THEORY AND OBSERVATION OF AURORAL SUBSTORMS - A MAGNETOHYDRODYNAMIC APPROACH

A theory of auroral substorm dynamics is constructed on the basis of M HD wave processes in the ionosphere-magnetosphere system. The basic vi ew is that the substorm commences in the nightside near-Earth magnetos phere through a collapse of plasma equilibrium. The collapse releases a significant amount of free energy embedded initially in a collection of compressional waves. It is suggested that substorm dynamics after the collapse are determined by the evolution of these waves. We first investigate the quantitative ramifications of the waves in a two-dimen sional box in the GSM yz cross section of the magnetotail. The model i s constructed to allow the study of radiation of substorm wave energy into the solar wind and also encompasses the essential elements of res onant interaction in the plasma sheet boundary layer. The natural boun dary condition leading to radiative loss is introduced. It is found th at wave radiation into the solar wind can relax the magnetospheric sys tem in less than a hour. The resonant Alfven modes driven by the norma l compressional modes in the box are studied through the construction of proper dispersion equation. By studying the held-aligned current ge nerated by resonances, we establish the auroral pattern expected to re sult from the coupling. Following the theoretical study, we examine an auroral substorm observed by the CANOPUS photometer array on February 20, 1990. It is found that, among the testable theoretical prediction s, there exists a general agreement with the observations. We did find , however, that electron- and proton-induced aurora oscillate essentia lly in phase, thus implying a more complicated precipitation process.