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 ∼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 ∼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 ∼105 km.

Observations by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) and simulations by the Whole Atmosphere Community Climate Model‐eXtended (WACCM‐X) are used to analyze the impacts of the January 2012 solar proton events (SPEs) on mesospheric ozone at high latitudes. The mesospheric ozone at high latitudes decreased evidently with the increasing proton flux and ionization rates during SPEs. The results of SABER and WACCM‐X both showed that maximum mesospheric ozone depletion reached 100% in the North Hemispheric (NH) high latitudes during SPEs while only 40% in the South Hemispheric (SH) high latitudes. The SPEs‐caused ozone changes simply occurred below 85 km and the ozone changes observed by SABER above 85 km may be associated with day‐to‐day variations. WACCM‐X simulations showed NOx (N, NO, and NO2) increased over 600% in both the northern and southern high latitudes during SPEs, while HOx (H, OH, and HO2) increased over 300% in the NH high latitudes and only over 30% in the SH high latitudes. HOx is the main ozone‐depleting substance because the reactions of NOx are less capable of depleting ozone than the reactions of HOx. Due to the difference in sunlight between the northern and southern hemispheres, ozone, HOx, and NOx exhibited significant northern and southern hemispheric asymmetries. Meanwhile, an increase in ozone after the SPEs was reported but not explained by previous studies. Here it is shown that the decreases in the HOx caused the increases in ozone after the SPEs.

Using the temperature data observed from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER), we investigate the response of the mesosphere and lower thermosphere (MLT) to two medium geomagnetic storms with disparate durations, on 20 April 2018 and 10 April 2022. The high-latitude MLT temperature increase in the Southern hemisphere can reach 40 K during April 2018 geomagnetic storm with a longer duration (Kp values greater than 4 for 15 h), while the temperature variations are less than 10 K for the April 2022 event (Kp values greater than 4 for 6 h). To investigate the different temperature responses to disparate geomagnetic storm durations and understand what physical process results in this difference, we simulated the two events using the thermosphere ionosphere mesosphere electrodynamics general circulation model (TIMEGCM). The simulations show that more particles and energy input in longer-duration geomagnetic storms produce larger ion drag force and pressure gradient force at ~130 km, and then the enhanced two forces cause faster horizontal wind, leading to larger horizontal divergence. Subsequently, the stronger downward vertical wind is transported to the MLT region (below 110 km) and ultimately makes greater temperature increases through adiabatic heating/cooling and vertical advection. Therefore, the effects of the storm’s duration on the MLT temperature are also important.

Using the temperature data observed from SABER (the Sounding of the Atmosphere using Broadband Emission Radiometry), we investigate the response of the mesosphere and lower thermosphere (MLT) to two medium geomagnetic storms with disparate durations, on 20 April 2018 and 10 April 2022. The high-latitude MLT temperature increases can reach 40 K during April 2018 geomagnetic storm with a longer duration (Kp greater than 4 for 15 hours), while the temperature variations are less than 10 K for the April 2022 storm (Kp greater than 4 for 6 hours). To investigate the different temperature responses to disparate geomagnetic storm durations and understand what physical process results in this difference, we simulated the two events using the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIMEGCM). The simulations show that more particles and energy input in longer-duration geomagnetic storms produce larger ion drag force and pressure gradient force at ~130 km, and then the enhanced two forces cause faster horizontal wind leading to larger horizontal divergence. Subsequently, the stronger downward vertical wind is transported to the MLT region (below 110 km) and ultimately makes greater temperature increases through adiabatic heating/cooling and vertical advection. Therefore, for the medium storms, the effects of the storm’s duration on the MLT temperature are more important than the storm’s intensity.