Possible influence of extreme magnetic storms on the thermosphere in the high latitudes

Solar and interplanetary events can create extreme magnetic storms, such as the Carrington storm in 1859 with intensity up to Dst similar to -1,760 nT. The influence of an idealized, smaller Carrington-type storm on the thermosphere has been simulated using the nonhydrostatic Global Ionosphere-Thermosphere Model. For the storm conditions we simulated, the solar wind B-Z and velocity were -50 nT and 1,000 m/s, respectively. The corresponding cross polar cap potential reached 360 kV, and the hemispheric power was 200 GW. Consequently, the hemispheric integrated Joule heating exceeded 3,500 GW, which is more than 70 times higher than normal conditions. The thermosphere variations at high latitudes were examined through the comparison of three cases: reference, storm with geomagnetic energy enhancement only, and storm with both solar and geomagnetic energy enhancement. At 400-km altitude, the neutral density increased by >20 times at certain locations and by >10 times globally averaged. The atmosphere experienced a temperature of 4000 K, more than 1,500 m/s horizontal wind, and exceeding 150 m/s vertical wind. In general, additional energy increase from solar irradiation resulted in 20-30% more perturbation in neutral density and temperature. The exobase (top boundary of the thermosphere) expanded to altitudes >1,000 km, and the buoyancy acceleration (difference between vertical pressure gradient force and gravity force) can be as large as 3 m/s(2). The results will help to determine possible extreme responses to interplanetary coronal mass ejections for various phenomena occurring in geospace.