In the topside ionosphere during the daytime, the thermal electrons are directly heated by photoelectron fluxes from the local and conjugate hemispheres. The heat is lost primarily through conduction to the regions below where collisions with the ions provide the cooling. Ions are heated by collisions with both the electrons and ions and are cooled primarily by conduction to lower altitudes where the heat is lost to collisions with the neutral gas. Here we examine the effects of ion composition on the ion and electron temperatures in the midlatitude topside ionosphere during daytime using data from the Defense Meteorological Satellite Program F15 satellite in 2004–2006. The variations of electron and ion temperature with plasma density and composition indicate how the heat is exchanged between the different constituent ions and the role played by the solar zenith angle and the solar ionizing flux (F10.7). The observations show that the daytime temperature difference between the O+ and electron is smallest when the fractional contribution of H+ is the highest. In such an environment the electrons transfer heat to the H+ then to the O+ through ion‐ion collisions.

Recently reported measurements from the Defense Meteorological Satellite Program (DMSP) indicate that the O+ temperature in the topside ionosphere is dependent on the fractional H+ density. This finding indicates that the mass‐dependent energy exchange rate between O+ and H+ plays an important role in the thermal balance of the topside ionosphere. In this study we utilize the SAMI2 model to retrieve both TH+ and TO+ and verify the previously observed dependence of ion temperature on ion composition. The model shows that in the topside at middle latitudes when a single ion is dominant, O+ or H+ is heated by electron collisions and cooled by conduction as expected. However, in the intervening altitude region where both O+ and H+ are present, O+ is heated by collisions with H+ and cooled by conduction, while H+ is heated by collisions with electrons and cooled by collisions with O+.

Plasma temperatures in the ionosphere are associated with both the dynamics and spatial distribution of the neutral and charge particles. During the daytime, temperatures are determined by solar energy inputs and energy exchange between charged and neutral particles. Plasma transport parallel to the magnetic field adds another influence on temperatures through adiabatic processes that are most evident during the nighttime. Previous observations suggest that the topside H+ temperature (TH+) should reside between the O+ temperature (TO+) and the electron temperature (Te), and further calculations confirm the preferential heat transfer from the electrons to H+ in the topside. In this work we implement a more sophisticated analysis procedure to extract individual mass‐dependent ion temperatures from the retarding potential analyzer measurements on the DMSP F15 satellite. The results show that the daytime 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. The ion temperatures and measured plasma flows present clear longitudinal variations that are associated with magnetic declination.