Investigation of the Structure of lmf Substorm Triggers Using Multi-Satellite Observations T.-S. Hsu and R.L. McPherron Institute of Geophysics and Planetary Physics, University of California, Los Angeles, 90095-1567 Tel: 310-825-1882 1 Fax: 310-206-8042 e-mail: thsu@im~.ucla.edu 1 rmcpherron@igpp.ucla.edu ABSTRACT An outstanding question in magnetospheric physics is whether substorms are always triggered extemally by changes in the interplanetary magnetic field (IMF) or solar wind plasma, or whether they sometimes occur spontaneously as a result of internal processes. An apparent association between northward turnings of the IMF and substorm onset has been frequently demonstrated, but it is also found that not all substorms are triggered. Previous studies have shown that the ratio of triggered and non-triggered substorm is about 60140. A surprising result is that triggered substorms exhibit a stronger response than non-triggered substonns. It has been suggested that this may be due to undetected small scale structures in the IMF which presumably have weak driving fields of short duration and hence transfer less energy to the magnetosphere. In this work we use a large database of 1978-1985 ISEE 2 and IMP8 IMF observations to examine whether small scale structures occur frequently enough to account for the 40% nontriggered substorms. We find that the probability of observing IMF small scale structures is less than 13%. This low probability (13%) does not match the occurrence frequency of 40% for non-triggered substorm onsets. It is thus unlikely that all non-triggered substorms can be attributed to small scale IMF structures missed by an upstream monitor. INTRODUCTION To understand the magnetospheric substorm, it is necessary to determine whether substorm onset is always extemally triggered by the interplanetary magnetic field (IMF) or whether substorm onset sometimes occurs spontaneously as a result of internal processes. Lyons [1995; 19961 argued that substorms must be triggered by external changes in the IMF andlor the solar wind. Specifically, Lyons [I9961 argued that events without apparent triggers were likely to be a nonsubstorm disturbance such as a convection bay [Pytte et a/., 19781. The hypothesis that most or perhaps all substorms are triggered has initiated considemble interest in substorm triggering studies. Over the past decade, several studies have demonstrated that a majority of substorms (-60%) appear to be triggered by the IMF. However, 40% of all substorms appear to begin without obvious IMF perturbations. A statistical study performed by Hsu and McPherron 120021 has demonstrated that the association between h kgers and substorm onsets is a real physical phenomenon. Another study that determined the average characteristics of triggered and non-triggered substorm in the magnetotail and at geosynchronous orbit was performed by Hsu and McPherron [2004]. It was found that the average response in the tail field and plasma suggests no qualitative difference between the two classes of events. However, the magnitude of the response is different. Triggered substorms exhibit a larger response than non-triggered ones. This surprising result has been suggested to be a manifestation of undetected small scale structures in the IMF. Small structures are presumed to have weak driving fields of short duration and hence transfer less energy to the magnetosphere. To investigate this hypothesis, multi-satellite observations are required to reduce the possibility of missing IMF trigger structures. In this study we will use multi-satellite observations to examine how frequently different IMF structure are observed at different locations in the solar wind. Specifically, data from 1978 to 1985 when two spacecrafts, ISEE2, and IMP8 were in the solar wind are used to examine the scale size of the structures that appear to trigger substorm onsets. DATA PRESENTATION AND ANALYSIS During the fall season of 1978-1985 ISEE 2 provided solar wind observations near the subsolar region, while IMP 8 was circling the earth. A schematic diagram showing the orbits is presented in Figure 1. An automatic procedure [Lyons et al., 19971 was used to identify possible IMF triggers of substorm onset in ISEE 2 and IMP 8 solar wind observations. Both ISEE 2 - m Shock Figure I: The approximate locations of ISEE2 and IMP8 are shown by the elliptical and circular trajectories.