A spacecraft in LEO receives electromagnetic radiation from three primary external sources. The largest source is the direct solar flux. The mean value of this solar flux at the mean SunEarth distance is called the “solar constant”. It is not really a constant but varies by about 3,4 % during each year because of the slightly elliptical orbit of the Earth about the Sun. The two others radiation sources are the fraction of the incident sunlight reflected off the planet, termed albedo, and the Earth infrared radiation.
Solar and geomagnetic activities are often described by indices. The UV radiation of the Sun, which strongly affects the Earth atmosphere, cannot be directly measured from the ground. But it was found to be strongly correlated with e.g. the sunspot number and the cm wavelength Sun radiation. For example, the widely used 10,7 cm radio flux index (F10.7) gives an indication of the solar UV radiation output which is highly variable over a solar cycle.
Geomagnetic indices typically describe the variation of the geomagnetic field over a certain time period. They provide a measure of the disturbance of the magnetosphere which has direct consequences for the charged particle space environment, or the external component of the geomagnetic field.
Solar and geomagnetic indices are used as input for upper atmosphere and other models of the near Earth space environment. They are provided for short durations or as long time averages. Predictions for future index values are usually provided at different confidence levels and they are available for complete solar cycles. The given data are mainly average values. For detailed thermal analyses or certain special applications, more detailed data and models are required. These are outside the scope of this Standard.