ON THE SEASONAL RESPONSE OF THE THERMOSPHERE AND IONOSPHERE TO GEOMAGNETIC STORMS

Ionosonde observations have provided the data to build a picture of th e response of the midlatitude ionosphere to a geomagnetic storm. The p articular characteristic of interest is the preference for ''negative storms'' (decrease in the peak electron density, Nmn) in summer and '' positive storms'' (increase in NmF2) in winter. A three-dimensional, t ime-dependent model of the coupled thermosphere and ionosphere is used to explain this dependence. During the driven phase of a geomagnetic storm the two main magnetospheric energy sources to the upper atmosphe re (auroral precipitation and convective electric field) increase dram atically. Auroral precipitation increases the ion density and conducti vity of the upper atmosphere; the electric field drives the ionosphere and, through collisions, forces the thermosphere into motion and then deposits heat via Joule dissipation. The global wind response is dive rgent at high latitudes in both hemispheres. Vertical winds are driven by the divergent wind field and carry molecule-rich air to higher lev els. Once created, the ''composition bulge'' of increased mean molecul ar mass is transported by both the storm-induced and background wind f ields. The storm winds imposed on the background circulation do not ha ve a strong seasonal dependence, and this is not necessary to explain the observations. Numerical computations suggest that the prevailing s ummer-to-winter circulation at solstice transports the molecule-rich g as to mid and low latitudes in the summer hemisphere over the day or t wo following the storm. In the winter hemisphere, poleward winds restr ict the equatorward movement of composition. The altered neutral-chemi cal environment in summer subsequently depletes the F region midlatitu de ionosphere to produce a ''negative storm''. In winter midlatitudes a decrease in molecular species, associated with downwelling, persists and produces the characteristic ''positive storm''.