A diagnosis of the plasma waves responsible for the explosive energy release of substorm onset

N M E Kalmoni,G Grubbs,C Forsyth,C E J Watt,M Samara,K R Murphy,I J Rae,R G Michell

doi:10.1038/s41467-018-07086-0

Abstract

During geomagnetic substorms, stored magnetic and plasma thermal energies are explosively converted into plasma kinetic energy. This rapid reconfiguration of Earth’s nightside magnetosphere is manifest in the ionosphere as an auroral display that fills the sky. Progress in understanding of how substorms are initiated is hindered by a lack of quantitative analysis of the single consistent feature of onset; the rapid brightening and structuring of the most equatorward arc in the ionosphere. Here, we exploit state-of-the-art auroral measurements to construct an observational dispersion relation of waves during substorm onset. Further, we use kinetic theory of high-beta plasma to demonstrate that the shear Alfven wave dispersion relation bears remarkable similarity to the auroral dispersion relation. In contrast to prevailing theories of substorm initiation, we demonstrate that auroral beads seen during the majority of substorm onsets are likely the signature of kinetic Alfven waves driven unstable in the high-beta magnetotail.

Highlights

During geomagnetic substorms, stored magnetic and plasma thermal energies are explosively converted into plasma kinetic energy
Auroral displays in the ionosphere can be used as an indication of the physical location in the nightside magnetosphere where stressed magnetic fields suddenly and explosively reconfigure
On 18 September 2012, a substorm onset was observed within the field-of-view of the MOOSE (Multi-spectral Observatory Of Sensitive EM-CCDs; e.g., ref. 23) all-sky imagers at Poker Flat in Alaska around 09:23:00 UT

Summary

Introduction

During geomagnetic substorms, stored magnetic and plasma thermal energies are explosively converted into plasma kinetic energy This rapid reconfiguration of Earth’s nightside magnetosphere is manifest in the ionosphere as an auroral display that fills the sky. In contrast to prevailing theories of substorm initiation, we demonstrate that auroral beads seen during the majority of substorm onsets are likely the signature of kinetic Alfven waves driven unstable in the high-beta magnetotail. We compare the outcome of this analysis to kinetic wave theory in a high-beta regime to determine substorm onset aurora is strongly associated with shear Alfvén waves of short perpendicular extent These waves accelerate electrons in the magnetosphere (e.g., ref.22) suggesting that the shear Alfven waves embedded in the auroral signature are not just modifying the aurora, but likely the cause of it

Methods

Results

Conclusion