Nonlinear Evolution of the Resonant Drag Instability in Magnetized Gas
  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.
