Heat Exchange Processes in the Topside Ionosphere at Low to Middle Latitudes
Plasma temperatures in the ionosphere are fundamental indicators of the heat balance between the charged and neutral species and are associated with both the dynamics and structure of the upper atmosphere. At low and middle latitudes the temperatures are determined by solar energy inputs and energy exchange between charged particles and neutrals. During the daytime the thermal electrons are heated by photoelectrons and cooled by conduction and collisions with ions. The ions are heated principally by collisions with electrons and ions and cooled by conduction to lower altitudes. During the nighttime, when the source of photoionization is absent, both ions and electrons are cooled by conduction to lower altitudes where heat is lost to the neutral gas. Thus, variations in both electron temperature and ion temperatures can be used to explore the heat exchange processes in the topside ionosphere. A numerical model called Sami2 is Another Model of the Ionosphere (SAMI2) and measurements from the Defense Meteorological Satellite Program (DMSP) F15 satellite are used in this study of plasma temperatures in the topside ionosphere. The effects of ion composition on the ion and electron temperatures in the mid-latitude topside ionosphere during daytime are examined by using data that has been previously derived from the DMSP F15 measurements in 2004–2006. The heat exchanged between the different constituent ions and the role played by the solar zenith angle and the solar ionizing flux (F10.7) is suggested from the variations of electron and ion temperature with electron number density and composition. The SAMI2 model adds further interpretation of the electron and ion temperature distribution by capturing the major mechanisms that control the heat flow in the topside ionosphere at middle latitudes. Due to the absence of constituent ion temperature information in the original DMSP dataset, this model study is also used to retrieve both TH+ and TO+ and verify the observed dependence of ion temperature on ion composition. The model calculation shows that topside TH+ should reside between TO+ and Te, and further confirms the preferential heat transfer from the electrons to H+ in the topside. Finally, an implementation of a more sophisticated analysis procedure to extract constituent ion temperatures from the Retarding Potential Analyzer (RPA) measurements is utilized to reexamine the DMSP F15 dataset. The result shows that in the daytime, when electrons are actively heated, TH+ is a few hundred degrees higher than TO+ at all longitudes. The nighttime temperature difference between TH+ and TO+ is indicative of mass dependent adiabatic heating and cooling processes across the equatorial region. These adiabatic processes present clear longitudinal variations in the ion temperatures and measured plasma flows that are associated with season and magnetic declination.