Geomagnetic Index for Intense Ionospheric Storm

From analysis of the geomagnetic activity indices aa and ap it is obtained that the index aa can be more adequate indicator of geomagnetic activity for the ionospheric parameters at periods of maxima of intense magnetic storms than ap. This is supported by analysis of the F2-layer critical frequency over Irkutsk station at the period of the extreme storm of 13-15.03.1989. It is shown that existence of the ap upper limit equal to 400 puts some limitations on using the ap index for extreme events.