On the Inexactness of the 10.7-cm Flux from the Sun as an Index of the Total Extreme Ultraviolet Radiation

Abstract

The 10.7-cm solar flux (S) is widely used for upper-atmosphere studies as an index of the solar extreme ultraviolet radiation (EUV), whose variations are responsible for major changes in properties above 200 km. A model was developed to calculate the density as a function of S for various local times and altitudes from 200 to 800 km. The accuracy was checked by comparison of calculated rates of satellite orbital decay with observed values for 7 satellites. This comparison revealed systematic differences between the calculated and observed decay values. From the analysis of these differences, it is concluded that the ?semi-annual? effect is not a real effect in 1958 to 1961; rather this effect arises because S is not an accurate index for the total EUV. Evidently, the EUV has two variable components: one radiated from active (sunspot) areas, correlated with S, and the other radiated as a background emission rather uniformly distributed over the entire sun and not correlated with S. A more accurate estimate of the relative variation of the EUV by months from 1958 to 1962 is presented in terms of a new parameter S?. While S appears to represent the year-to-year variation of the sun's EUV fairly well from 1958 to 1960, S? averages much less than S in 1961 and 1962, indicating that S cannot be used to describe the long-term (solar cycle) variation of the EUV. Variations of the isothermal temperature above 400 km and the mean molecular weight from 400 to 800 km have been calculated for various S? and local times with the generalized hydrostatic equation and constraints consistent with diffusive equilibrium. The average relative contribution of magnetic heating to the atmosphere is about 10 per cent of the total heating affecting the 100 to 200 km layer from July 1958 to September 1960.