Understanding the behaviors of thermospheric nitric oxide cooling during the 15 May 2005 geomagnetic storm

The behaviors of thermospheric nitric oxide (NO) cooling during the 15 May 2005 intense geomagnetic storm are studied using measurements by the Sounding of the Atmosphere using Broadband Emission Radiometry instrument on board the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics satellite and simulations by the Thermosphere-Ionosphere-Electrodynamics General Circulation Model. The geomagnetic storm was the most intense (Dst = -247 nT) of 2005 with a short and rapid main phase and long-lasting recovery (more than 3 days). NO cooling responded globally to the geomagnetic storm within 2 hr after the onset of storm main phase. The most significant NO cooling increases occurred at middle and low latitudes in the Northern Hemisphere and at middle latitudes in the Southern Hemisphere. The model outputs agree with observations in general but overestimate the NO cooling at high latitudes and underestimate the NO cooling elsewhere. Furthermore, observations show a significant upward shifting of the NO cooling peak altitude in the storm main phase and a significant downward shifting of the NO cooling peak altitude during the storm recovery phase at low latitudes. An unusual double-peak structure in the NO cooling rate appeared during storm main phase and recovery phase. By investigating the NO cooling vertical profiles, we suggest that the horizontal equatorward transport plays an important role in inducing these significant variations of the NO cooling peak altitude.