The composition of the low latitude topside ionosphere as a function of longitude, season, local time, and solar cycle

The composition of the low latitude topside ionosphere is examined in detail. The variations of the constituent ion species and the total concentrations are examined extensively using data from the Defense Meteorological Satellite Program (DMSP) F10 for solar local times near 09:30 and 21:30. DMSP provides coverage of all longitudes over about a three day period, which allows for the averaging of the data by longitude and latitude for an entire month. We first examine the longitudinal, seasonal, and local time variations in the composition of the topside low-latitude ionosphere for the months of June, September, and December 1993. The variations observed are consistent with the effects of the neutral winds at F-region heights modulating the composition of the topside by inducing interhemispheric plasma flow. At longitudes where the magnetic field has a significant declination, 150{dollar}spcirc{dollar}E to 270{dollar}spcirc{dollar}E and 300{dollar}spcirc{dollar}E to 360{dollar}spcirc{dollar}E, the zonal winds are shown to be significant even at solstice, when the meridional winds are the dominant source of modulation of the topside composition. The winds at night were found to modulate not only the height of the F-peak, but also the plasma decay rate.; We next extend our study of the effects of the winds to a four year portion of a solar cycle, when the 10.7 cm solar flux was decreasing rapidly. The altitude of the {dollar}rm Osp+ - Hsp+{dollar} transition height is seen to be above the spacecraft altitude 800 km in 1991, when the F{dollar}sb{lcub}10.7{rcub}{dollar} solar flux values are high. As solar activity decreases, the transition height moves down in altitude so that O{dollar}sp+{dollar} is the dominant ion observed at 800 km only in certain locations in the morning. The F-region zonal and meridional neutral winds are thought to be approximately constant under all levels of solar activity, but their effects vary greatly with solar activity. Under conditions of high solar activity, the hemispherical asymmetry in the O{dollar}sp+{dollar} concentrations is low due to neutral wind induced interhemispheric transport. The amount of hemispherical asymmetry in the O{dollar}sp+{dollar} concentrations increases with decreasing solar activity as the O{dollar}sp+{dollar} that is raised in the upwind hemisphere charge exchanges with neutral hydrogen, and more H{dollar}sp+{dollar} is transported across the dip equator at 800 km. He{dollar}sp+{dollar} is a major light ion under high solar activity conditions, but is below the RPA's threshold of detectability much of the time in 1994. The longitudinal variations in the He{dollar}sp+{dollar} distributions are seen to be consistent with previous studies of the effects of neutral winds and electromagnetic drifts.; Finally, we examine significant variations in the distributions of the plasma in the longitude region 0{dollar}spcirc{dollar}E to 150{dollar}spcirc{dollar}E, where the magnetic declination at the dip equator is small. These variations can be as much as an order of magnitude in some cases. We attribute these variations to the combination of a hemispherical asymmetry in the magnetic declination near 60{dollar}spcirc{dollar}E and a minimum in the cross-sectional area of the flux tubes at the dip equator near 100{dollar}spcirc{dollar}E. Both these factors affect the influence of neutral zonal winds on the field-aligned transport of plasma.