Latitude variations of exospheric hydrogen and the polar wind

Abstract

Several satellite experiments have measured the solar Lyman-α line, either in scattering from upper atmospheric atomic hydrogen (the Lyman-α airglow) or directly at line center (which determines the hydrogen column density along the line of sight). Recent analyses of data from the above experiments consistently reveal the presence of an atomic hydrogen depletion at high latitudes. In situ determinations of hydrogen at lower altitude show no evidence of such behaviour. This has led us to postulate two mechanisms which may be more effective in reducing the high-latitude density at the high altitudes of the exospheric measurements (500–2000 km). The first is the polar wind loss of protons, which depletes atomic hydrogen through a charge exchange reaction. The second is a high-latitude magnetospheric heating of protons, followed by charge exchange. Opposing the above loss mechanisms are the influences of ballistic lateral flow and mean meriodional winds. We have shown by means of a three-dimensional exospheric transport model that none of the above mechanisms can reconcile the disparate results in the two altitude regimes, nor can they provide the large outward hydrogen fluxes and the correct seasonal variations observed at high latitudes.