Density variations in the lower thermosphere from analysis of the AE-C accelerometer measurements

Density measurements at 140, 160, 180 and 200km from the AE-C accelerometer experiment during 1974 are analyzed by least-squares multiple linear regression. The resulting empirical model is compared to the widely-used Jacchia (1971) model which is primarily based on satellite orbital decay data above 200 km. The semiannual and geomagnetic (K_p) variations derived from the AE-C data set are in good agreement with Jacchia's predictions. However, individual magnetic storm responses often exceed such average representations of the geomagnetic effect by more than a factor of two. The density is more variable with respect to the daily 10.7cm solar flux than Jacchia's model predicts. The regression analysis reveals a positive correlation with the absolute value of the geographic latitude. The statistical weakness of this latter effect reflects the transient, aperiodic nature of high-latitude heating events related to magnetic activity, which can only be properly analyzed on an individual basis. Finally, the sub-solar bulge effect enters the regression equation with about half the amplitude predicted by the Jacchia model at these heights.