Abstract
The diurnal variation of the upper atmosphere as revealed from satellite drag measurements has been further investigated on the basis of a simultaneous integration of the heat conduction equation and the hydrostatic law. In addition to the beat source due to absorption of solar extreme ultraviolet radiation and the hypothetical “second heat source,” the heating due to absorption of solar radiation in the Schumann-Runge range by oxygen molecules has been included. Furthermore, the effects of time-dependent variations in the boundary conditions on the phase and amplitude of the diurnal variation in the upper thermosphere and exosphere have been investigated. Also the effects of lateral heat conduction and lateral convective heat transport on the diurnal variation of density and temperature are discussed.
The main purpose of the paper is to investigate several possibilities which could be thought to eliminate the requirement for the “second heat source.” It is shown that neither the inclusion of absorption of solar radiation in the Schumann-Runge band by O2 molecules in our heat source nor diurnal variations of the boundary conditions at 120 km can be invoked in order to explain the diurnal variation on the basis on an EUV heat source exclusively. Further the effect of horizontal conduction is found in a simplified analysis to be quantitatively insufficient to account for an energy transport toward the west large enough to explain the observed diurnal variation under the presumption that all heating comes from the solar EUV radiation.
