  Squire & Hopkins (2017) 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 which 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
Alfven) 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 which appear when Lorentz forces are strong and dust streams
super-Alfvenically along field lines. We discuss astrophysical applications in
the warm ISM, CGM/IGM, HII regions, SNe ejecta/remnants, Solar corona,
cool-star winds, GMCs, and AGN.
