Structure of the thermosphere

Abstract

The vertical distribution of the density in the thermosphere, deduced from satellite observations, must be explained by an increase of the scale height with altitude. A varying gradient of the scale height cannot be interpreted by assuming an increase of the temperature gradient with altitude. An examination of the interrelationships between the absolute values of density in a dark atmosphere and diurnal conditions of heat conduction reveals that the varying gradient of the scale height above 200 km is essentially due to the decrease of the molecular weight, mg, of the atmospheric constituents subject to diffusion. In the night atmosphere the isothermy above a certain altitude (>200 km) is the critical factor characterizing the vertical distribution of density. The temperature of the isothermal region, resulting from conduction, is related to the ultra-violet heating which was available during the day. The effect of diffusion has been clearly shown by establishing a thermo-isobaric relation connecting the temperature of the isothermal region with an isobaric level where atomic oxygen has a specific concentration. From observational data on the variation of the night-time density at high levels, it is possible to deduce the variation of the temperature of the isothermal region. The gradient of temperature in a sunlit atmosphere is related to the fraction of the ultra-violet Solar energy absorbed, which determines the magnitude of the variation of the scale height with altitude. Since heat transport is a function of the atomic or molecular concentrations and the square of the distance, it is shown that anomalies in the temperature gradient cannot be permanent.