Impact of the ambipolar diffusion in the structuration of the magnetic Rayleigh Taylor instability with oblique magnetic field

TL;DR

This study explores how ambipolar diffusion influences the development and anisotropy of the magnetic Rayleigh-Taylor instability under oblique magnetic fields, revealing that it allows a broader range of unstable modes and amplifies directional anisotropy.

Contribution

We developed a bi-fluid MHD framework with an analytic dispersion relation to analyze ambipolar diffusion effects on RTI with oblique magnetic fields, highlighting the role of neutral fluids and coupling regimes.

Findings

Oblique magnetic fields allow a wider range of unstable modes.

Ambipolar diffusion enhances anisotropy in instability development.

Maximum effect occurs at intermediate charge-neutral coupling.

Abstract

Aims. We investigate the impact of ambipolar diffusion on the development of the Rayleigh-Taylor instability (RTI) with an oblique magnetic field in the incompressible limit. Methods. We developed a general bi-fluid framework comprising charges and neutrals with the specific feature of differing gravity between charges and neutrals. We derived the perturbed magnetohydrodynamic (MHD) equations and obtained an analytic dispersion relation for an oblique magnetic field. The growth rate was then evaluated using a numerical integration of the dispersion relation. In particular, we focussed on the anisotropy in the mode growth induced ambipolar diffusion. Results. In contrast to the case of a magnetic field within the interface, an oblique magnetic field is much less restrictive with respect to the wavenumbers that can develop; rather than a sharp cut-off, it results in a selection of preferred scales for mode growth. The presence of a second neutral fluid that is insensitive to gravity tends to amplify the anisotropy in the possible direction of the instability development. In particular, we show that this effect is maximal when the coupling is in an intermediate range between full and no charge-neutral coupling, specifically in the region where the ambipolar diffusion is highest.