Vortices at the magnetic equator generated by hybrid Alfven resonant waves

TL;DR

This study uses 3D magnetohydrodynamic simulations to explore how hybrid Alfven resonant waves generate vortices at the magnetic equator, shedding light on auroral arc brightening during substorms.

Contribution

It reveals the nonlinear behaviors of Alfven resonant modes, including vortex formation and current structure deformation, in a coupled magnetosphere-ionosphere system.

Findings

Vortices form at the magnetic equator due to hybrid Alfven resonant modes.

Secondary flow shear instability occurs at the magnetic equator.

Resonant modes contribute to auroral brightening during substorm onset.

Abstract

We performed three-dimensional magnetohydrodynamic simulations of shear Alfven waves in a full field line system with magnetosphere-ionosphere coupling and plasma non-uniformities. Feedback instability of the Alfven resonant modes showed various nonlinear features under the field line cavities: i) a secondary flow shear instability occurs at the magnetic equator, ii) trapping of the ionospheric Alfven resonant modes facilitates deformation of field-aligned current structures, and iii) hybrid Alfven resonant modes grow to cause vortices and magnetic oscillations around the magnetic equator. Essential features in the initial brightening of auroral arc at substorm onsets could be explained by the dynamics of Alfven resonant modes, which are the nature of the field line system responding to a background rapid change.