3.2              Terms specific to the present standard

3.2.1                      Ap, Kp indices

geomagnetic activity indices to describe fluctuations of the geomagnetic field

NOTE              Values of Ap range from 0 to 400 and they are expressed in units of nT (nanotesla). Kp is essentially the logarithm of Ap.

3.2.2                      absorbed dose

energy absorbed locally per unit mass as a result of radiation exposure which is transferred through ionization and excitation

NOTE              A portion of the energy absorption can result in damage to the lattice structure of solids through displacement of atoms, and this is now commonly referred to as Non-Ionizing Energy Loss (NIEL).

3.2.3                      accommodation coefficient

measure for the amount of energy transfer between a molecule and a surface

3.2.4                      albedo

fraction of sunlight which is reflected off a planet

3.2.5                      atmospheric albedo neutrons

neutrons escaping from the earth’s atmosphere following generation by the interaction of cosmic rays and solar particles

NOTE              Atmospheric albedo neutrons can also be produced by other planetary atmospheres and surfaces.

3.2.6                      bremsstrahlung

high-energy electromagnetic radiation in the X-γ energy range emitted by charged particles slowing down by scattering of atomic nuclei

NOTE              The primary particle is ultimately absorbed while the bremsstrahlung can be highly penetrating. In space, the most common source of bremsstrahlung is electron scattering.

3.2.7                      contaminant

molecular and particulate matter that can affect or degrade the performance of any component when being in line of sight with that component or when residing onto that component

3.2.8                      contaminant environment

molecular and particulate environment in the vicinity of and created by the presence of a spacecraft

3.2.9                      current

the rate of transport of particles through a boundary

NOTE              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 ¼ f normally in each direction across the plane. Current is often used in the discussion of radiation transport.

3.2.10                  direct flux

free stream or outgassing molecules that directly impinge onto a critical surface, i.e. without prior collisions with other gas species or any other surface

3.2.11                  distribution function f(x,v)

function describing the particle density of a plasma in 6-D space made up of the three spatial vectors and the three velocity vectors, with units s3 m-6

NOTE              For distributions that are spatially uniform and isotropic, it is often quoted as f(v), a function of scalar velocity, with units s m-4, or f(E) a function of energy, with units J-1m-3. This can be converted to flux as follows:

(3‑1)

or

(3‑2)

where

v     is the scalar velocity;

E     is the energy;

m     is the particle mass.

3.2.12                  dose

quantity of radiation delivered at a position

NOTE              In its broadest sense this 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.

3.2.13                  dose equivalent

radiation quantity normally applied to biological effects and includes scaling factors to account for the more severe effects of certain kinds of radiation

3.2.14                  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)

3.2.15                  Earth infrared

thermal radiation emitted by the Earth

NOTE              It is also called outgoing long wave radiation.

3.2.16                  energetic particle

particles which, in the context of space systems radiation effects, can penetrate outer surfaces of spacecraft

NOTE              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.

3.2.17                  equivalent fluence

quantity which attempts to represent the damage at different energies and from different species

NOTE 1      For example: For solar cell degradation it is often taken that one 10 MeV protons is “equivalent” to 3 000 electrons of 1 MeV. This concept also occurs in consideration of Non-ionizing Energy Loss effects (NIEL).

NOTE 2      Damage coefficients are used to scale the effect caused by particles to the damage caused by a standard particle and energy.

3.2.18                  exosphere

part of the Earth’s atmosphere above the thermosphere for which the mean free path exceeds the scale height, and within which there are very few collisions between atoms and molecules

NOTE 1      Near the base of the exosphere atomic oxygen is normally the dominant constituent.

NOTE 2      With increasing altitude, the proportion of atomic hydrogen increases, and hydrogen normally becomes the dominant constituent above about 1 000 km. Under rather special conditions (i.e. winter polar region) He atoms can become the major constituent over a limited altitude range.

NOTE 3      A small fraction of H and He atoms can attain escape velocities within the exosphere.

3.2.19                  external field

part of the measured geomagnetic field produced by sources external to the solid Earth

NOTE              the external sources are mainly: electrical currents in the ionosphere, the magnetosphere and coupling currents between these regions.

3.2.20                  F10.7 flux

solar flux at a wavelength of 10.7 cm in units of 104 Jansky (one Jansky equals 10-26 Wm-2Hz-1)

3.2.21                  fluence

time-integration of the flux

3.2.22                  flux

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-2s-1) above a certain threshold energy

NOTE              The directional flux is the differential with respect to solid angle (e.g. particles cm-2 steradian-1s-1) while the “differential” flux is differential with respect to energy (e.g. particles cm-2 MeV-1s-1). In some cases fluxes are also treated as a differential with respect to Linear Energy Transfer (see 3.2.32).

3.2.23                  free molecular flow regime

condition where the mean free path of a molecule is greater than the dimensions of the volume of interest (characteristic length)

3.2.24                  geocentric solar magnetospheric coordinates (GSM)

elements of a right-handed Cartesian coordinate system (X,Y,Z) with the origin at the centre of the Earth

NOTE              X points towards the Sun; Z is perpendicular to X, lying in the plane containing the X and geomagnetic dipole axes; Y points perpendicular to X and Z and points approximately towards dusk magnetic local time (MLT).

3.2.25                  heterosphere

Earth’s atmosphere above 105 km altitude where the neutral concentration profiles are established due to diffusive equilibrium between the species

NOTE              N2 is normally dominant below approximately 200 km, O is normally dominant from approx 200 km to approx. 600 km, He is dominant above 600 km altitude, and H dominant at very high altitudes. These conditions depend on solar and geomagnetic activity, and the situation may be quite variable at high altitudes during major geomagnetic disturbances.

3.2.26                  homosphere

Earth’s 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

NOTE              The homopause (or turbopause) marks the ceiling of the homosphere.

3.2.27                  indirect flux

molecules impinging on a critical surface, after collision with, or collision and sojourn on other surfaces

3.2.28                  internal field

part of the measured geomagnetic field produced by sources internal to the solid Earth, primarily due to the time-varying dynamo operating in the outer core of the Earth

3.2.29                  interplanetary magnetic field

solar coronal magnetic field carried outward by the solar wind, pervading the solar system

3.2.30                  isotropic

property of a distribution of particles where the flux is constant over all directions

3.2.31                  L or L shell

parameter of the geomagnetic field, often used to describe positions in near-Earth space

NOTE              L or L shell has a complicated derivation based on an invariant of the motion of charged particles in the terrestrial magnetic field (see Annex E). However, it is useful in defining plasma regimes within the magnetosphere because, for a dipole magnetic field, it is equal to the geocentric altitude in Earth-radii of the local magnetic field line where it crosses the equator.

3.2.32                  linear energy transfer (LET)

rate of energy deposit from a slowing energetic particle with distance travelled in matter, the energy being imparted to the material

NOTE              Normally used to describe the ionization track caused by passage of an ion. LET is material-dependent and is also a function of particle energy. For ions involved in space radiation effects, it increases with decreasing energy (it also increases at high energies, beyond the minimum ionizing energy). LET allows different ions to be considered together by simply representing the ion environment as the summation of the fluxes of all ions as functions of their LETs. This simplifies single-event upset calculation. The rate of energy loss of a particle, which also includes emitted secondary radiations, is the stopping power.

3.2.33                  magnetic local time (MLT)

parameter analogous to longitude, often used to describe positions in near-Earth space

NOTE              Pressure from the solar wind distorts the Earth magnetic field into a comet-like shape. This structure remains fixed with its nose towards the Sun and the tail away from it as the Earth spins within it. Hence longitude, which rotates with the Earth, is not a useful way of describing position in the magnetosphere. Instead, magnetic local time is used. This has value 0 (midnight) in the anti-sunward direction, 12 (noon) in the sunward direction and 6 (dawn) and 18 (dusk) perpendicular to the sunward/anti-sunward line. This is basically an extension of the local solar time on Earth, projected vertically upwards into space although allowance is made for the tilt of the dipole.

3.2.34                  mass flow rate

mass (g) of molecular species crossing a specified plane per unit time and unit area (g cm-2s-1)

3.2.35                  Maxwellian distribution

plasma distribution functions described in terms of scalar velocity, v, by the Maxwellian distribution below:

(3‑3)

where

n        is the density;

k         is the Boltzmann constant;

T        is the temperature.

NOTE              The complete distribution is therefore described by a pair of numbers for density and temperature. This distribution is valid in thermal equilibrium. Even non-equilibrium distributions can often be usefully described by a combination of two Maxwellians.

3.2.36                  meteoroids

particles in space which are of natural origin

NOTE              nearly all meteoroids originate from asteroids or comets.

3.2.37                  meteoroid stream

meteoroids that retain the orbit of their parent body and that can create periods of high flux

3.2.38                  molecular column density (MCD)

integral of the number density (number of molecules of a particular species per unit volume) along a specified line of sight originating from a (target, critical, measuring, reference) surface

3.2.39                  molecular contaminant

contaminant without observable dimensions

3.2.40                  nano-Tesla

standard unit of Geomagnetism

NOTE              An older unit, not widely used now, is the Gauss, which is 105 nT.

3.2.41                  omnidirectional flux

scalar integral of the flux over all directions

NOTE              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 1/√π). An omnidirectional flux is not to be confused with an isotropic flux.

3.2.42                  outgassing rate

mass of molecular species evolving from material per unit time and unit surface area (g cm-2s-1)

NOTE              Outgassing rates can also be given in other units, such as in relative mass unit per time unit: (g s-1), (% s-1) or (% s-1cm-2).

3.2.43                  particulate contaminant

solid or liquid contaminant particles

3.2.44                  permanent molecular deposition (PMD)

molecular matter that permanently sticks onto a surface (non-volatile under the given circumstances) as a result of reaction with surface material, UV-irradiation or residual atmosphere induced reactions (e.g. polymerization, formation of inorganic oxides)

3.2.45                  plasma

partly or wholly ionized gas whose particles exhibit collective response to magnetic and electric fields

NOTE              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 than the Debye length plasmas are electrically neutral.

3.2.46                  radiation

transfer of energy by means of a particle (including photons)

3.2.47                  return flux

molecules returning to the source or a surface which is not in direct view of the incoming flux

NOTE              The cause can be:

•      collisions with other residual natural atmospheric species (ambient scatter) or with other identical or different contaminant species (self scatter) before reaching the critical surface;

•      ionization or dissociative ionization of the molecules under radiation (e.g. UV or particles) and subsequent attraction to a charged surface

3.2.48                  single-event upset (SEU), single-event effect (SEE), single-event latch-up (SEL)

effects resulting from the highly localized deposition of energy by single particles or their reaction products and where the energy deposition is sufficient to cause observable effects

3.2.49                  sporadic flux

random flux with no apparent pattern

3.2.50                  solar constant

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

NOTE              1 AU = average Earth-Sun distance

3.2.51                  solar flare

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

NOTE              There is some controversy about the causal relationship between solar flares and the arrival of large fluxes of energetic particles at Earth. Therefore, it is more consistent to refer to the latter as Solar Energetic Particle Events (SEPEs).

3.2.52                  sticking coefficient

parameter defining the probability that a molecule, colliding with a surface, stays onto that surface for a time long compared to the phenomena under investigation

NOTE              It is a function of parameters such as contamination/surface material pairing, temperature, photo-polymerization, and reactive interaction with atomic oxygen.

3.2.53                  surface accommodation

situation which occurs when a molecule becomes attached to a surface long enough to come into a thermal equilibrium with that surface

3.2.54                  thermosphere

Earth’s atmosphere between 120 km and 250 km to approximately 400 km (depending on the activity level), where temperature has an exponential increase up to a limiting value T∞ at the thermopause (where T∞ is the exospheric temperature)

3.2.55                  trackable objects

objects regularly observed and catalogued by ground-based sensors of a space surveillance network (typically objects larger than about 10 cm in LEO and larger than about 1 m in GEO)

3.2.56                  VCM-test

screening thermal vacuum test to determine the outgassing properties of materials

NOTE              The test is described in ECSS-Q-ST-70-02 [RD.23] and ASTM-E595 [RD.24]. The test results are:

•      TML - Total Mass Loss, measured ex-situ as a difference of mass before and after exposure to a vacuum under the conditions specified in the outgassing test, normally expressed in % of initial mass of material.

•      CVCM - Collected Volatile Condensable Material, measured ex-situ on a collector plate after exposure (to a vacuum) under the conditions specified in the outgassing test, normally expressed in % of initial mass of material.

3.2.57                  world magnetic model

revised every five years by a US-UK geomagnetic consortium, primarily for military use