Positive and negative ionospheric disturbances at middle latitudes during geomagnetic storms

The morphology of middle latitude ionospheric disturbances in response to g eomagnetic storms has been investigated to determine the phenomenological d ifferences between positive and negative ionospheric storm effects, using f oF2 observations from azimuthal chain of stations. To better organize the d isturbance signatures, two ionospheric indices were introduced to describe the maximum positive (Dfu-index) and negative (Dfl-index) deviation observe d during an ionospheric storm. A systematic appearance of nighttime positiv e effects was determined with a 24-hour recurrence. The thermospheric-ionos pheric view associated with positive and negative storm effects proposed by Prolss (1993) was extended to encompass our observations. This test clearl y demonstrates that such a model can capture most of the basic aspects of i onospheric storms, nevertheless the prominent feature of large nighttime en hancements in the ionization density have yet to be explained.