F2-region response to geomagnetic disturbances

The F2-region response to a geomagnetic storm usually called a ionospheric storm is a rather complicated event. It consists of the so-called positive an negative phases, which have very complicated spatial and temporal behavior. During the recent decade there was significant progress in understanding this behavior. The principal features of the positive and negative phase distribution and variations have been explained on the basis of the principal concept: during a geomagnetic disturbance there is an input of energy into the polar ionosphere, which changes thermospheric parameters, such as composition, temperature and circulation. Composition changes directly influence the electron concentration in the F2 region. The circulation spreads the heated gas to lower latitudes. The conflict between the storm-induced circulation and the regular one determines the spatial distribution of the negative and positive phases in various seasons. There are still problems unsolved. The most acute ones are: the appearance of positive phases before the beginning of a geomagnetic disturbance, the occurrence of strong negative phases at the equator, the role of vibrationally excited nitrogen in forming the negative phase, and the relation of positive phases to the dayside cusp. There are indications that the f(o)F2 long-term trends revealed during the recent years may be explained by long-term trends of the number of negative ionospheric disturbances due to secular variations of the geomagnetic activity. (C) 2001 Elsevier Science Ltd. All rights reserved.