Simultaneous Hα line profile and radar measurements at Arecibo

Abstract

High resolution measurements of the geocoronal Balmer alpha (Hα) emission were made with a Fabry‐Perot interferometer at Arecibo, Puerto Rico, in six observing campaigns between March 1980 and November 1984. Four of these included simultaneous radar measurements of the topside F region. The nighttime variation of the zenith Hα intensity is highly symmetric about solar midnight. However, effective hydrogen temperatures near the morning exobase are generally hotter than near the evening exobase, implying a depletion of exobase hydrogen density in the early morning hours. The exospheric temperature is derived from the Doppler line profiles and is compared to the topside F region O+ temperature. The cooling of H relative to O+ due to the escape of hot, energetic H is not obvious in our data, and it appears that a downward flux of hot protons often causes heating of the bound neutral H population by charge exchange during low to moderate solar activity conditions. This downward flux is usually strongest between 0200 and 0600 LT, and occurs throughout the night in the late fall and early winter. We have detected a deficiency of inbound high‐energy hydrogen relative to the high‐energy outbound population in radial velocity Hα profiles. We have also detected large distortions of the Hα profile wings during low to moderate solar activity periods and during periods of large downward ion speeds. We attribute these wing distortions to efficient charge exchange of H with H+. A narrowing of zenith profile widths near midnight demonstrates the gravitational cooling of H due to the inverse relationship between geocentric distance and the kinetic energy required for escape. We have also searched for evidence that satellite hydrogen populations with small orbital eccentricities may be contributing to narrow line widths.