Improved thermosphere mass density recovery during the 5 April 2010 geomagnetic storm by assimilating NO cooling rates in a coupled thermosphere-ionosphere model

The recovery of thermosphere mass density following geomagnetic storms is a result of competing heating and cooling processes. Simulations often underestimate the speed of the recovery. In this study, for the first time, we report that assimilating the Thermosphere Ionosphere Mesosphere Energetics and Dynamics Sounding of the Atmosphere using Broadband Emission Radiometry nitric oxide (NO) cooling rate profiles into a coupled thermosphere-ionosphere model via the ensemble Kalman filter improves the thermosphere mass density recovery following a geomagnetic storm. This is due to the impact of the assimilation on both the cooling processes and the thermosphere circulation. The dynamical changes due to the assimilation include stronger upwelling and equatorial transport. These lead to an effective increase in NO at all altitudes at mid-high latitudes, resulting in the improved recovery. The improved representation of cooling processes in the storm's main phase also results in improved >24 hr forecasts of the density recovery. Plain Language Summary Since nitric oxide (NO) infrared emission plays the most important role in cooling down the thermosphere during storm times, it is plausible to attribute the slow neutral mass density recovery to an inaccurate representation of NO cooling in the simulations. In this study, for the first time, we assimilate the Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry NO cooling rate profiles into a coupled thermosphere-ionosphere model. We found that the improved density recovery is obtained due to the impact of the assimilated observations on the general circulation and cooling processes. The improved representation of cooling processes in the storm's main phase further improves forecasts of the neutral density recovery.