This is the same box as here [“Magnetic RDIs in a Piece of an HII Region: Larger Box with a Spectrum of Grain Sizes”] this is an example of the RDIs in magnetized gas. Specifically, the movie shows an xyz projection, with the initial magnetic field in the vertical direction (z), the acceleration of the grains in the x-z (width-height) direction, and the mutually perpendicular direction is depth (y). The colors show the logarithm of the gas density, relative to mean, measured on slices on the xyz surfaces in the periodic box, while the black points show the dust in the same slice.
This particular box is motivated by parameters typical in HII regions, at roughly ~0.1 pc from a bright O star, with the box size being something like ~0.1 pc on a side. Here the dust is charged and feels both magnetic/Lorentz and drag/Coulomb forces, and it is drifting mildly super-sonically in a medium with plasma beta of ~10. The difference from the previous box[es] is that this includes a broad spectrum of grain sizes, a factor of ~100, from grains ~nanometer in size to ~0.1 micron. The different grains respond differently to various forces. Here, the grain charge scales with grain size, but — unlike the previous movie — we assume the grains are all large in size relative to the incident radiation so they are all in the “geometric absorption” limit. This means the acceleration depends on grain size, in such a way that the “terminal velocity” becomes independent of grain size. As a consequence, the “separation” of different grain sizes is less dramatic, but small grains actually clump more strongly than large grains (unlike the previous case).