TY - JOUR

T1 - STOCHASTIC PARTICLE ACCELERATION in TURBULENCE GENERATED by MAGNETOROTATIONAL INSTABILITY

AU - Kimura, Shigeo S.

AU - Toma, Kenji

AU - Suzuki, Takeru K.

AU - Inutsuka, Shu Ichiro

N1 - Funding Information:
This work is partly supported by JST grant Building of Consortia for the Development of Human Resources in Science and Technology (SSK and KT) and JSPS Grants-in-Aid for Scientific Research 15H05437 (KT).

PY - 2016/5/10

Y1 - 2016/5/10

N2 - We investigate stochastic particle acceleration in accretion flows. It is believed that magnetorotational instability (MRI) generates turbulence inside accretion flows and that cosmic rays (CRs) are accelerated by the turbulence. We calculate equations of motion for CRs in the turbulent fields generated by MRI with the shearing box approximation and without back reaction to the field. Our results show that the CRs randomly gain or lose their energy through interaction with the turbulent fields. The CRs diffuse in the configuration space anisotropically: the diffusion coefficient in the direction of the unperturbed flow is about 20 times higher than the Bohm coefficient, while those in the other directions are only a few times higher than the Bohm. The momentum distribution is isotropic and its evolution can be described by the diffusion equation in momentum space where the diffusion coefficient is a power-law function of the CR momentum. We show that the shear acceleration works efficiently for energetic particles. We also cautiously note that in the shearing box approximation, particles that cross the simulation box many times along the radial direction undergo unphysical runaway acceleration by the Lorentz transformation, which needs to be taken into account with special care.

AB - We investigate stochastic particle acceleration in accretion flows. It is believed that magnetorotational instability (MRI) generates turbulence inside accretion flows and that cosmic rays (CRs) are accelerated by the turbulence. We calculate equations of motion for CRs in the turbulent fields generated by MRI with the shearing box approximation and without back reaction to the field. Our results show that the CRs randomly gain or lose their energy through interaction with the turbulent fields. The CRs diffuse in the configuration space anisotropically: the diffusion coefficient in the direction of the unperturbed flow is about 20 times higher than the Bohm coefficient, while those in the other directions are only a few times higher than the Bohm. The momentum distribution is isotropic and its evolution can be described by the diffusion equation in momentum space where the diffusion coefficient is a power-law function of the CR momentum. We show that the shear acceleration works efficiently for energetic particles. We also cautiously note that in the shearing box approximation, particles that cross the simulation box many times along the radial direction undergo unphysical runaway acceleration by the Lorentz transformation, which needs to be taken into account with special care.

KW - acceleration of particles

KW - accretion, accretion disks

KW - galaxies: nuclei

KW - turbulence

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U2 - 10.3847/0004-637X/822/2/88

DO - 10.3847/0004-637X/822/2/88

M3 - Article

AN - SCOPUS:84969531726

VL - 822

JO - Astrophysical Journal

JF - Astrophysical Journal

SN - 0004-637X

IS - 2

M1 - 88

ER -