Case study of the mesospheric and lower thermospheric effects of solar X-ray flares: coupled ion-neutral modelling and comparison with EISCAT and riometer measurements

Two case studies of upper mesospheric and lower thermospheric (UMLT) high-latitude effects of solar X-ray flares are presented. Sodankyl ¨ a Ion-neutral Chemistry Model (SIC) electron density profiles agree with D-region EISCAT and riometer observations, provided that the profiles of the most variable ionisable component, nitric oxide, are adjusted to compensate for NOx production during preceding geo- magnetically active periods. For the M6-class flare of 27 April 2006, following a quiet period, the agreement with cos- mic noise absorption observed by the Sodankylriometers was within reasonable limits without adjustment of the (NO) profile. For the major (X17-class) event of 28 October 2003, following high auroral activity and solar proton events, the NO concentration had to be increased up to on the order of 10 8 cm 3 at the D-region minimum. Thus (NO) can in prin- ciple be measured by combining SIC with observations, if the solar spectral irradiance and particle precipitation are ad- equately known. As the two case events were short and modelled for high latitudes, the resulting neutral chemical changes are insignif- icant. However, changes in the model ion chemistry occur, including enhancements of water cluster ions.