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Local two-dimensional particle-in-cell simulations of the collisionless magnetorotational instability

Author(s): Riquelme, MA; Quataert, E; Sharma, P; Spitkovsky, Anatoly

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Abstract: The magnetorotational instability (MRI) is a crucial mechanism of angular momentum transport in a variety of astrophysical accretion disks. In systems accreting at well below the Eddington rate, such as the central black hole in the Milky Way (SgrA*), the plasma in the disk is essentially collisionless. We present a nonlinear study of the collisionless MRI using first-principles particle-in-cell plasma simulations. We focus on local two-dimensional (axisymmetric) simulations, deferring more realistic three-dimensional simulations to future work. For simulations with net vertical magnetic flux, the MRI continuously amplifies the magnetic field, B, until the Alfvén velocity, vA , is comparable to the speed of light, c (independent of the initial value of vA /c). This is consistent with the lack of saturation of MRI channel modes in analogous axisymmetric MHD simulations. The amplification of the magnetic field by the MRI generates a significant pressure anisotropy in the plasma (with the pressure perpendicular to B being larger than the parallel pressure). We find that this pressure anisotropy in turn excites mirror modes and that the volume-averaged pressure anisotropy remains near the threshold for mirror mode excitation. Particle energization is due to both reconnection and viscous heating associated with the pressure anisotropy. Reconnection produces a distinctive power-law component in the energy distribution function of the particles, indicating the likelihood of non-thermal ion and electron acceleration in collisionless accretion disks. This has important implications for interpreting the observed emission - from the radio to the gamma-rays - of systems such as SgrA*.
Publication Date: 25-Jul-2012
Electronic Publication Date: 10-Aug-2012
Citation: Riquelme, MA, Quataert, E, Sharma, P, Spitkovsky, A. (2012). Local two-dimensional particle-in-cell simulations of the collisionless magnetorotational instability. Astrophysical Journal, 755 (1), 10.1088/0004-637X/755/1/50
DOI: doi:10.1088/0004-637X/755/1/50
ISSN: 0004-637X
EISSN: 1538-4357
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Type of Material: Journal Article
Journal/Proceeding Title: Astrophysical Journal
Version: Final published version. Article is made available in OAR by the publisher's permission or policy.

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