In situ measurements of hydrogen concentration and flux between 160 and 300 km in the thermosphere

Abstract

Thermospheric concentrations of neutral atomic hydrogen near and below the F peak are directly related to H+, O+, and atomic oxygen concentrations through the charge exchange equilibrium that is established between hydrogen and oxygen at these altitudes. This chemical relationship, together with in situ measurements of ionospheric and neutral atmospheric concentrations by instrumentation on board the Atmosphere Explorer C satellite, is utilized to investigate properties of neutral hydrogen at altitudes below 200 km where vertical diffusion strongly affects the hydrogen distribution. Data are discussed for a set of satellite orbits during quiet geomagnetic and solar conditions in February 1974; the resultant altitude variation of the derived hydrogen concentrations applies specifically to early afternoon at low latitudes between the winter solstice and the spring equinox. A hydrogen concentration of (2.55 ± 0.5) × 105 atoms/cm³ is observed for these conditions at 300 km. At lower altitudes the concentration profiles are interpreted in terms of vertical hydrogen flow. The resultant daytime flux in the thermosphere is estimated to be (3.2 ± 1.0) × 108 atoms/cm² s. The present observations thus support theoretical estimates and model calculations of large hydrogen flows upward from the region below 100 km. They also support the concept of daytime thermospheric loss processes of greater magnitude than the traditional evaporative escape mechanism.