Simulating diverse instabilities of dust in magnetized gas

Abstract

Recently Squire & Hopkins (2018b) showed that charged dust grains moving through magnetized gas under the influence of any external force (e.g. radiation pressure, gravity) are subject to a spectrum of instabilities. Qualitatively distinct instability families are associated with different Alfvén or magnetosonic waves and drift or gyro motion. We present a suite of simulations exploring these instabilities, for grains in a homogeneous medium subject to an external acceleration. We vary parameters such as the ratio of Lorentz-to-drag forces on dust, plasma β, size scale, and acceleration. All regimes studied drive turbulent motions and dust-to-gas fluctuations in the saturated state, can rapidly amplify magnetic fields into equipartition with velocity fluctuations, and produce instabilities that persist indefinitely (despite random grain motions). Different parameters produce diverse morphologies and qualitatively different features in dust, but the saturated gas state can be broadly characterized as anisotropic magnetosonic or Alfvénic turbulence. Quasi-linear theory can qualitatively predict the gas turbulent properties. Turbulence grows from small to large scales, and larger-scale modes usually drive more vigorous gas turbulence, but dust velocity and density fluctuations are more complicated. In many regimes, dust forms structures (clumps, filaments, sheets) that reach extreme over-densities (up to 10 9 times mean), and exhibit substantial sub-structure even in nearly-incompressible gas. These can be even more prominent at lower dust-to-gas ratios. In other regimes, dust self-excites scattering via magnetic fluctuations that isotropize and amplify dust velocities, producing fast, diffusive dust motions.

Formation of Dust Filaments in the Diffuse Envelopes of Molecular Clouds

“…It has been predicted that negatively charged grains may affect the star formation efficiency due to gyroresonance effects with MHD waves propagating inside MCs (Pilipp et al 1987;Nakano 1998;Falceta-Gonçalves et al 2003;Hopkins et al 2020). For instance, Chapman & Wardle (2006) demonstrated that C-type MHD shocks profiles are deeply affected by the presence of a population of charged grains; Pandey & Vladimirov (2019) have recently shown that the Kevin-Helmholz instability in MCs depends on the dust charge density and size distribution; and Wardle (2007) and Pinto et al (2008) derived the modified coefficients for non-ideal MHD effects (Ohmic resistivity, and ambipolar and Hall diffusion) in the presence of charged dust grains.…”

The impact of astrophysical dust grains on the confinement of cosmic rays

“…This effect was seen and diagnosed in detail in the CR streaming-instability simulations of Holcomb & Spitkovsky (2019), where they noted that CRs with an initial drift velocity that is too large (approaching c) are inefficiently self-confined, because the wave scattering only isotropizes particles with one sign of ξ . However, the non-linear saturation of the dust instability presents significant uncertainties; if the saturation is quasi-turbulent in nature, as suggested by the simulations of Seligman, Hopkins & Squire (2019), Hopkins et al (2020c) (albeit in a different regimes), it seems implausible that only one wave polarization would be present in the saturated state. In that case, fluctuations induced by the dust could more efficiently isotropize CRs.…”

“…The most obvious candidate is by exciting some type of turbulence, which scatters dust particles sufficiently to shut off the instability's drive. The simulations of Hopkins et al (2020c) suggest that a simple saturation criterion is when a characteristic turnover time at the scale of interest is equal to the instability growth rate, which leads to an estimate of the power in small scale parallel fluctuations δ B 2 /B 2 ∼ ( /ω A ) 2 , where /ω A is the growth rate of the dust instability normalized by the Alfvén frequency at the chosen scale. This implies the CR diffusivity…”

“…turbulent fluctuations of both polarizations) even if one polarization reaches higher amplitude. Unfortunately, the extremely small scales of interest were not directly probed by the simulations of Hopkins et al (2020c) (the 'CGM' simulation approaches similar conditions). Further simulations or more detailed analytic theory are clearly needed, but we proceed anyway for lack of a more accurate estimate.…”

Physical models of streaming instabilities in protoplanetary discs

“…Collins 1989), plasma instabilities (e.g. Melrose 1980), or related dust instabilities (Moseley, Seligman, Hopkins & Squire 2019;Hopkins, Squire & Seligman 2020).…”