http://arxiv.org/abs/2112.00752
We investigate the possibility of cosmic ray (CR) confinement by charged dust grains through resonant drag instabilities (RDIs). We perform magnetohydrodynamic particle-in-cell simulations of magnetized gas mixed with charged dust and cosmic rays, with the gyro-radii of dust and GeV CRs on $\sim\mathrm{AU}$ scales fully resolved. As a first study, we focus on one type of RDI wherein charged grains drift super-Alfv\’enically, with Lorentz forces strongly dominating over drag forces. Dust grains are unstable to the RDIs and form concentrated columns and sheets, whose scale grows until saturating at the simulation box size. Initially perfectly-streaming CRs are strongly scattered by RDI-excited Alfv\’en waves, with the growth rate of the CR perpendicular velocity components equaling the growth rate of magnetic field perturbations. These rates are well-predicted by analytic linear theory. CRs finally become isotropized and drift at least at $\sim v_\mathrm{A}$ by unidirectional Alfv\'{e}n waves excited by the RDIs, with a uniform distribution of the pitch angle cosine $\mu$ and a flat profile of the CR pitch angle diffusion coefficient $D_{\mu\mu}$ around $\mu = 0$, without the “$90$ degree pitch angle problem.” With CR feedback on the gas included, $D_{\mu\mu}$ decreases by a factor of a few, indicating a lower CR scattering rate, which contradicts predictions of standard self-confinement theory. Our study demonstrates that the dust-induced CR confinement can be very important under certain conditions, e.g., the dusty circumgalactic medium around quasars or superluminous galaxies.
S. Ji, J. Squire and P. Hopkins
Fri, 3 Dec 21
34/81
Comments: 13 pages, 10 figures, submitted to MNRAS