Theory and observation of auroral substorms: A magnetohydrodynamic approach

Abstract

A theory of auroral substorm dynamics is constructed on the basis of MHD wave processes in the ionosphere-magnetosphere system. The basic view is that the substorm commences in the nightside near-Earth magnetosphere 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-dimensional box in the GSM yz cross section of the magnetotail. The model is constructed to allow the study of radiation of substorm wave energy into the solar wind and also encompasses the essential elements of resonant interaction in the plasma sheet boundary layer. The natural boundary condition leading to radiative loss is introduced. It is found that wave radiation into the solar wind can relax the magnetospheric system in less than a hour. The resonant Alfvtn modes driven by the normal compressional modes in the box are studied through the construction of proper dispersion equation. By studying the field-aligned current generated by resonances, we establish the auroral pattern expected to result 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 predictions, there exists a general agreement with the observations. We did find, however, that electron-and proton-induced aurora oscillate essentially in phase, thus implying a more complicated precipitation process. intensification with enhanced auroral emissions in the 4861/• but quantitatively underdeveloped models of substorm. The (HIS) line caused by adiabatically heated protons [Samson et present work is part of our efforts to create a new line of subal., 1993]. Whereas the OAP and the magnetospheric MHD waves responsible for them have been successfully used as a constraint on the spatial origin of the substorm, one is further Copyright 1995 by the American Geophysical Union. Paper number 945A02036. 0148-0227/95/945A-02036505.00storm modeling. Our primary attention will be focused on forming testable quantitative predictions on the basis of physical principles governing the system of our construct. The present paper will deal with the evolution of the auroral substorm after its onset. In the magnetospheric context, the period of interest is normally called the recovery phase of substorm whereby the magnetosphere relaxes into a quieter state. In the ionosphere, however, this same period features 79

Optical signatures of auroral arcs produced by field line resonances: comparison with satellite observations and modeling

“…The source of energy for monochromatic, ULF FLRs is believed to be compressional, near-monochromatic fast MHD waves formed in magnetospheric waveguides or cavities (Samson et al, 1992a;Liu et al, 1994). This is still a very active area of study, and results are not conclusive.…”

“…This cutoff marks the field lines mapping to the inner edge of the electron plasma sheet . These undulations may indicate that the FLR was driven by a magnetotail cavity mode (Liu et al, 1994), not the flank MHD waveguide postulated by Samson et al (1992a). The apparent westward propagation (cf.…”

Multiple current sheets in a double auroral oval observed from the MAGION-2 and MAGION-3 satellites

“…As shown by J. Samson and his group, both from observational and theoretical grounds (see Samson et al, 1992;Samson and Rankin, 1994;Rankin et al, 1994;Liu et al, 1995), non-linear standing lowfrequency waves in the mHz range can be intimately related to the arcs' formation and their multiplicity. In this theory the resonances are fed by broad-band noise in the system, and it was shown that auroral electrons accelerated by FA electric field due to large k ⊥ in the Alfven wave can reach typical auroral energies in a steady state.…”

Stationary magnetospheric convection on November 24, 1981. 1. A case study of "pressure gradient/minimum-<i>B</i>" auroral arc generation

“…Finally we may comment about the model developed by Samson et al (1992) and Liu et al (1995), which considers the slow time evolution of a homogeneous arc. We suppose that the MFM could be a stable location of a slowly oscillating arc.…”

“…Another theory is that being developed by Samson et al (1992) (see also, Liu et al, 1995), stimulated by high resolution ground-based optical observations of arcs from the Canadian CANOPUS network. Oscillations and splitting of homogeneous arcs with periods of order of 10±20 min are often observed, basely distinguishable by the naked eye.…”