A statistical study of the spatial structure of interplanetary magnetic field substorm triggers and their associated magnetic response

Abstract

An outstanding question in magnetospheric physics is whether substorms are always triggered externally 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 nontriggered substorms is about 60/40. A surprising finding is that substorms classified as triggered exhibit a stronger response than nontriggered substorms. 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 IMP 8 IMF observations to investigate 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 nontriggered substorm onsets. It is thus unlikely that all nontriggered substorms can be attributed to small‐scale IMF structures missed by an upstream monitor. Another interesting finding is that the small‐scale IMF Bz triggers do not seem to create significant AL response but the large‐scale IMF Bz triggers do. The median curve shows a sudden sharp drop of the AL index at the time the IMF trigger is expected to arrive at the magnetopause. This suggests that some substorms must be associated with the IMF triggers.