Solar Cycle Variation of Local Time Dependence in Frequency of Occurrence of Pi2

Abstract

Geomagnetic Pi2 pulsations are observed simultaneously with the onset of magnetic substorms. Individual Pi2 pulsations generally have a one—to—one correspondence with individual substorms (Saito, 1969). Therefore, the frequency of occurrence of the individual substorm can be estimated from the frequency of occurrence of the individual Pi2. In order to find some solar cycle variations in the frequency of occurrence of Pi2, Saito and Matsushita (1968) analyzed the data of Pi2 pulsations observed at Onagawa from 1956 to 1964. Together with the report by Afanasieva (1961), they pointed out that the local time of the maximum frequency of occurrence of Pi2 shows some solar cycle variation so that it tends to have a peak in pre—midnight hours during declining and minimum phases, and around midnight during maximum phases. Their analyses were limited in the data for two solar cycles. The purpose of the present paper is to examine the validity of their conclusion for solar cycle Nos. 20, 21 and the former half of 22.