A comparative study of sawtooth events and substorm onsets triggered by interplanetary shocks

Abstract

A comparison of solar wind conditions, geomagnetic response, and auroral boundary movement, using a similar number of sawtooth events and shock induced substorm triggers is carried out. 81% of the sawtooth onsets were triggered at low latitudes compared to 33% for onsets during shocks. Results of superposed epoch analysis indicated that the mean interplanetary magnetic field (IMF) B z remained strongly southward during sawtooth events while it was southward during the loading period and turned northward 20 minutes prior to the onset, during the shock triggered events. During both the event types, the mean energy available in the solar wind was found to be above the substorm threshold level. Relatively high levels of solar wind density indicated to the magnetosphere being compressed to a larger degree during substorms initiated by shocks. The high latitude indices were elevated during sawtooth events with 2-3 hr fluctuations was observed in the AL and PCN indices but AU index after the onset remained at similar levels. SYM-H remained below storm time values during shock related onsets but was much stronger and remained above the storm threshold during sawtooth events, that they occurred embedded in a geomagnetic storm. Comparatively higher mean values of ASYM-H indicated to a much stronger asymmetric ring current during sawtooth events. Mean boundary locations during sawtooth events were located a few degrees equatorward of those during the shock related onsets. Oval in the dawn, dusk, and midnight sectors was much thicker during sawtooth events with clear widening around the onset during both event groups but the thickness in noon sector was similar and remained steady during the epoch window during both the event groups. Elevated driving during sawtooth events was also indicative in a greater stretching and relaxation of the magnetic field lines at geosynchronous orbit in the midnight sector.