Simulating the Diverse Instabilities of Dust in Magnetized Gas. (arXiv:1904.11494v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Hopkins_P/0/1/0/all/0/1">Philip F. Hopkins</a> (Caltech), <a href="http://arxiv.org/find/astro-ph/1/au:+Squire_J/0/1/0/all/0/1">Jonathan Squire</a> (Otago), <a href="http://arxiv.org/find/astro-ph/1/au:+Seligman_D/0/1/0/all/0/1">Darryl Seligman</a> (Yale)
Recently Squire & Hopkins 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 Alfvenic 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 $beta$, 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 Alfvenic 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 $gg 10^{9}$ 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.
Recently Squire & Hopkins 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 Alfvenic 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 $beta$, 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 Alfvenic 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 $gg 10^{9}$ 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.
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