Modeling the dominance of the gradient drift or Kelvin-Helmholtz instability in sheared ionospheric E x B flows

TL;DR

This study investigates the conditions under which gradient drift instability (GDI) or Kelvin-Helmholtz instability (KHI) dominate in sheared ionospheric E×B flows, revealing the roles of collisions, altitude, and velocity shear.

Contribution

Introduces a novel model to analyze localized ionospheric instabilities and delineates the dominance regimes of GDI and KHI based on collisionality and altitude.

Findings

KHI dominates in low collisionality regimes

GDI dominates in high collisionality regimes

Both instabilities can coexist in intermediate conditions

Abstract

Studies have shown that in sheared $\mathbf{E}\times\mathbf{B}$ flows in an inhomogeneous ionospheric plasma, the gradient drift (GDI) or the Kelvin-Helmholtz (KHI) instability may grow. This work examines the conditions that cause one of these instabilities to dominate over the other using a novel model to study localized ionospheric instabilities. The effect of collisions with neutral particles plays an important role in the instability development. It is found that the KHI is dominant in low collisionality regimes, the GDI is dominant in high collisionality regimes, and there exists an intermediate region in which both instabilities exist in tandem. For low collisionality cases in which the velocity shear is sufficiently far from the density gradient, the GDI is found to grow as a secondary instability extending from the KHI vortices. The inclusion of a neutral wind driven electric field in the direction of the velocity shear does not impact the dominance of either instability. Using data from empirical ionospheric models, two altitude limits are found. For altitudes above the higher limit, the KHI is dominant. For altitudes below the lower limit, the GDI is dominant. In the intermediate region, both instabilities grow together. Increasing the velocity shear causes both limits to be lower in altitude. This implies that for ionospheric phenomena whose density and velocity gradients span large altitude ranges, such as subauroral polarization streams, the instabilities observed by space-based and ground-based observation instruments could be significantly different.