THE DISTANT TAIL AT 200 R-E - COMPARISON BETWEEN GEOTAIL OBSERVATIONSAND THE RESULTS FROM A GLOBAL MAGNETOHYDRODYNAMIC SIMULATION

This paper reports a comparison between Geotail observations of plasma s and magnetic fields at 200 R-E in the Earth's magnetotail with resul ts from a time-dependent, global magnetohydrodynamic (MHD) simulation of the interaction of the solar wind with the magnetosphere. The study focuses on observations from July 7, 1993, during which the Geotail s pacecraft crossed the distant tail magnetospheric boundary several tim es while the interplanetary magnetic field (IMF) was predominantly nor thward and was marked by slow rotations of its clock angle. Simultaneo us IMP 8 observations of solar wind ions and the IMF were used as driv ing input for the MHD simulation, and the resulting time series were c ompared directly with those from the Geotail spacecraft. The very good agreement found provided the basis for an investigation of the respon se of the distant tail associated with the clock angle of the IMF. Res ults from the simulation show that the stresses imposed by the draping of magnetosheath field lines and the asymmetric removal of magnetic f lux tailward of the cusps altered considerably the shape of the distan t tail as the solar wind discontinuities convected downstream of Earth . As a result, the cross section of the distant tail was considerably flattened along the direction perpendicular to the IMF clock angle, th e direction of the neutral sheet following that of the IMF. The simula tion also revealed that the combined action of magnetic reconnection a nd the slow rotation of the IMF clock angle led to a braiding of the d istant tail's magnetic field lines along the axis of the tail, with th e plane of the braid lying in the direction of the IMF.