Observations of magnetospheric substorms occurring with no apparent solar wind/IMF trigger

An outstanding topic in magnetospheric physics is whether substorms are always externally triggered by disturbances in either the interplanetary magnetic field (IMF) or solar wind, or whether they can also occur solely as the result of an internal magnetospheric instability. Over the past decade, arguments have been made on both sides of this issue. Horwitz [1985] and McPherron et al. [1986] have shown examples of substorm onsets which they claimed were not externally triggered. However, as pointed out by Lyons [1995, 1996], there are several problems associated with these studies that make their results somewhat inconclusive. In particular, in the McPherron et al. study, fluctuations in the B y component were not considered as possible triggers. Furthermore, Lyons suggests that the sharp decreases in the AL index during intervals of steady IMF/solar wind are not substorms at all but rather that they are just enhancements of the convection driven DP 2 current system that are often observed to occur during steady magnetospheric convection events. In the present study, we utilize a much more comprehensive data set (consisting of particle data from the Los Alamos energetic particle detectors at geosynchronous orbit, IMP 8 magnetometer and plasma data, Viking UV auroral imager data, midlatitude Pi 2 pulsation data, ground magnetometer data, and ISEE 1 magnetic field and energetic particle data) to show as unambiguously as possible that typical substorms can indeed occur in the absence of an identifiable trigger in the solar wind/IMF.