Thermospheric hydrogen: The long‐term solar influence

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Abstract
We describe an investigation of the long‐term behavior of neutral hydrogen in the thermosphere as influenced by changes associated with the solar cycle. Hydrogen concentrations are derived from application of charge exchange equilibrium to ionospheric measurements in the low‐latitude F region with the AE‐C and AE‐E satellites. The adopted data base spans the years, 1974–1979, of increasing solar activity and daily average exospheric temperatures ranging between 700° and 1300°K. Statistical analyses yield the diurnal relative variations and mean concentrations of hydrogen at a reference height of 300 km. These mean concentrations generally agree with other measurements over smaller temperature intervals, with minor differences at the higher temperatures. The associated daily average evaporative escape flux tends toward a constant value above ∼1000°K; this behavior is consistent with theory for the total vertical flux, but the magnitude, near 108 cm−2 s−1, is smaller by a factor of ∼3 than values deduced by other means. The observed diurnal variation has an average near 2.5 at the low temperatures, decreasing to about 2 as the temperature increases, in agreement with some but generally larger than most reported measurements. Current theoretical models require the presence of neutral winds and/or diurnally varying charge exchange fluxes into and out of the plasmasphere to provide diurnal ratios as large as those reported here.