Multispacecraft observations of sudden impulses in the magnetotail caused by solar wind pressure discontinuities: Wind and IMP 8 (original) (raw)

1998, Journal of Geophysical Research

Two upstream solar wind pressure discontinuities that were associated with storm sudden commencements have been examined to determine their effect on the geomagnetic tail lobe field. During the two events, occurring on March 9, 1995, and August 17, 1995, the Wind spacecraft was located in the upstream region monitoring the solar wind, and the IMP 8 spacecraft was in the geomagnetic tail lobe observing the tail response. The two events occurred during periods with northward or weak southward interplanetary magnetic field. In each case, the data suggest that the magnetic field in the tail lobe increased in magnitude directly in response to the external solar wind pressure increase. It is shown that a simple model in which a uniform magnetic field is compressed by a step function constriction accurately predicts characteristic timesca!es, which are of the order of a couple minutes, and the magnetic field profiles. The inferred flaring angles are COnsistent with model predictions, and the changes in the flaring angle across the discontinuities correspond to expectations based on changes in the subsolar magnetopause position and tail width. Overall, the results of this study indicate that the magnetotail maintains an approximate MHD equilibrium even as it responds rapidly to interplanetary pressure discontinuities. 1. Introduction Geomagnetic sudden commencements are observed as rapid perturbations in magnetic fields measured at low-latitude observatories of the order of 1-6 min and with magnitudes of several tens of nanotesla [Jursa, 1985]. They are produced by the passage of a solar wind pressure discontinuity or shock and its associated magnetospheric compression. The resulting sudden changes in geomagnetic field strength are observed throughout the magnetosphere, including in the magnetotail lobe [Kawano et al., 1992] where a "sudden increase" in the lobe magnetic field strength is observed [Sugiura et al., 1968] because of the magnetotail compression caused by the pressure enhancement. Currently, in the International Solar Terrestrial Physics (ISTP) era it is possible to study the effect of these pressure discontinuities on the tail lobes routinely by using multiple spacecraft. This study employs data from the IMP 8 and Wind spacecraft when Wind was in the upstream region monitoring the solar wind and observed a solar wind pressure increase and when IMP 8 was in the tail lobe. To avoid complications potentially introduced by the response of the lobe magnetic field strength to a southward interplanetary magnetic field (IMF)