Mid-latitude ionosphere during two great geomagnetic storms

The ionospheric disturbances observed at many European ionosonde stations in association with the severe geomagnetic storms occurring on 19-20 December 1980 and 11-12 April 1981 were investigated by using the available ionospheric and geomagnetic data. During these storms the ionospheric F region underwent major changes at all mid-latitudes. However, the variations from storm to storm were much larger at lower mid-latitudes. These results support the view that even in assessing the response of the mid-latitude ionosphere to severe geomagnetic storms it is necessary to distinguish carefully between global, regional and local behaviour.