Modelling of composition changes during F-region storms

A recalculation of the global changes of thermospheric gas composition, resulting from strong heat inputs in the aurora1 ovals, shows that (contrary to some previous suggestions) widespread increases of mean molecular mass are produced at mid-latitudes. in summer and at equinox. Decreases of mean molecular mass occur at mid-latitudes in winter. Similar results are given by both the ‘UCL’ and ‘NCAR TIGCM’ three-dimensional models. The computed composition changes now seem consistent with the local time and seasonal response observed by satellites. and can broadly account for ‘negative storm effects’ in the ionospheric F&layer at mid-latitudes.