A Preliminary Model for the Variations of Upper Air Densities

Abstract

SATELLITE observations have shown a significant variability of the density of the upper air above a height of 200 km. During the period 1957–60, several major effects must be distinguished, in addition to dependence on altitude. The construction of general representative atmospheric models is therefore extremely difficult, as has been found by several authors¹. Hence it seems to be more useful to give a model for the variability, from which the density of the air can be calculated for special cases. Three influences seem to be predominant²: (1) The solar W-effeet produced by the variable short wave-length solar ultra-violet radiation. This is demonstrated by a strong correlation with the sunspot number³ or with the solar radio emission in the decimetre wave-length region⁴. (2) The day–night effect, the amplitude of which increases strongly with height⁵. (3) The annual effect. This effect shows a pronounced annual and a semi-annual period². It is demonstrated by the fact that for different satellites the day–night variation is not constant in phase and amplitude if it is normalized for a constant solar W-effect⁶. This third effect is probably caused by the interaction with interplanetary plasma, the density of which varies along the Earth's orbit perhaps due to an interstellar wind⁷.