A Modeling Study on the Responses of the Mesosphere and Lower Thermosphere (MLT) Temperature to the Initial and Main Phases of Geomagnetic Storms at High Latitudes

Joule heating and radiative cooling usually play key roles in high-latitude thermospheric temperature changes during geomagnetic storms. In the mesosphere and lower thermosphere (MLT), however, the causes of storm-time temperature changes at high latitudes are still elusive. Here, we elucidate the nature and mechanisms of MLT temperature variations at high latitudes during the 10 September 2005 storm by diagnostically analyzing the MLT thermodynamics in the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIMEGCM) simulations. In the storm's initial and main phases, the MLT temperature decreases at 0:00 local time (LT)-12:00 LT, but increases in the 12:00 LT-24:00 LT sector at high latitudes. Afterward, the temperature decrease disappears and temperature increase occurs at all local times in the high latitudes. Adiabatic heating/cooling and vertical advection associated with vertical winds are the main drivers of high-latitude temperature changes in the entire altitude range of the MLT region. However, around the auroral oval and above similar to 100 km, the Joule heating rate is comparable to the heating caused by vertical advection and adiabatic heating/cooling associated with vertical winds and becomes one of the major contributors to total heating in the high-latitude MLT region. The effects of Joule heating can penetrate down to similar to 95 km. Horizontal advection also plays a key role in storm-time MLT temperature changes inside the polar cap and becomes larger than the adiabatic heating/cooling above similar to 105 km. Plain Language Summary In previous works, Joule heating and radiative cooling were proposed to be the most important processes to determine the temperature changes in the thermosphere during storms. However, we found that Joule heating is important near the auroral oval above 95 km. Joule heating changes the temperature and then the pressure gradient drives horizontal wind changes in the thermosphere. The divergent and convergent winds associated with horizontal wind changes cause vertical winds, which are transmitted to the MLT region. The MLT downward vertical winds bring warm air and upwelling winds bring cold air if the temperature profile increases with altitude. The vertical winds cause adiabatic heating and cooling by convergence and divergence. The two physical mechanisms dominate MLT total heating and then change MLT temperature. The MLT temperature increase and decrease can be observed at 12:00-24:00 and 0:00-12:00 LT, respectively. Of course, the effect of Joule heating is an important heating term near the auroral oval, but it is a minor contributor to adiabatic heating/cooling and vertical advection below 100 km. The horizontal advection has an additional effect on the temperature changes in the polar cap. The rest of the other physical processes can be negligible, including radiative cooling.