Topside variations of ion mass across the magnetic equator during the northern winter solstice based upon a plasma temperature model applicable to sunspot maximum conditions

Abstract

An effective plasma temperature model appropriate to sunspot maximum conditions has been constructed for low latitudes taking into consideration previously reported incoherent backscatter and satellite probe measurements of electron and ion temperatures. Using this temperature model and electron density latitude profiles, obtained from an analysis of ISIS A topside ionograms (see Walker and Chan, 1976), the variations of mean ion mass at heights of 800, 900 and 1000 km over a range of dip latitude 30°S to 20°N have been estimated during the daytime (1100–1600 LST, long. 105°E) for the period November 1969–January 1970. The results show 1. (1) that there has been a general increase in mean ion mass with increase in solar activity, 2. (2) a latitude crest of ion mass peaking near the magnetic equator to a value ~16 a.m.u., indicating a predominance of O + ions at a height of 800 km and 3. (3) that the mean ion mass estimates (and consequently O + ion concentrations) were consistently higher in the summer hemisphere compared with those of similar dip locations in the winter hemisphere. The predominance of O + ions over the magnetic equator is clearly related to the (E × B) upthrust mechanism at this location giving rise to the ‘equatorial anomaly’ in the ionosphere while the predominance of O + ions in the summer hemisphere can be explained by a meridional wind blowing from the summer to the winter hemisphere.