Study of the thermospheric-ionospheric response to intense geomagnetic storms at middle latitudes

The ionospheric response to enhanced geomagnetic activity was studied in te rms of two ionospheric parameters, f(o)F2 and h'F2, aiming the investigatio n of the middle latitude positive and negative ionospheric storm effects ch aracteristics. Simultaneous observations from a chain of several middle lat itude stations distributed in longitude around the earth during two intense geomagnetic storms (Dst < -100 nT), were analyzed in this work. The correl ation results between the f(o)F2 and h'F2 disturbances and the Dst geomagne tic index verified that the morphology of middle latitude ionospheric distu rbances in response to geomagnetic storms shows a great degree of variabili ty and a strong dependence on local time observation. Negative deviations c onstitute the typical ionospheric response. In general it is reported that the F2 layer virtual height h'F2, which is observed only during daytime in conjunction with Fl laver presence, shows clear response to magnetic storm activity. On the other hand, positive ionospheric disturbances are accompan ied by weak height enhancements. Real observations were also compared to th e thermospheric-ionospheric view associated with positive and negative stor m effects proposed by Prolss (1993). This test clearly demonstrates that su ch a model can capture most of the basic aspects of ionospheric storms, tak ing into consideration that any new burst of activity is associated to a ne w disturbance zone, resulting to the formation of a large zone of disturban ce that covers the post-midnight and the forenoon sector as well. This mech anism may be the cause of long lasting negative storm effects observed as t he typical ionospheric response to geomagnetic storm activity. (C) 2001 Els evier Science Ltd. All rights reserved.