The response of ionospheric electron temperatures to geomagnetic activity has been simulated using the Thermosphere-Ionosphere Nested Grid (TING) model. The cause of this response has been analyzed using a new postprocessor for the TING model that looks at the individual physical terms that drive the electron energy equation. It is found that (1) electron temperatures are significantly enhanced in regions of depleted electron densities, especially inside the middle-latitude electron density trough. The most pronounced electron temperature enhancement occurs at the equatorward edge of the trough; (2) this enhancement is produced by heat flux from the plasmasphere, coupled with the effect of the relatively low thermal electron gas heat capacity there and the inefficient heat conduction in the bottomside of the F region, where electron densities have almost vanished; (3) in regions of enhanced electron densities, electron temperatures are significantly decreased as a result of enhanced energy loss to the ions; this prevents the electron temperature "morning overshoot" from occurring in the middle and low latitudes; (4) upwelling of molecular-species-rich air from the lower thermosphere to higher altitudes during the storm increases mixing ratios of the neutral molecular species in the F region, thus enhances the relative contribution of the neutral molecular species to the overall electron cooling there; (5) ion frictional heating increases high-latitude F region ion temperatures during geomagnetic storms, causing the ions to transfer energy to the electrons and thus enhancing electron temperatures; and (6) there are no significant electron temperature increases in the E region during the storm because of the rapid energy loss from the electrons to the neutrals.