Dynamics of the magnetotail during magnetic storms: Review of ISEE 3 and GEOTAIL observations

Abstract

This paper reviews ISEE 3 and GEOTAIL observations of the distant magnetotail during geomagnetically disturbed intervals. Although the probability of encountering the magnetotail is reduced because non-radial solar wind flow associated with interacting solar wind streams moves a compressed tail away from the nominal position during disturbed intervals, a series of temporal entries of spacecraft into the tail at positions of larger than 100 R E downstream from the earth are often observed during the storm main phase. In such cases the magnitude of lobe magnetic field becomes at times more than 30 nT. The magnetic field of as large as 53 nT was observed in the distant tail lobe by GEOTAIL at -182 R E on March 9, 1993, when a moderate magnetic storm was in progress. The characteristic time scale of magnetospheric period is approximately 20 minutes. These entries into the magnetotail are usually preceded by a strong southward component of sheath magnetic field. It is confirmed that pressure balance is approximately holds at times of magnetopause crossings during enhanced magnetosheath conditions. This indicates that the strong lobe magnetic fields are due to the compression associated with the increase of solar wind thermal and magnetic pressures. Observations of large lobe field events suggest that the addition of magnetic flux to the magnetotail caused by the merging process near the earth during magnetic storms constrain the magnetotail radius from decreasing the nominal value significantly even at distances beyond ∼ 150 R E , although the magnetotail is in a compressed state due to the increase in exterior pressure.