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

Tushar Andriyas

doi:10.4401/ag-7481

Abstract

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.

Highlights

A substorm is a short lived disturbance with a life time of 2-4 hours [Tanskanen et al 2002] in the magnetosphere-ionosphere coupled system that can deposit large quantity of energy into the auroral ionosphere
Given that the passage of Interplanetary shocks (IPS) through the magnetosphere can lead to large increases in geomagnetic activity and that sawtooth events are found to occur during elevated solar wind driving, the similarities and differences between the two event types are investigated
As shocks lead to elevated geomagnetic activity and sawtooth events are found to occur during elevated solar wind driving, a comparative study between sawtooth events and substorm onsets triggered by shocks was carried out

Summary

Introduction

A substorm is a short lived disturbance with a life time of 2-4 hours [Tanskanen et al 2002] in the magnetosphere-ionosphere coupled system that can deposit large quantity of energy into the auroral ionosphere. Substorms occur at time scales that are much faster when compared to a geomagnetic storm. Three phases have been ascertained as occurring during a substorm [McPherron et al 1986]. The growth phase is generally initiated after the southward turning of solar wind interplanetary magnetic field (IMF) Bz. The expansion phase occurs when instabilities inherent to the magnetotail cause the diversion of the cross-tail current through the polar regions. The expansion phase occurs when instabilities inherent to the magnetotail cause the diversion of the cross-tail current through the polar regions This is recorded as a fluctuations in the local magnetic field that can reach levels above 1000 nT. Substorms play an important role in energy circulation through the magnetosphere through dayside reconnection, energy storage in magnetotail, and release of this energy to relax the field lines to a more dipolar shape

Results

Discussion

Conclusion