Atmospheric density and temperature variations from the Explorer XVII satellite and a further comparison with satellite drag
Hundreds of direct, in situ atmospheric density measurements were made at satellite altitudes during the three month interval from April to June 1963. At northern mid-latitudes the density varied by at least a factor of four at 270 km and a factor of ten at 450 altitude. At 360 km altitude the average density had a diurnal variation of a factor of five with a maximum value between 1200 and 1500 hr. The diurnal temperature varied between a maximum of 1000°K and a minimum near 500°K. A large variation in density scale height was observed for altitudes above 400 km near the time of local sunrise. A greater degree of heating of the atmosphere occurred for small variations in the planetary geomagnetic index than was previously expected. For example, an increase of 70°K would result from a variation of Ap, from two to eight ; this change correlates with a solar wind speed change from 350 to 430 km/sec. The drag-determined densities were found to be a factor of two greater than the averaged gauge densities. It was concluded that a latitude dependence of the density in the interval 35°N to 55°N must be less than a factor of two. An indication was found of a significant amount of hydrogen in the atmosphere above 800 km altitude. Fluctuations of a factor of two in the density were observed which do not correlate with known activity indices such as 10.7 cm solar index or daily geomagnetic planetary index. One such fluctuation correlates with a reported increased intensity of the day airglow 6300 Å oxygen line. These fluctuations suggest that caution should be used in interpreting results obtained by combining measured quantities with model atmosphere values.