On the geomagnetic storm response and recovery timescales of the thermosphere

The temperature of the Earth's thermosphere can be substantially increased during geomagnetic storms mainly due to high-latitude Joule heating induced by magnetospheric convection and auroral particle precipitation. The main cooling mechanism controlling the recovery of neutral temperature and density to geomagnetic activity is the infrared emission from nitric oxide (NO) emission at 5.3 micrometers. NO is produced by both solar and auroral activity, the first due to solar EUV and X-rays the second due to particle dissociation of N2, and has a typical lifetime of 12 to 24 hours in the mid and lower thermosphere. NO cooling in the thermosphere peaks between 150 and 200 km altitude. In this paper, a global, three-dimensional, time-dependent, non-linear coupled model of the thermosphere, ionosphere, plasmasphere, and electrodynamics (CTIPe) has been used to determine the response and recovery timescale of the upper atmosphere to geomagnetic activity. In these simulations, realistic NO storm increases are defined by the three-dimensional nitric oxide empirical model (NOEM) based on measurements from the Student Nitric Oxide Explorer (SNOE) scientific satellite. The F10.7 index is used to define solar EUV heating. The magnetospheric energy input into the system is characterized by the time variation of the solar wind velocity, the interplanetary magnetic field (IMF) magnitude and direction, and the auroral precipitation index derived from the TIROS/NOAA satellite observations. The solar wind parameters and auroral indices are used to define the magnetospheric convection electric field and auroral ionization/heating rates. The energy is subsequently lost from the system primarily by infrared radiation, particularly by NO cooling. The source is therefore the time integral of the electromagnetic energy input and the loss is radiative cooling. Together they combine to provide the characteristic response and recovery of the system to geomagnetic activity. Comparisons of the neutral density observed by the CHAMP satellite with predictions of CTIPe are presented for selected geomagnetic storm events.