Loss of MHD equilibrium caused by the enhancement of the magnetic By component in Earth's magnetotail

Abstract

The purpose of this paper is to propose that an enhancement of the magnetic By component in Earth's magnetotail might lead to the disruption of the near‐Earth cross‐tail currents and thus to the onset of a magnetospheric substorm. We show, within the context of two‐dimensional MHD equilibrium theory, that the magnetic field energy density, By²/2μo, acts as an additional pressure in the tail plasma sheet that is able to shear magnetotail field lines and, thus, to reduce the normal magnetic field component in the equatorial plane, Bze, perpendicular to the tail current sheet. We solve the Grad‐Shafranov equation for two‐dimensional (∂/∂y = 0) magnetospheric MHD equilibria with a nonzero By component. We compute sequences of quasi‐static equilibria by increasing either |By(A)| or the thermal plasma pressure p(A) in a magnetosphere including the Earth's dipolar field. As a result, the magnetosphere can change from a more “dipolar” tail structure to a tail configuration with highly sheared and/or stretched field lines. Furthermore, increasing By beyond critical values may lead to catastrophic configurations in the near‐Earth plasma sheet whereby the equatorial component, Bze, exhibits a minimum or becomes zero. Such magnetic field configurations can also be achieved by an enhancement of the plasma pressure via the process of adiabatic sunward magnetotail convection. Loss of MHD equilibrium occurs when a quasi‐static magnetic field sequence reaches a critical configuration beyond which no acceptable equilibrium solution exists. Such a senario, as a result of an enhanced (toroidal) shear magnetic field, By, has been proposed as a possible mechanism for driving solar flares or prominence eruptions.