Auroral vortex street formed by the magnetosphere-ionosphere coupling instability

TL;DR

This paper uses 3D nonlinear MHD simulations to demonstrate how magnetosphere-ionosphere coupling instabilities lead to the formation of auroral vortex streets before substorm onset, highlighting the role of Alfven eigenmodes.

Contribution

It introduces a novel simulation approach showing the formation mechanism of auroral vortex streets via magnetosphere-ionosphere coupling and identifies a critical electric field threshold.

Findings

Auroral vortex streets form from initial arc splitting and deformation.

A critical convection electric field is necessary for Alfven eigenmode growth.

Vortex streets result from coupling of magnetospheric Alfven waves and ionospheric density waves.

Abstract

By performing three-dimensional nonlinear MHD simulations including Alfven eigenmode perturbations most unstable to the ionospheric feedback effects, we reproduced the auroral vortex street that often appears just before substorm onset. We found that an initially placed arc splits, intensifies, and rapidly deforms into a vortex street. We also found that there is a critical convection electric field for growth of the Alfven eigenmodes. The vortex street is shown to be a consequence of coupling between the magnetospheric Alfven waves carrying field-aligned currents and the ionospheric density waves driven by Pedersen/Hall currents.