Observations of optical aurora modulated by resonant Alfvén waves

Abstract

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‐Å and 4709‐Å emission lines, by resonant shear Alfvén 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° 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° to 73° 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°. 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 Alfvén waves play a direct role in controlling the luminosity variation of the optical aurora.