Empirical studies of magnetosheath and tail field and current structure, and model comparisons

In this dissertation, we present the observational results obtained with IMP-8 in the magnetosheath and magnetotail, and compare them with the results from the gas dynamic (GD) and global magnetohydrodynamic (MHD) models.;Although the magnetotail has been investigated by many others, the magnetosheath is one of the least studied regions of space despite its importance in solar wind-magnetosphere coupling. In this study, by utilizing a large magnetometer data set obtained simultaneously from IMP-8 and ISEE-3, we construct average magnetic field vector maps of the magnetosheath and tail in the cross-sectional plane. We determine how this average structure is influenced by the IMF, the Earth's dipole tilt angle, and geomagnetic activity. For equatorial IMFs, the magnetosheath field drapes asymmetrically around the magnetosphere causing a rotation of the field draping pattern, the tail current sheet twists, and plasma sheet field lines cease to show north-south symmetry. The magnetic field structure for north/south IMFs is also shown to exhibit different properties including irregularities in the sheath (for southward) and placement of the dipolar and flaring regions in the tail. An IMF related perturbation field is observed in the tail which is strongest on the plasma sheet dawn and dusk sides for equatorial IMF, and on the flanks for north-south IMF.;The Earth's dipole tilt with respect to the geomagnetic equator affects the magnetotail current structure by causing its displacement from the geomagnetic equator. We examine the structure of the current sheet shape and location extensively using 16-years of IMP-8 magnetic field data. We show how geomagnetic activity affects this displacement. Many other field and current properties of the tail structure are also investigated such as the spatial distribution of the current density vectors and the (J x B) forces.;The GD magnetosheath model results for the northward IMFs in the cross-sectional plane are found to agree with IMP-8 data on the large scale except near the magnetopause. Global MHD model comparisons are shown to be consistent with: the observed sheath draping near the magnetopause, the tail current sheet rotation with IMF, and the localization of the IMF related field perturbations over the tail cross-section. These results demonstrate the potential usefulness of the latter for understanding of the solar wind-magnetosphere coupling.