Steady-state magnetic-field models and tearing-mode instabilities for the Earth's magnetotail

This thesis addresses a fundamental question in magnetospheric physics, which is whether steady-state convection could theoretically exist in the Earth's plasma sheet. By constructing a numerical two-dimensional magnetohydrostatic equilibrium magnetosphere that is consistent with the condition of steady, lossless, adiabatic, plasma-sheet convection, assertions made by others concerning the non-existence of a physical steady-state solution within the ideal MHD limit (isotropic pressure, perfect conductivity). The computed steady-state magnetic field model, however, is different both from averaged observations and from standard magnetic-field models in that the equatorial magnetic field strength Bze exhibits a very deep broad minimum in the inner plasma sheet. To study tearing instability of the configurations that possess a Bze-minimum in the near-earth plasma sheet in the presence of resistivity, a time-dependent numerical resistive MHD code was developed. The calculations suggest that the magnetotail structure associated with adiabatic earthward convection is intrinsically unstable even if the solar wind condition is precisely steady, which may explain why magnetospheric substorms should occur and why the neutral line associated with the substorm should occur in the near-earth plasma sheet