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Isadore Harris and Wolfgang Priester

Abstract

Several possible mechanisms are investigated which could be invoked to explain the observed semiannual density variation in the thermosphere and exosphere. A variation of the height of the mixtopause leads to a large density variation for heights above 700 km. Below that height, however, the density is essentially invariant to this process. This invariance is to some degree caused by the neglect of downward heat transport by eddy diffusion at the bottom of the thermosphere. The limitations of using the simple mixtopause scheme in this context are discussed.

Another mechanism can be ruled out on the grounds that it fails to explain the observed amplitude at a height of 200 km. This mechanism is a small permanent heat flux conducted into the lower exosphere from above. A variation of this flux by 3×10−2 erg cm−2 sec−1 would yield a sufficiently large density variation only for heights above 300 km. The recent observations at heights below 200 km indicate that the temperature and density at the bottom of the thermosphere (90–120 km) vary with a semiannual period.

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Isadore Harris and Wolfgang Priester

Abstract

The physical properties of the upper atmosphere are determined mainly by heat conduction, the heat sources and the barometric law. An analysis of the integro-differential equation which describes these physical processes has been carried out. It is found that heating of the thermosphere due to absorption of the solar extreme ultraviolet (EUV) radiation alone cannot explain the observed diurnal variation of density and temperature, since it would yield a maximum of these properties at about 17h local time, instead of 14h where it is observed. Secondly, if the EUV flux is adjusted to give the observed average temperature, then the diurnal variation in density would be much too large compared with the observed amplitude. Thirdly, it would require an extremely high efficiency for the conversion of EUV radiation into heat, if we compare the required flux with Hinteregger's measurements of the EUV flux. Thus, it is necessary to have another heat source in addition to the heating due to absorption of EUV radiation. If an additional beat source is used, which has a maximum at about 9h local time and a flux of 1 erg cm−1 sec−1, a time-dependent model of the upper atmosphere is obtained that is in good agreement with the observed densities. There is evidence that this additional heat source derives its energy ultimately from the solar corpuscular radiation.

In this paper we present the results of calculations for a model in the equatorial and temperature zones of the earth, for those times when the average solar activity corresponds to a solar radiation flux of 200×110−22 Wm−2 (cps)−1 at 10.7-cm wavelength. The physical properties (temperature, density, pressure, scale height, mean molecular weight and the number densities of N2, O2, O, He and H) are given as a function of local time and for the altitudes between 120 km and 2050 km.

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Isadore Harris and Wolfgang Priester

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.

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