Latitudinal Patterns in Plasma Scale Heights & Ion Transition Altitudes in the Low Latitude Topside Ionosphere

Abstract

Trans-equatorial passes of the Alouette-2 satellite covering ±60° dip are examined for latitude and altitude variations of electron density during typical solstice and equinox periods. Clear seasonal differences are seen between dip-conjugate locations in the two hemispheres up to an altitude of 1200 km, very pronounced in the solstices and milder in equinox. Above 1200 km, altitude profiles of <i>N_e</i>, from all latitudes within ± 40° merge into uniformity. Interpolated <i>N-h</i> data at closely spaced height intervals are used to compute scale heights, and hence to compute ion change-over altitudes at these low latitudes. The 0⁺ -He⁺ change-over altitude seems to occur at 600-800 km with higher values in the summer hemisphere. The He⁺-H⁺ change-over shows more variability ranging between 1200 and 2000 km. Field-aligned values of electron density show a smooth variation with considerable symmetry about the equator even during solstices. These features are interpreted in terms of physical processes in the topside.