# Shear flow-interchange instability in nightside magnetotail causes auroral beads as a signature of substorm onset

## TL;DR

This paper models shear flow-interchange instability in the nightside magnetotail, linking it to auroral beads and substorm onset, through a geometric wedge model and wave equation analysis.

## Contribution

It introduces a wedge model-derived wave equation for shear flow-interchange waves, providing insights into the instability's role in substorm onset and auroral bead formation.

## Key findings

- Shear flow-interchange instability likely triggers substorm onset.
- Wave equation analysis yields rough dispersion relations and growth rates.
- Nonlinear effects are essential for accurate instability scale predictions.

## Abstract

A geometric wedge model of the near-earth nightside plasma sheet is used to derive a wave equation for low frequency shear flow-interchange waves which transmit $\vec{E} \times \vec{B}$ sheared zonal flows along magnetic flux tubes towards the ionosphere. Discrepancies with the wave equation result used in Kalmoni et al. (2015) for shear flow-ballooning instability are discussed. The shear flow-interchange instability appears to be responsible for substorm onset. The wedge wave equation is used to compute rough expressions for dispersion relations and local growth rates in the midnight region of the nightside magnetotail where the instability develops, forming the auroral beads characteristic of geomagnetic substorm onset. Stability analysis for the shear flow-interchange modes demonstrates that nonlinear analysis is necessary for quantitatively accurate results and determines the spatial scale on which the instability varies.