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

¹ Laboratoire Univers et Particules de Montpellier (LUPM) Université Montpellier, CNRS/IN2P3, CC72, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
² Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
³ Laboratoire Univers et Théories, Observatoire de Paris, Universite PSL, Universite de Paris, CNRS, Meudon, France
⁴ Laboratoire de Physique de l’École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 75005 Paris, France

^★ Corresponding author: edouard.callies@umontpellier.fr

Received: 7 February 2025
Accepted: 9 April 2025

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.