Ionosphere-thermosphere system response to extreme geomagnetic storms

Recent works have shown that the ionosphere-thermosphere (IT) response during intense and extreme storms has unique characteristics in terms of the time and region where the response initiates, the time it takes for the response to propagate to the global system and the time it takes for the system to recover. We study the global IT response by means of superposed epoch analysis of the thermosphere mass density response and the Poynting flux during geomagnetic storms of different intensities. We calculate Poynting flux from the Defense Meteorological Satellite Platform (DMSP) satellites and thermospheric density from the CHAMP, GRACE, and GOCE low-Earth orbit satellites for the period of September 2001 to October 2013. There are over 300 storms during that period of time. The zero epoch time for each storm is defined as the onset of the main phase and we include 24 hours before the onset and 72 hours after the onset of the storm in our superposed epoch analysis. The thermosphere density observations from the different satellites are inter-calibrated against the Jacchia-Bowman empirical model in a novel way with a 15th-degree polynomial expansion. For the average storm we find that density increases first at high latitude regions (auroral zones and polar cap) in both hemispheres, and the enhancement subsequently spreads out toward the magnetic equator in a time interval of 3 hours. During extreme storm events the high-latitude response occurs quicker and over a wider range of latitudes and local times. The heating also propagates from high-latitude regions to the equator within approximately 1 hour, a significantly shorter time than during average storms. Similarly, the thermosphere cools off abruptly at all latitudes approximately 18 hours after the storm onset, significantly faster than during average storms. We attribute this fast cooling to the high level production of Nitric Oxide, a very important cooling factor in the thermosphere.