The Characteristics of Storm-Time Substorms and Non-Storm Substorms

It has been suggested that there may be a fundamental difference between substorms that occur during magnetic storms and those that occur at other times. Baumjohann presented evidence that there is no obvious change in lobe field in “quiet-time ” substorms, but “storm-time ” substorms exhibit the classic pattern of storage and release of lobe field energy [Kamide et al., 1997]. This result led him to speculate that the former is caused by current sheet disruption while the latter is caused by reconnection of lobe flux. In this paper we examine this hypothesis with a much larger data set using both his definition of the two types of substorms as well as additional more restrictive definitions of the classes of event. Our results show that the only difference between the various classes is the absolute value of the lobe field and the size of the changes. When the data are normalized to unit field amplitude we find that the percent change during storm-time and non storm-time substorms is nearly the same. The above conclusions are demonstrated with superposed epoch analysis of lobe field (Bt and Bz) for four classes of substorms: active times (Dst <-50 nT, mostly recovery phase), main phase substorms, non-active times (Dst>-25 nT), quiet-time substorms (no evidence of storm in Dst). Epoch zero for the analysis was taken as the main substorm onset (Pi 2 onset closest to sharp break in AL index). These preliminary results were obtained using the position of the spacecraft to define the location of the plasma sheet. For a subset of the magnetic field data there is also plasma data. For this subset we separately examine events inside and outside the plasma sheet boundary as defined by plasma pressure. For events inside the plasma sheet we also infer the lobe field behavior from pressure balance. Our results suggest that there is no qualitative distinction between the various classes of substorms and so they are all likely to be caused by the same mechanism.