Solar activity variations in the composition of the low‐latitude topside ionosphere

Abstract

Ion composition data from the retarding potential analyzer onboard the Defense Meteorological Satellite Program (DMSP) F10 have been analyzed for the months of June and September for the years 1991–1994 when the solar F10.7 flux changed from near 200 to less than 100. Low‐latitude composition data have been averaged by dip latitude and geographic longitude for morning and evening passes to investigate variations attributable to solar variability. In 1991 the dominant ion is O+ in both the morning and the evening, but by 1994, O+ is the dominant ion only at certain locations in the morning. The O+ −H+ transition height is well above the DMSP altitude (800 km) in 1991, but the transition height is near or below 800 km in 1994. Neutral wind induced longitude variations in the topside are present under all levels of solar activity, but the differing role of interhemispheric plasma transport at different solar activity levels dramatically changes the latitude distributions resulting from the winds. At high solar activity the O+ −H+ transition height is well above 800 km, and interhemispheric transport of O+ in the flux tubes connecting the northern and southern hemispheric topside reduces the latitude asymmetry in O+ while producing the minimum observable H+ asymmetries at night. At lower solar activity levels, H+ dominates the topside ionosphere above 800 km, and larger latitudinal asymmetries in the O+ concentration are observed, while the H+ latitude distribution remains quite symmetric at all local times.