Ubiquitous instabilities of dust moving in magnetized gas

Abstract

Squire & Hopkins showed that coupled dust–gas mixtures are generically subject to ‘resonant drag instabilities’ (RDIs), which drive violently growing fluctuations in both. But the role of magnetic fields and charged dust has not yet been studied. We therefore explore the RDI in gas that obeys ideal MHD and is coupled to dust via both Lorentz forces and drag, with an external acceleration (e.g. gravity, radiation) driving dust drift through gas. We show this is always unstable, at all wavelengths and non-zero values of dust-to-gas ratio, drift velocity, dust charge, ‘stopping time’ or drag coefficient (for any drag law), or field strength; moreover, growth rates depend only weakly (sub-linearly) on these parameters. Dust charge and magnetic fields do not suppress instabilities, but give rise to a large number of new instability ‘families,’ each with distinct behavior. The ‘MHD-wave’ (magnetosonic or Alfvén) RDIs exhibit maximal growth along ‘resonant’ angles where the modes have a phase velocity matching the corresponding MHD wave, and growth rates increase without limit with wavenumber. The ‘gyro’ RDIs are driven by resonances between drift and Larmor frequencies, giving growth rates sharply peaked at specific wavelengths. Other instabilities include ‘acoustic’ and ‘pressure-free’ modes (previously studied), and a family akin to cosmic ray instabilities that appear when Lorentz forces are strong and dust streams super-Alfvénically along field lines. We discuss astrophysical applications in the warm ISM, circum-galactic medium/inter-galactic medium (CGM/IGM), H II regions, SNe ejecta/remnants, Solar corona, cool-star winds, GMCs, and AGN.