A three-dimensional modelling study of the processes leading to mid latitude nitric oxide increases in the lower thermosphere following periods of high geomagnetic activity

The processes leading to enhancements in mid latitude nitric oxide (NO) densities following geomagnetic storms have been investigated using the University College London (UCL) Coupled Middle Atmosphere and Thermosphere (CMAT) general circulation model. A comparison of calculated storm time and quiet time NO densities at 110 km altitude reveals the presence of aurorally produced NO at both high and mid latitudes for several days after subsidence of activity. At 150 km, the NO enhancements are shorter lived and remain for up to approximately 2 days after the storm. By separating the contribution of chemical production and loss, horizontal and vertical advection, and molecular and eddy diffusion in the calculation of NO densities, we show that at 150 km altitude, horizontal transport must be taken into consideration if post-storm mid latitude enhancements are to be reproduced. Chemical production of NO at high latitudes continues for up to 2 days after subsidence of a storm at altitudes of around 150 km. We show that equatorward winds at this altitude are sufficiently strong to transport the aurorally produced NO to mid latitudes. Vertical diffusion transports NO from altitudes of 150 km and above, to lower altitudes where it is longer lived. At 110 km altitude, chemical, diffusive and advective terms must all be included in the calculation of NO density in order to simulate realistic mid latitude enhancements. We propose that it is the combined effects of increased chemical production, downward diffusion from altitudes of 150 km and above, and transport by winds that lead to increases in mid latitude NO density at altitudes of around 110 km. This is the first detailed study of the causes of post-storm mid latitude NO enhancements to use a three-dimensional general circulation model.