Nighttime behavior of the equatorial topside ionosphere

The high altitude region of the ionosphere above the F2 peak is called the topside ionosphere. In this region, the effects of diffusion are comparable to production and loss processes. The behavior of this region is dominated by transport and chemical processes which help determine the relative amounts of O+ and H+ which are the dominant ions in the topside. In order to understand the dynamics and energetics of this region, it is also necessary to understand the roles of E x B drifts and F region neutral winds. This region of the ionosphere has been studied for the last 30 years using in-situ measurements from instruments on spacecraft, ground-based radars, mathematical and computational methods, etc. In our study we use data from the Defense Meteorological Satellite Program (DMSP) F10 satellite which orbits in a sun-synchronous polar orbit with an orbital inclination of about 98°. We have examined latitude profiles of ion temperatures and densities at 2100 hours LT and at an altitude of 800 km to discover the influence of field-aligned plasma transport induced by F region neutral winds. Such dependencies are readily seen by contrasting observations at different seasons, longitudes, and magnetic declinations. Our initial study involves examination of the temperature and density variations in 1991 under high solar activity levels. Data show strong evidence for adiabatic heating effects produced by interhemispheric plasma transport. This heating manifests itself as a local temperature maximum that appears in the winter hemisphere during solstices and is generally absent during equinox. A longitudinal variation in the appearance of this maximum is consistent with the roles of meridional and zonal winds in modulating the field-aligned plasma velocities. The data also show a local temperature minimum near the dip equator. However, it is not so easy to attribute this minimum to adiabatic cooling. This initial study is followed by a further study involving solar activity dependencies in the topside. A study of plasma temperatures under moderate solar activity conditions in 1992 indicates features similar to those in 1991, except that overall temperatures are lower. Evidence for ion cooling and heating by adiabatic expansion and compression seen in 1991 are also seen in 1992, and are attributed to interhemispheric transport of plasma. In this case, temperature and density data in 1992 are examined and compared with similar data set in 1991. It is found that the adiabatic effects are strongly dependent on the location of the transition height. In contrast to high solar activity levels, both, the temperature maximum and the temperature minimum manifest themselves much closer to the dip equator under moderate levels of solar activity. Note that, for both years O+ is the dominant ion at latitudes where the heating effect is seen.