Diurnal and seasonal latitudinal variations in the upper atmosphere

Abstract

The variations with solar and geomagnetic activity have been eliminated by the use of appropriate models, and the diurnal variation has been isolated in the densities derived from the drag of seven artificial satellites with perigees between 250 and 650 km. An analysis of the diurnal variations leads to the following conclusions: 1. A. Contrary to previous suggestions, the diurnal bulge does migrate in latitude in phase with the subsolar point at all heights. 2. B. The diurnal bulge peaks at 2 pm. local solar time (LST) irrespective of solar activity; the minimum density occurs between 3 and 4 am. 3. C. The amplitude of the diurnal variation seems to fluctuate in a manner that cannot be entirely accounted for by variations in solar activity. 4. D. The significant high-latitude residuals from the diurnal-variation model observed on the drag of Explorer 19 and Explorer 24 may be explained by the formation at exospheric heights of a region of greater helium density above the poles during the winter months, caused by a seasonal variation in the height of the turbopause.