Modelling of the ballooning instability in the near-earth magnetotail.

Abstract

In recent years, many alternative models of the substorm process have been proposed to explain different aspects of this magnetospheric phenomenon. Some features in these competing models are compatible while others, such as the nature and location of substorm onset, remain controversial. The objective of this thesis is to assess the viability of the ballooning instability as a mechanism for initiating substorms. A review of the history and development of magnetospheric substorm research as well as a review of substorm models is presented. In these models, the crosstail current disruption responsible for the onset of the expansion phase is usually ascribed to the onset of some microinstability. An alternative triggering mechanism is a macroscopic magnetohydrodynamic instability such as the ballooning instability. To derive a threshold condition for the ballooning instability, a simplified magnetotail geometry with cylindrical symmetry near the equatorial plane is assumed. In such circumstances, the torsion of the magnetic field lines is zero and they can be characterised by their curvature. The hydromagnetic equations with isotropic pressure are linearised to find the dispersion relation. This leads to a threshold condition which depends on the pressure and magnetic field intensity gradients. In order to obtain realistic numerical results for the threshold condition, a quasistatic, self-consistent, two-dimensional numerical model of the magnetotail during conditions typical of substorm growth phase is used. The model involves solving the Grad-Shafranov equation with appropriate boundary conditions. It provides time-dependent magnetospheric magnetic field configurations that are characterised by the development of a minimum in Bz in the equatorial plane. Calculations of the detailed configuration of the magnetotail during onset allow an estimate of the instability criterion. In a model which does not allow an increase of pressure with radius, it is found that the magnetotail is not unstable to ballooning. Part of this work has been presented at a conference, viz.: Dormer, L.A. and A.D.M. Walker, Investigation of local MHD instabilities in the magnetotail using a two-dimensional magnetospheric convection model. Poster presented at the 39th annual South African Institute of Physics conference, University of Bophuthatswana, 1994.