Linear theory of the current sheet shear instability

Abstract

AbstractThe present study investigates the linear properties of the current sheet shear instability (CSSI) based on the two‐fluid equations. The mode is typically excited in the thin current layer formed around the X line during a quasi‐steady phase of collisionless reconnection and is considered to give rise to the anomalous momentum transport. The linear analyses are carried out for a realistic current sheet as evolved in collisionless reconnection, where the current density profile is produced by the nonuniform ion and electron flows and the pressure balance is maintained due to the temperature gradients. The density profile is assumed to be uniform, so that the lower hybrid drift instability is mostly suppressed. We confirm that the eigenfunctions in the numerical analysis are well consistent with the profiles in a kinetic simulation, implying that the two‐fluid approximation is valid for the CSSI. The mass ratio dependencies of the wave number and growth rate are remarkable for the electron‐scale current sheet, indicating that both the electrons and ions contribute to the wave generation. From the analytical analysis, it is found that these mass ratio dependencies originate from the fact that the ion momentum balance is coupled with the electron dynamics in the electron‐scale current layer. In particular, the electron inertia and electron flow shear play a significant role in generating the CSSI through the induction electric and magnetic fields.