Linear Vlasov theory of a magnetised, thermally stratified atmosphere

TL;DR

This paper investigates the stability of collisionless, magnetised, thermally stratified plasmas, revealing new kinetic-scale instabilities like the electron MTI and analyzing their implications for galaxy cluster atmospheres.

Contribution

It introduces the kinetic electron MTI (eMTI) driven by electron temperature gradients and compares fluid and kinetic scale instabilities in magnetised plasmas.

Findings

Identification of the electron MTI (eMTI) with growth rates exceeding the standard MTI.

Finite-Larmor-radius effects tend to stabilize the plasma at intermediate scales.

Both fluid and kinetic instabilities can be driven by the same temperature gradient.

Abstract

The stability of a collisionless, magnetised plasma to local convective disturbances is examined, with a focus on kinetic and finite-Larmor-radius effects. Specific application is made to the outskirts of galaxy clusters, which contain hot and tenuous plasma whose temperature increases in the direction of gravity. At long wavelengths (the "drift-kinetic" limit), we obtain the kinetic version of the magnetothermal instability (MTI) and its Alfv\'enic counterpart (Alfv\'enic MTI), which were previously discovered and analysed using a magnetofluid (i.e. Braginskii) description. At sub-ion-Larmor scales, we discover an overstability driven by the electron temperature gradient of kinetic-Alfv\'en drift waves -- the electron MTI (eMTI) -- whose growth rate is even larger than the standard MTI. At intermediate scales, we find that ion finite-Larmor-radius effects tend to stabilise the plasma. We discuss the physical interpretation of these instabilities in detail, and compare them both with previous work on magnetised convection in a collisional plasma and with temperature-gradient-driven drift-wave instabilities well-known to the magnetic-confinement-fusion community. The implications of having both fluid and kinetic scales simultaneously driven unstable by the same temperature gradient are briefly discussed.