Properties of the Ionosphere during an Extreme Storm

Resumo

Based on an analysis of the F2-layer critical frequency (foF2) at Irkutsk station during the extreme storm of March 13–15, 1989, it is found that geomagnetic activity index aa is a more adequate indicator than ap for foF2 during the maximum of an extreme storm. This is related to the existence of the upper limit of changes in ap (ap = 400) and this fact imposes some limitations on using the ap index for extreme events. In the initial period of the recovery phase of the considered storm, a rapid decrease of geomagnetic activity occurred. A strong deviation of the experimental values of foF2 from the values calculated by the semi-empirical model, which considers the dependences of the thermospheric temperature and composition on geomagnetic activity by the NRLMSISE-00 atmospheric model, was observed during that period. It is assumed that this could be related to the effect of disturbance of thermospheric wind velocity due to the rapid decrease of geomagnetic activity. This assumption is rather qualitative and requires a special study.