On the occurrence of the largest geomagnetic storms per solar cycle

The predictive method of Gumbel's first asymptotic distribution is used to estimate modal extreme Values and return intervals related to the largest geomagnetic storms, characterized by the Dst index, which occurred in the last four solar cycles. A repeat interval of seven solar cycles has been obtained for storms with maximum \Dst\ index values of approximately 600 nT (similar to the extremely large event of 14 March 1989). Copyright (C) 1996 Elsevier Science Ltd.