Nitric oxide response to geomagnetic storms: A superposed epoch analysis study

Abstract:

During geomagnetic storms large amounts of magnetospheric energy and momentum enter the ionosphere-thermosphere (IT) system, causing prompt heating and upwelling of thermospheric neutral molecules due to ion/neutral collisions. This effect consequently enhances neutral mass density levels at Low Earth Orbit (LEO) altitudes leading to stronger drag effects on satellites there. However, storm activity can also produce nitric oxide (NO) molecules which exert a very powerful cooling effect on the IT system. Moreover, the concomitant effects caused by thermosphere heating and NO cooling, well-known as the thermostat effect, are not currently well understood in part due to the lack of concurrent NO and density composition observations in the thermosphere, nor are they captured by current thermospheric empirical models. In this work, we attempt to fill this gap by performing a superposed epoch analysis (SEA) study of TIMED/SABER NO data during geomagnetic storms with different intensities and compare it to similarly constructed neutral density SEA for the same storms. We group the storms with respect to storm intensities, ranging from weak to extreme, and line up the data with respect to storm main phase onset of each event. Results show that NO production at altitudes higher than 100 km is intense at auroral latitudes and enhanced NO is seen to propagate to lower latitudes, the cycle being very strongly correlated with the storm cycle. For the intense and extreme storms the NO production is directly correlated with the fast cooling observed during such storms. For altitudes lower than 100 km, no clear patterns before and after storm are discernible. In summary, the superposed NO patterns show good agreement with cooling patterns of thermospheric neutral density during the recovery phase of magnetic storms reported by previous studies, particular in the case of extreme events (minimum SYM-H < -250 nT).