Nonlinear Evolution of the Resonant Drag Instability in Magnetized Gas

TL;DR

This paper presents the first nonlinear MHD simulations of magnetized resonant drag instabilities, revealing dust organization, turbulence, and magnetic dynamo effects, with results matching linear theory initially and evolving into complex nonlinear states.

Contribution

It introduces the nonlinear evolution of magnetized RDIs through detailed MHD simulations, highlighting new behaviors distinct from purely acoustic RDIs.

Findings

Dust forms coherent sheets and structures.

Development of Alfvénic and compressible turbulence.

Magnetic dynamo saturates at energy equipartition.

Abstract

We investigate, for the first time, the nonlinear evolution of the magnetized "resonant drag instabilities" (RDIs). We explore magnetohydrodynamic (MHD) simulations of gas mixed with (uniform) dust grains subject to Lorentz and drag forces, using the GIZMO code. The magnetized RDIs exhibit fundamentally different behaviour than the purely acoustic RDIs. The dust organizes into coherent structures and the system exhibits strong dust-gas separation. In the linear and early nonlinear regime, the growth rates agree with linear theory and the dust self-organizes into two-dimensional planes or "sheets." Eventually the gas develops fully nonlinear, saturated Alfv\'enic and compressible fast-mode turbulence, which fills the under-dense regions with a small amount of dust, and drives a dynamo which saturates at equipartition of kinetic and magnetic energy. The dust density fluctuations exhibit significant non-Gaussianity, and the power spectrum is strongly weighted towards the largest (box-scale) modes. The saturation level can be understood via quasi-linear theory, as the forcing and energy input via the instabilities becomes comparable to saturated tension forces and dissipation in turbulence. The magnetized simulation presented here is just one case; it is likely that the magnetic RDIs can take many forms in different parts of parameter space.