I. Tsagouri et al., Positive and negative ionospheric disturbances at middle latitudes during geomagnetic storms, GEOPHYS R L, 27(21), 2000, pp. 3579-3582
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.