The occurrence rate of magnetospheric‐substorm onsets: Random and periodic substorms

Abstract

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 Δt between substorm onsets is determined. In this manner, 1001 values of Δt are obtained. A statistical analysis of the Δt values finds that the most‐probable time between substorms onsets is Δt ≈ 2.75 hours: this is interpreted to be the period between substorms when substorms occur cyclically. The statistical analysis of the Δt 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 are periodic and about half occur randomly.

The magnetosphere as a complex system

“…The spectral brake in the power spectrum of the AL index has also been related to the nonlinear behavior of substorms and have been studied in detail by many authors (Borovsky et al, 1993;Tsurutani et al, 1990;Vassiliadis et al, 1995;Valdivia et al, 1996). Consolini (1997) took the AL index as a global representation of the energy dissipation, and showed a well-defined power law distribution for "burst strength" events for over 4 decades, (Uritsky andPudovkin, 1998).…”

A superposed epoch investigation of the relation between magnetospheric solar wind driving and substorm dynamics with geosynchronous particle injection signatures

“…[30] Initial inspection of Figure 1a suggests that a secondary peak is reached in the category 2 F pc at approximately −3 h, with smaller fluctuations occurring after onset at approximately +2, +3, and +4 h. Borovsky et al [1993] studied the waiting time between substorms identified from particle injection events. They found an average waiting time of ∼2.75 h for substorms occurring cyclically and a "random" waiting time of ∼5 h for events occurring outside of these intervals.…”

Energetic electron precipitation during sawtooth injections

“…The sawtooth is associated with stretching and dipolarisation of the magnetic field and displays similar characteristics to periodic substorms (e.g. Borovsky et al, 1993;Belian et al, 1995) and in the past individual teeth have been studied as substorms (Henderson, 2004). Huang et al (2003a) used observations from three geomagnetic storms to show that continuous energy transfer to the magnetosphere is necessary to maintain sawtooth behaviour.…”