Joule Heating in the Thermosphere

High-latitude Joule heating is an important energy source for thermospheric dynamics and composition. It is influenced by winds, plasma turbulence, variable electric fields, and conductivity modifications by strong electric fields. The height-integrated heating can be estimated from the Poynting flux above the ionosphere. Most energy is deposited near the morning and afternoon/evening sides of the auroral oval and in the cusp region. Multiinstrument data assimilation can help quantify complex spatial/temporal variations of Joule heating. Rapid changes of heating launch gravity waves that propagate globally. Within several hours, a global circulation sets up that reduces horizontal variations of the pressure scale height, causing spatial correlation between the temperature and the mean molecular mass. The distributions of temperature and density in the upper thermosphere often show little relation to the distribution of Joule heating. Vertical winds decrease the O/N-2 ratio in regions of heating and increase the ratio in regions of subsidence. The upper thermosphere is affected more strongly by the fraction of Joule heating deposited above 150 km than by the larger amount of Joule heating deposited below 150 km.