovered in 1968 by a research group at the University of Iowa led by Professor J. A. Van Allen.

Radio emission

Emission of the Sun in radio wavelengths from centimeters to decameters, under both quiet and disturbed conditions. Some patterns, known variously as noise storms, bursts, and sweeps, are identified as described below. These types of emission are subjectively rated on an importance scale of 1 to 3, 3 representing the most intense.

Type I: A noise storm composed of many short, narrow-band bursts in the meter wavelength range (300-50 MHz), of extremely variable intensity. The storm may last from several hours to several days.
Type II: Narrow-band emission (sweep) that begins in the meter range (300 MHz) and sweeps slowly (tens of minutes) toward decameter wavelengths (10 MHz). Type II emissions occur in loose association with major flares and are indicative of a shock wave moving through the Solar atmosphere.
Type III: Narrow-band bursts that sweep rapidly (seconds) from decimeter to decameter wavelengths (500-0.5 MHz). They often occur in groups and are an occasional feature of complex Solar active regions.
Type IV: A smooth continuum of broad-band bursts primarily in the meter range (300-30 MHz). These bursts occur with some major flare events; they begin 10 to 20 minutes after the flare maximum and can last for hours.
Type V: Short-duration (a few minutes) continuum noise in the decameter range usually associated with Type III bursts.

Radiometry

The detection and measurement of radiant electromagnetic energy, usually in the infrared. The radiometer on the Magellan spacecraft, for instance, measured the thermal radiation of the surface of Venus.

Rayleigh-Taylor instability

A fluted or ripple-like instability that can develop on a fluid or plasma boundary surface and propagate along it. This instability is often invoked to explain phenomena in the ionosphere and magnetosphere.

Reconnection

A process by which differently directed field lines link up, allowing topological changes of the magnetic field to occur, determining patterns of plasma flow, and resulting in conversion of magnetic energy to kinetic and thermal energy of the plasma. Reconnection is invoked to explain the energization and acceleration of the plasmas that are observed in Solar flares, magnetic substorms, and elsewhere in the Solar system.

Recurrence

Used especially to express a tendency of some Solar and geophysical parameters to repeat a trend and sometimes the actual value of the parameter itself every 27 days (the approximate rotation period of the Sun).

Red dwarf

A small star, on the order of 100 times the mass of Jupiter.

Red line

An intense coronal emission line at 637.4 nm from Fe X (an iron atom from which nine electrons have been stripped). It identifies relatively cooler regions of the corona.

Refraction

The deflection or bending of electromagnetic waves when they pass from one kind of transparent medium into another.

Region number

A number assigned by the Space Environment Services Center to a plage region or sunspot group if one of the following conditions exists: (1) the region is a group of at least sunspot classification C; (2) two or more separated optical reports confirm the presence of smaller spots; (3) the region produces a solar flare; (4) the region is clearly evident in H alpha and exceeds 5 heliographic degrees in either latitude or longitude. (See also active region.)

Regression

A functional relationship between two or more correlated variables that is often empirically determined from data and is used especially to predict values of one variable when values of the others are given.

Resolution

Ability to distinguish visual detail, usually expressed in terms of the size (in kilometers) of the smallest features that can be distinguished.

Resonance

A relationship in which the orbital period of one body is related to that of another by a simple integer fraction, such as 1/2, 2/3, 3/5.

Retrograde

Backwards; as applied to an orbit, moving in the opposite sense from the great majority of Solar system bodies.

Revolution

Movement of a planet around the Sun; more generally, orbital motion about a point.

Right ascension

The angular distance of a celestial object measured in hours, minutes, and seconds along the celestial equator eastward from the vernal equinox.

Rigidity

A measure of how easily a particle is deflected by a magnetic field, expressed in megavolts (MV) per nucleon. It is the momentum per unit charge. The integral proton spectrum of a flare can be expressed as an exponential function of rigidity rather than a power function of energy.

Ring current

A very spread-out electric current circling around the Earth, carried by trapped ions and electrons. The current is produced by the gradient and curvature drift of the trapped charged particles. The ring current is greatly augmented during magnetic storms because of the hot plasma injected from the magnetotail. The ring current causes a worldwide depression of the horizontal geomagnetic field during a magnetic storm.

Riometer

Relative Ionospheric Opacity meter. A specially designed ground-level radio receiver for continuous monitoring of cosmic noise. The absorption of cosmic noise in the polar regions is very sensitive to the Solar low-energy cosmic ray flux. Absorption events are known as PCAs (polar cap absorption) and are primarily associated with major Solar flares.

Rotation

Turning of a body on its axis.

Rudimentary

A type of sunspot penumbra characterized by granular (rather than filamentary) structure, brighter intensity than the umbra, and narrow extent, and possibly only partially surrounding the umbra. Penumbrae are typically rudimentary during the sunspot formative and decay phases.

S

Satellite

A body that revolves around a larger body. A natural or an artificial moon. Earth-orbiting spacecraft are called satellites. While deep-space vehicles are technically satellites of the Sun or of another planet, or of the galactic centre, they are generally called spacecraft instead of satellites.

Satellite anomaly

The usually undesirable response of spacecraft systems to variations in the space environment. High energy particles cause detector noise and/or physical damage to Solar cells, electronics, and memory devices (single event upsets or "bitflips"). Large and varying low-to-medium energy particle fluxes can result in a charge buildup between spacecraft components, especially during the eclipse season and during spacecraft maneuvers. Atmospheric drag on spacecraft below approximately 1,000 km can increase during geomagnetic storms, resulting in cross-track and in-track orbit errors and orientation problems. Various communication interference problems result during Solar radio bursts from flares when the Sun is within the field of view of the ground tracking dish. Ionospheric irregularities during geomagnetic storms can cause radio telemetry scintillation and fading.

Saturn

Sixth planet from the Sun, a gas giant or Jovian planet.

S-band

A range of microwave radio frequencies in the neighbourhood of 2 to 4 GHz.

S component

The slowly varying (weeks or longer) fluctuation observed in Solar radio emission at microwave frequencies (wavelengths from 3 to 100 cm).

Scintillation

Describing a degraded condition of radio propagation characterized by a rapid variation in amplitude and/or phase of a radio signal (usually on a satellite communication link) caused by abrupt variations in electron density anywhere along the signal path. It is positively correlated with spread F and to a lesser degree, sporadic E. Scintillation effects are the most severe at low latitudes, but can also be a problem at high latitudes, especially in the auroral oval and over the polar caps.

Sector boundary

In the Solar wind, the area of demarcation between sectors, which are large-scale features distinguished by the predominant direction of the interplanetary magnetic field, toward the Sun (a negative sector), or away from the Sun (a positive sector). The sector boundary separating fields of opposite polarity is normally narrow, passing the Earth within minutes to hours as opposed to the week or so needed for passage of a typical sector. The Solar wind velocities in the boundary region are typically among the lowest observed.

Semi-major axis

Half the distance of an ellipse's maximum diameter, the distance from the centre of the ellipse to one end.

SEU, SEE, SEL

Single-event upset, single-event effect, single-event latch-up. These effects are the result of the highly localised deposition of energy by single particles or their reaction products. The energy deposition is sufficient to cause observable effects. An upset is an unexpected change in the logic state of a single digital bit. SEUs can be either "soft" (the microcircuit is not damaged and can be rewritten to either state), or a latch-up, which cannot easily be reset.

Shepherd moon

Moon which gravitationally confines ring particles.

Shock

A discontinuity in pressure, density, and particle velocity, propagating through a compressible fluid or plasma.

Short wave fade (SWF)

An abrupt decrease of HF radio signal strength, lasting from minutes to hours, caused by increased day-side ionization from some Solar flares. An SWF is one effect under the broad category of sudden ionospheric disturbances (SIDs).

Sidereal

Referring to a coordinate system fixed with respect to the distant stars.

Sidereal time

Time relative to the stars other than the Sun.

Simultaneous flares

Unrelated solar flares that occur at nearly the same time. Compare sympathetic flares.

Solar activity

Transient perturbations of the Solar atmosphere as measured by enhanced x-ray emission (see x-ray flare class), typically associated with flares. Five standard terms are used to describe the activity observed or expected within a 24-h period:

Very low: x-ray events less than C-class.
Low: C-class x-ray events.
Moderate: isolated (one to 4) M-class x-ray events.
High: several (5 or more) M-class x-ray events, or isolated (one to 4) M5 or greater x-ray events.
Very high: several (5 or more) M5 or greater x-ray events.

Solar array

Photovoltaic panel.

Solar constant

The electromagnetic radiation from the Sun that falls on a unit area of surface normal to the line from the Sun, per unit time, outside the atmosphere, at one astronomical unit.

Solar coordinates

Specifications for a location on the Solar surface. The location of a specific feature on the Sun (for example, a sunspot) is complicated by the fact that there is a tilt of 7.25 degrees between the ecliptic plane and the Solar equatorial plane as well as a true wobble of the Solar rotational axis. (Only twice a year are the Solar north pole and the celestial north pole aligned.) Consequently, to specify a location on the Solar surface, three coordinates (P, B, L) are necessary to define a grid. Daily values for the coordinates in Coordinated Universal Time (UTC) are listed in The Astronomical Almanac published annually by the U.S. Naval Observatory. The terms used to refer to the coordinates are defined as follows:

P-angle (or P): The position angle between the geocentric north pole and the Solar rotational north pole measured eastward from geocentric north. The range in P is +/- 26.3l °.
B_o: Heliographic latitude of the central point of the Solar disk; also called the B-angle. The range of B_o is +/- 7.23 °, correcting for the tilt of the ecliptic with respect to the Solar equatorial plane. Example: If (P, B_o) = (-26.21 °, -6.54 °), the heliographic latitude of the central point on the Solar disk is -6.54 ° (the north rotational pole is not visible), and the angle between the projection onto the disk of the geocentric north pole and the Solar north rotational pole is 26.21 ° to the west.
L_o: Heliographic longitude of the central point of the Solar disk. The longitude value is determined with reference to a system of fixed longitudes rotating on the Sun at a rate of 13.2 °/day (the mean rate of rotation observed from central meridian transits of sunspots). The standard meridian on the Sun is defined to be the meridian that passed through the ascending node of the Sun's equator on 1 January 1854 at 12:00 UTC and is calculated for the present day by assuming a uniform sidereal period of rotation of 25.38 days.

Once P, B_o, and L_o are known, the latitude, central meridian distance, and longitude of a specific Solar feature can be determined as follows:

Latitude. The angular distance from the Solar equator, measured north or south along the meridian.
Central meridian distance (CMD). The angular distance in Solar longitude measured from the central meridian. This position is relative to the view from Earth and will change as the Sun rotates; therefore, this coordinate should not be confused with heliographic positions that are fixed with respect to the Solar surface.
Longitude. The angular distance from a standard meridian (0 ° heliographic longitude), measured from east to west (0 ° to 360 °) along the Sun's equator. It is computed by combining CMD with the longitude of the central meridian at the time of the observation, interpolating between ephemeris values (for 00:00 UT) by using the synodic rate of Solar rotation (27.2753 days, 13.2 °/day).

Solar flare

An emission of optical, UV and X-radiation from an energetic event on the Sun.

Solar maximum

The month(s) during the sunspot cycle when the smoothed sunspot number reaches a maximum.

Solar minimum

The month(s) during the sunspot cycle when the smoothed sunspot number reaches a minimum.

Solar nebula

The large cloud of gas and dust from which the Sun and planets condensed 4.6 billion years ago.

Solar rotation rate

(1) synodic: l3.39 ° - 2.7 ° sin²(Solar latitude)/day. (2) sidereal: 14.38 ° - 2.7 sin²(Solar latitude)/day. The difference between sidereal and synodic rates is the Earth orbital motion of 0.985 °/day.

Solar sector boundary (SSB)

The boundary between large-scale unipolar magnetic regions on the Sun's surface, as determined from inversion lines mapped using filaments and filament channels, or large-scale magnetograms. The supposed Solar signature of an interplanetary sector boundary.

Solar wind

The Solar wind is part of the corona, the Sun's outer atmosphere. The high temperature of the plasma near the Sun causes it to expand outwards against gravity, carrying the Solar magnetic field along with it. It starts at the Sun as a hot, dense, slowly moving plasma but accelerates outwards to become cool, rare and supersonic near the Earth. Most of the acceleration takes place near the Sun.

Solstice

A point on the ecliptic where the Sun reaches its greatest absolute declination. There are two of these points, halfway between the equinoxes; they mark the beginning of summer and winter.

South Atlantic Anomaly

A region of the Earth centered near 25 degrees S 50 degrees W (geographic coordinates, near the Atlantic coast of Brazil) of low geomagnetic field intensity owing to the fact that the geomagnetic field axis is offset from the centre of the Earth. One consequence of the SAA is that trapped particles in the plasmasphere drift closer to the Earth's surface and can more easily be lost into the atmosphere. The result is that the F region is highly variable in this region, and satellites in low Earth orbits suffer greater radiation doses when they pass through the SAA. There is a corresponding location of maximum geomagnetic field intensity in Southeast Asia.

Spacecraft charging

A term that encompasses all the charging effects on a spacecraft due to the environment in space (See Spacecraft charging background page).

Space debris

Man-made objects or parts thereof in space which do not serve any useful purpose.

Space tether

An experiment in which a satellite was released from the space shuttle at the end of a long insulated cable. The plan was for the dynamo process due to the motion of the tether through the Earth's magnetic field to generate a large current in it.

Space weather

The popular name for energy-releasing phenomena in the magnetosphere, associated with magnetic storms, substorms and shocks.

Spectroscopy

The use of spectrum analysis.

Spectrum

A particular distribution of wavelengths and frequencies.

Spicules

Rapidly changing, predominantly vertical, spike-like structures in the Solar chromosphere observed above the limb. Spicules appear to be ejected from the low chromosphere at velocities of 20 to 30 km s^-1 reaching a height of about 9000 km and then falling back or fading. The total lifetime is 5 to 10 minutes.

Sporadic E (E_s)

Transient, localized patches of relatively high electron density in the E region of the ionosphere, which significantly affect radiowave propagation. Sporadic E can occur during daytime or nighttime, and it varies markedly with latitude. E_s can be associated with thunderstorms, meteor showers, Solar activity and geomagnetic activity.

Spray (SPY)

Luminous material ejected from a Solar flare with sufficient velocity to escape the Sun (675 km/s). Sprays are usually seen in H alpha with complex and rapidly changing form. There is little evidence that sprays are focused by magnetic fields. Compare surge.

Spread F

A condition of the F region of the ionosphere caused by patches of ionization that scatter or duct radio signals, characterized on ionograms by a wide range of heights of reflected pulses. In equatorial latitudes spread F is most commonly observed at night and may be negatively correlated with geomagnetic activity; at high latitudes spread F occurs throughout the daytime and is positively correlated with magnetic activity. The latitude of minimum occurrence of spread F is near 30 degrees magnetic latitude.

S_q

The diurnal variation of the geomagnetic field. The S_q variation is explained in terms of Solar tidal motions of the ionosphere and thermally driven ionospheric winds.

Stopping power

The rate of energy loss of a particle, which also includes emitted secondary radiation.

Stratosphere

Part of the homosphere between 12 and 50 km where the temperature increases with altitude from a minimum of 218 K at the tropopause to a maximum at the stratopause (280 K); The cold region of a planetary atmosphere above the convecting regions (the troposphere), usually without vertical motions but sometimes exhibiting strong horizontal jet streams.

Subcarrier

Modulation applied to a carrier which is itself modulated with information-carrying variations.

Substorm

A geomagnetic perturbation lasting 1 to 2 hours, which tends to occur during local post-midnight nighttime. The magnitude of the substorm is largest in the auroral zone, potentially reaching several thousand nanoteslas. A substorm corresponds to an injection of charged particles from the magnetotail into the auroral oval.

Sudden commencement (SC, or SSC for Storm Sudden Commencement)

An abrupt increase or decrease in the northward component of the geomagnetic field, which marks the beginning of a geomagnetic storm. SCs occur almost simultaneously worldwide but with locally varying magnitudes.

Sudden impulse (SI + or SI -)

A sudden perturbation, positive or negative, of several nT in the northward component of the low-latitude geomagnetic field, not associated with a following geomagnetic storm (An SI becomes an SC if a storm follows).

Sudden ionospheric disturbance (SID)

Any of several radio propagation anomalies due to ionospheric changes resulting from Solar flares. Anomalies include short wave fades, enhancements of atmospherics, phase shifts, cosmic noise absorptions, and signal enhancements.

Sunspot

An area seen as a dark spot on the photosphere of the Sun. Sunspots are concentrations of magnetic flux, typically occurring in bipolar clusters or groups. They appear dark because they are cooler than the surrounding photosphere. Larger and darker sunspots sometimes are surrounded (completely or partially) by penumbrae. The dark centers are umbrae. The smallest, immature spots are sometimes called pores.

Sunspot Cycle

The recurring, eleven-year rise and fall in the number of sunspots. The polarity pattern of the magnetic field reverses with each cycle. Other Solar phenomena, such as the 10.7-cm Solar radio emission, exhibit similar cyclical behavior.

Sunspot number

Index of the activity of the entire visible disk of the Sun.
See indices background page.

Supergranulation

A system of large-scale velocity cells that does not vary significantly over the quiet Solar surface or with phase of the Solar cycle. The cells are presumably convective in origin with weak upward motions in the centre, downward motions at the borders, and horizontal motions of typically 0.3 to 0.4 km s^-1. Magnetic flux is more intense along the borders of the cells.

Super high frequency (SHF)

That portion of the radio frequency spectrum from 3 GHz to 30 GHz.

Superior planet

Planet which orbits farther from the Sun than Earth's orbit.

Superior conjunction

Alignment between Earth and a planet on the far side of the Sun.

Surge

A jet of material from active regions that reaches coronal heights and then either fades or returns into the chromosphere along the trajectory of ascent. Surges typically last 10 to 20 minutes and tend to recur at a rate of approximately 1 per hour. Surges are linear and collimated in form, as if highly directed by magnetic fields. Compare spray.

Sympathetic flares

Solar flares in different active regions that apparently occur as the common result of activation of a coronal connection between the regions. Compare simultaneous flares.

Synodic

Referring to a coordinate system fixed on the Earth.

Synoptic chart

A map of the whole Sun in absolute heliographic coordinates, displaying an integrated view of Solar features observed during a Carrington rotation.

T

Tail lobe: The two bundles of nearly-parallel magnetic field lines which stretch into the magnetotail, on opposite sides of the plasma sheet. The northern lobe contains field lines entering the north polar region of Earth, while the southern lobe contains lines emerging from the southern polar region.
Telemetry: Radio signals from a spacecraft used to encode and transmit data to a ground station.
Tenflare: A Solar flare accompanied by a 10-cm radio noise burst of intensity greater than 100% of the pre-event 10-cm flux value.
Tera- (T): SI prefix indicating 10¹².
Terrestrial Planet: Any of the four planets closest to the Sun: Mercury, Venus, Earth, Mars.
Thermosphere: The Earth atmosphere between 120 and 250 to 400 km (depending on the Solar and geomagnetic activity levels), where temperature has an exponential increase up to a limiting value T_exo at the thermopause. The temperature T_exo is called the exospheric temperature.
Three-way: Coherent communications mode wherein a DSS receives a downlink whose frequency is based upon the frequency of an uplink provided by another DSS.
Torus: Solid geometrical figure with the shape of a doughnut or innertube, as in the toroidal hydrogen cloud surrounding Saturn near the orbit of Titan.
Total electron content (TEC): The number of electrons along a ray path between a transmitter and a receiver. Units are electrons m^-2. This number is significant in determining ionospheric effects such as refraction, dispersion, and group delay on radio waves, and can be used to estimate critical frequencies. The TEC is strongly affected by Solar activity and geomagnetic activity.
Trailing side: For a satellite that keeps the same face toward the planet, the hemisphere that faces backwards, away from the direction of motion.
Transducer: Device for changing one kind of energy into another, typically from heat, position, or pressure into a varying electrical voltage or vice-versa, such as a microphone or speaker.
Transition region: That region of the Solar atmosphere lying between the chromosphere and the corona where the temperature rises from 10000 K to 1000000 K. The transition region is only a few thousand kilometers thick.
Transponder: Electronic device which combines a transmitter and a receiver.
Transverse: Component of magnetic field vector perpendicular to the direction of view, parallel to the Solar surface at the disk centre.
Troposphere: The lower Earth atmosphere between 0 and 12 km, with a nearly uniform composition. The temperature decreases with altitude and shows a local minimum at the tropopause (218 K); The lower regions of a planetary atmosphere where convection keeps the gas mixed. Most clouds are in the troposphere.
True anomaly: The angular distance of a point in an orbit past the point of periapsis, measured in degrees.
Two-ribbon flare: A flare that has developed as a pair of bright strands (ribbons) on both sides of an inversion line of the Solar magnetic field.
Two-way: Communications mode consisting of downlink received from a spacecraft while uplink is being received at the spacecraft.

U

U-burst: A radio noise burst associated with some flares. It has a U-shaped appearance in an intensity versus frequency plot. The minimum intensity falls roughly between 500 and 2000 MHz. A U-burst is sometimes called a Castelli U.
Ultrahigh frequency (UHF): That portion of the radio frequency spectrum from 300 MHz to 3 GHz.
Ultraviolet: Radiation with wavelengths shorter than visible light but longer than X-rays, in the neighbourhood of 100 nm wavelength.
Umbra: The dark core or cores (umbrae) in a sunspot with penumbra, or a sunspot lacking penumbra.
Unipolar magnetic region (UMR): A large-scale photospheric region where the magnetic elements are predominantly of one polarity (for example, the Solar polar regions).
Universal Time (UT): A shortened form of the more correct Coordinated Universal Time (UTC).
Unsettled: With regard to geomagnetic activity, a descriptive word between quiet and active specifically meaning that the A_k index is between 8 and 16.
Uplink: Signal sent to a spacecraft.
Uranus: Seventh planet from the Sun, a gas giant or Jovian planet.

V

Venus: Second planet from the Sun, a terrestrial planet.
Vernal equinox: The equinox that occurs in March. Compare autumnal equinox.
Very high frequency (VHF): That portion of the radio frequency spectrum from 30 to 300 MHz.
Very low frequency (VLF): That portion of the radio frequency spectrum from 3 to 30 kHz.
Volcano: An opening in the Earth's crust that allows magma to reach the surface.
Volcanism: Volcanic force or activity.

W

Wavelength: The distance that a wave from a single oscillation of electromagnetic radiation will propagate during the time required for one oscillation.
White light (WL): The sum of all visible wavelengths of light (400-700 nm) so that all colors are blended to appear white to the eye. No pronounced contribution from any one spectral line (or light-emitting element) is implied.
White-light flare: A major flare in which small parts become visible in white light. This rare continuum emission is caused by energetic particle beams bombarding the lower Solar atmosphere. Such flares are usually strong x-ray, radio, and particle emitters.
Wing: Portion of a spectroscopic absorption (or emission) line between the core of the line and the continuum adjacent to the line.
Wolf number: A historic term for sunspot number. In 1849, R. Wolf of Zurich originated the general procedure for computing the sunspot number. The record of sunspot numbers that he began has continued to this day.
See indices background page.

X

X-band

A range of microwave radio frequencies in the neighbourhood of 8 to 12 GHz.

X-ray

Electromagnetic radiation in the neighbourhood of 100 picometers wavelength.

X-ray background

A daily average background x-ray flux in the 0.1 to 0.8 nm range. It is a midday minimum given in terms of x-ray flare class.

X-ray burst

A temporary enhancement of the x-ray emission of the Sun. The time-intensity profile of soft x-ray bursts is similar to that of the H alpha profile of an associated flare. Soft x-rays are those of energies less than 20 keV, or wavelengths longer than 0.05 nm.

X-ray flare class

Rank of a flare based on its x-ray energy output. Flares are classified according to the order of magnitude of the peak burst intensity (I) measured at the Earth in the 0.1 to 0.8 nm band as follows:

B: I < 10.0^-06 W m^-2
C: 10.0^-06 <= I < 10.0^-05
M: 10.0^-05 <= I < 10.0^-04
X: I >= 10.0^-04

X-ray flare termination

The end time is defined as the time the flux has decayed to 1/2 the peak flux of the event.

Y

Yellow line: A coronal emission line at 569.4 nm from Ca XV (a calcium atom from which 14 electrons have been stripped). It identifies the hottest regions of the corona.

Z

Zeeman effect: The splitting of spectral emission lines due to the presence of a strong magnetic field. Briefly, the lines split into three or more components of characteristic polarization; the components are circular if the local magnetic field is parallel to the line of sight, and linear if the field is perpendicular to the line of sight. The amount of splitting is proportional to the strength of the field.
Zenith: The point on the celestial sphere directly above the observer. Opposite the nadir.

The definitions in this glossary were retrieved from various sources. These include:

Last update: Tue, 10 Sep 2024