THE OCCURRENCE RATE OF MAGNETOSPHERIC-SUBSTORM ONSETS - RANDOM AND PERIODIC SUBSTORMS

Particle-injection events are monitored on three geosynchronous satellites to determine the occurrences of magnetospheric substorms: for every consecutive pair of substorms found, the time interval DELTAt between substorm onsets is determined. In this manner, 1001 values of DELTAt are obtained. A statistical analysis of the DELTAt values finds that the most-probable time between substorms onsets is DELTAt almost-equal-to 2.75 hours: this is interpreted to be the period between substorms when substorms occur cyclically. The statistical analysis of the DELTAt values also finds a random probability for the occurrence of substorms with a mean time between random substorms of about 5 hours: it is speculated that this random occurrence may be caused by a property of the solar wind that varies randomly with an approximately 5-hour time scale. About 1500 substorms occur per year: about half axe periodic and about half occur randomly.