Ion transition heights from topside electron density profiles

Abstract

Theoretical electron density profiles are calculated for the topside ionosphere to determine the major factors controlling the profile shape. Only the mean temperature, the vertical temperature gradient and the O + H + ion transition height are important. Vertical proton fluxes alter the ion transition height but have no other effect on the profile shape. Diffusive equilibrium profiles including only these three effects fit observed profiles, at all latitudes, to within experimental accuracy. Values of plasma temperature, temperature gradient and ion transition height h tT were determined by fitting theoretical models to 60,000 experimental profiles obtained from Alouette l ionograms, at latitudes of 75°S–85°N near solar minimum. Inside the plasmasphere h T varies from about 500 km on winter nights to 850 km on summer days. Diurnal variations are caused primarily by the production and loss of O + in the ionosphere. The approximately constant winter night value of h T is close to the level for chemical equilibrium. In summer h T is always above the equilibrium level, giving a continual production of protons which travel along lines of force to aid in maintaining the conjugate winter night ionosphere. Outside the plasmasphere h T is 300–600 km above the equilibrium level at all times. This implies a continual near-limiting upwards flux of protons which persists down to latitudes of about 60° at night and 50° during the day.