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

Mario A. Riquelme, Eliot Quataert, Prateek Sharma, Anatoly Spitkovsky

Research output: Contribution to journalArticlepeer-review

59 Scopus citations

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*.

Original languageEnglish (US)
Article number50
JournalAstrophysical Journal
Volume755
Issue number1
DOIs
StatePublished - Aug 10 2012

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Keywords

  • accretion
  • accretion disks
  • dynamo
  • magnetic fields
  • plasmas

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