6.1.2 Electromagnetic radiation and indices
6.1.2.1 Solar constant
The solar constant is defined as the radiation 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 (1 AU = average EarthSun distance). The currently measured 1-sigma standard deviation in the composite dataset is approximately 0,6 Wm-2 and there is a long-term (yearly) smoothed solar cycle minimum to maximum relative variation about the mean value of approximately 1,4 kWm-2[RD.2] (largest during the period of maximum solar activity).
6.1.2.2 Solar spectrum
6.1.2.2.1 Soft X-rays or XUV (0,1 nm to 10 nm)
Usually associated with solar coronal phenomena, flares, million-degree temperatures, and atomic dissociation. The corona extends from about 21 000 km to 1 400 000 km above the photosphere. X-ray flares are responsible for enhancements in the D and E regions of the Earth's ionosphere.
6.1.2.2.2 Extreme ultraviolet or EUV (10 nm to 121 nm)
EUV has emission lines that come from the upper chromosphere (near-coronal temperatures), transition region, and lower corona. This spectral band is responsible for ionization and heating in the E and F regions of the ionosphere.
6.1.2.2.3 Ultraviolet or UV (100 nm to 400 nm)
UV solar flux is emitted primarily from the base of the sun's chromosphere layer, and has components due to active and quiet solar conditions. This band is responsible for only 1% of the total solar irradiance, but it is important because below 300 nm, it is completely absorbed by ozone and diatomic oxygen atoms in the earth's upper atmosphere.
6.1.2.2.4
Visible, optical or VIS (380 nm to 760 nm)
Visible light comes from the solar photosphere, which is only about 400 km thick, has a temperature of approximately 5 000 to 6 000 degrees Kelvin, and yet is responsible for the greatest percentage of the total solar radiation.
6.1.2.2.5 Infrared or IR (0,70 mm to 1 mm)
Solar infrared in this range is responsible for the direct heating of the Earth's lower atmosphere, through absorption by H2O, and has an effect on minor species constituents in the Earth's mesosphere and thermosphere.
NOTE The ISO 21348 Standard on determining solar irradiances provides more details on the solar spectrum [RD.3].
6.1.2.3 Indices
6.1.2.3.1 General
Solar and geomagnetic indices are used to describe the activity levels of the Sun and the disturbance of the geomagnetic field. Most activity indices are given for short periods and as long duration averages. They are also used for long range predictions of solar activities. Many space environment models require activity index values as input parameters.
6.1.2.3.2 Solar activity indices
• Sunspot number (R)
The sunspot number (R, alternatively called Ri or Rz) is a daily index of sunspot activity, defined as
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(6‑1) |
where
s is the number of individual spots,
g the number of sunspot groups, and
k is an observatory factor.
NOTE R12 (Rz12) is the 12-month running mean of the sunspot number R.
• F10.7
F10.7 (abbreviated F10) is the
traditional solar energy proxy that is used both for NRLMSISE-00
[RN.4] and JB-2006 [RN.5] atmosphere models. It corresponds to the solar radio
flux emitted by the sun at 2 800 megaHertz (10,7 cm wavelength).
NOTE The physical units of F10.7 are 10-22 Wm-2Hz-1; the numerical value is used without the multiplier as is customarily done and expressed as solar flux units (sfu). In other words, a 10,7 cm radio emission of 150x10-22 W m-2Hz-1 is simply referred to as F10.7 = 150 sfu.
F10.7 and the sunspot number, R, are correlated. Averaged (over one month or longer) values can be converted by the following expression:
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(6‑2) |
• S10.7
S10.7 (abbreviated S10) [RD.122] is a solar energy proxy that is used for the JB-2006 atmosphere model [RN.5] It is measured by SOHO with the Solar Extreme-ultraviolet Monitor (SEM) in the 26–34 nm solar EUV range, and normalized using a mean value of 1,9955x1010 photons cm-2s-1. It is expressed in sfu, as F10.7.
• M10.7
M10.7 (abbreviated M10) [RD.122] is a solar energy proxy that is used for JB-2006 atmosphere model. It is measured by NOAA 16 and NOAA 17 operational satellites with the Solar Backscatter Ultraviolet (SBUV) spectrometer. It is a measure of chromospheric and some photospheric solar active region activity and is expressed in sfu, as F10.7 and S10.7.
• IG12
IG12 [RD.147] is a solar activity index derived from the monthly
mean noon f0F2 data available from thirteen ionospheric observations. The IG
index has been confirmed as an alternative to sunspot number when predictions
of f0F2 are being prepared with the aid of the CCIR atlas of ionospheric
characteristics. IG12 is used for the IRI-2007 ionospheric model.
• Solar wind speed
The solar wind speed is used in external magnetic field model computation. It is the outward flux of solar particles and magnetic fields from the sun. Typically, solar wind velocities are near 350 km s-1.
6.1.2.3.3 Geomagnetic activity indices
Geomagnetic activity indices are used to describe fluctuations of the geomagnetic field.
• Kp and ap
Most widely used planetary indices are Kp and ap. They are based on 3hour measurements from 13 ground stations. Values of aprange from 0 to 400 and are expressed in units of 2 nT. Kp is essentially the logarithm of ap, its scale is 0 to 9 expressed in thirds of a unit, e.g. 5- is 4 2/3, 5o is 5 and 5+ is 5 1/3.The conversion from Kp to ap is given in Table 6‑1 (taken from [RD.100]. A daily index, Ap, is obtained by averaging the eight values of ap for each day. The index Ap can have values intermediate to those of ap.
Dst (Disturbance Storm Time) is a geomagnetic index used in external magnetic field model computation. It describes variations in the equatorial ring current and is derived from hourly scalings of low-latitude horizontal magnetic variation. It is expressed in nT.
• IMF
IMF (Interplanetary Magnetic Field) is a geomagnetic index used in external magnetic field model computation. It corresponds to the part of the Sun's magnetic field that is carried into interplanetary space by the solar wind. The three orthogonal components of the IMF are Bx, By and Bz. Bx and By are oriented parallel to the ecliptic.
The IMF is a weak field, varying in strength near the Earth from 1 to 37 nT, with an average value of about 6 nT.