Optical aurora and magnetometer data from the Canadian Auroral Network for the OPEN Unified Study (CANOPUS) array in Canada are used in the study of the modulation of optical aurora in the 5577-Jk and 4709-]k emission lines, by resonant shear Alfvtn waves, in the frequency range of 1-4 mHz. A total of four events, carefully chosen to represent different characteristics of timing and location, are analyzed. Typical of these events, the power spectra featured discrete spectral peaks usually near 1.3, 1.9, and 3.1 mHz. Furthermore, latitudinal phase shifts of about 180 ø were typically observed across the latitudinal maximum of a given frequency peak. These observations point irrevocably to the field line resonance as a major factor in the modulation of precipitation and possibly in the acceleration of electrons in forming auroral arcs. Our study demonstrates that the modulation phenomena are common features occurring in the auroral oval, observable in an extensive latitude range, from 66 ø to 73 ø invariant latitude. Of the events, one shows the modulation process accompanied by an inverted V structure of electron precipitation in the evening sector near 72 ø. Two events are observed in the equatorward region of the auroral oval just before substorms onset, and maybe related to the energetic electron arcs which are the precursor of substorm intensification. The fourth event is seen in the morning sector at an equatorward latitude and occurs in the recovery phase of a substorm. The diversity of these modulation events allows us to further infer that resonant Alfvtn waves play a direct role in controlling the luminosity variation of the optical aurora. 1. INTRODUCTION Ultralow frequency (ULF) magnetohydrodynamic (MHD) waves are important energy carriers in the process leading to the coupling of the Earth's magnetosphere and ionosphere [Goertz and Boswell, 1979]. In addition to magnetosphere-ionosphere coupling and the transmission of energy to the auroral ionosphere, a number of authors have suggested that MHD waves might also contribute directly to the energization of electrons which produce optical aurora [Hasegawa, 1976; Goertz, 1984; Samson et al., 1991]. MHD field line resonances have very thin latitudinal scale sizes [Walker et al., 1992] which are comparable with the latitudinal scale size of some discrete arcs, on the order of 50 km or less. If the transverse (latitudinal) scale sizes of these resonances in the upper ionosphere or magnetosphere become comparable to the ion gyroradius or the electron skin depth, then the resonances produce parallel (to the geomagnetic field) electric fields which can accelerate electrons to several hundred eV [HasegawaPaper number 93JA00435. 0148-0227/93/93 JA-00435505.00 important to note that MHD waves can modulate electron precipitation by a number of other mechanisms, in addition to electron inertia and kinetic Alfvtn waves. These additional mechanisms include modulation of the growth rates of ELF/ VLF waves which might be produced by compressional MHD waves [Sato et al., ...

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