The distribution of great and small geomagnetic storms in the sunspot cycle

Abstract

The distributions of (1) great geomagnetic storms, (2) small storms with “sudden commencement” (SC), and (3) small non-SC storms through the sunspot cycle are examined. There is a close accordance between the averaged sunspot curve for the past seven cycles and the averaged curve for geomagnetic storms (1) and (2), which is shared by the giant sunspots, solar flares of greatest magnitude, and notable auroral displays. For the small non-SC storms, the distribution within the sunspot cycle and also the extent of the relationship with sunspots at storm onset differ from those of the previous phenomena. The broad division of small storms according to the SC criterion appears to be of intrinsic significance (probably related to their solar origin), since this division is supported by the differences found in (1) distribution in the sunspot cycle, (2) relation to sunspots, and (3) characteristics (in addition to the SC criterion), including a statistical difference in the tendency to recur at intervals of a synodic rotation (27 days), as announced by E. and O. Thellier in 1948 and confirmed in the present paper. The Greenwich analysis of 1928, showing the statistical non-recurrence of great storms, is brought up to date.