Particle Acceleration in Stratified 2D MRI Turbulence: From Reconnection to First- and Second-Order Fermi and Shear Acceleration
Abstract
Low-luminosity accretion disks around black holes are weakly collisional, potentially enabling nonthermal particle acceleration. Local two-dimensional particle-in-cell simulations of the stratified magnetorotational instability (MRI) in pair plasmas by Sandoval et al. (2024; hereafter S24) revealed nonthermal populations with maximum energies proportional to the scale-separation ratio ωc,0/Ω0, where ωc,0 and Ω0 are the initial particle cyclotron and Keplerian frequencies, respectively. We identify the acceleration mechanisms involved and quantify their scale-separation dependence. Acceleration occurs primarily within large-scale current sheets associated with an MRI dynamo. Particles are initially injected by non-ideal electric fields during reconnection driven by plasmoid splitting, attaining energies independently of scale separation. They subsequently undergo rapid first-order Fermi-like acceleration occurring within converging flows between newly formed, separating plasmoids. During this stage, energies grow at a rate proportional to the instantaneous non-relativistic cyclotron frequency, independently of ωc,0/Ω0. This fast acceleration rate implies a maximum energy proportional to ωc,0/Ω0. Particles then continue interacting repeatedly with plasmoids, undergoing slower, predominantly second-order Fermi acceleration, with a subdominant contribution from shear acceleration. This stage increases particle energies by only a factor of {\it a few} over several orbits, regardless of ωc,0/Ω0. The combination of these three stages implies a maximum energy proportional to ωc,0/Ω0, explaining the scaling found by S24 and suggesting that the MRI may accelerate particles to ultrarelativistic energies in low-luminosity disks. Testing these results in three dimensions remains an important next step.
Related
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BibTeX
@misc{sandoval2026particle,
title = {{Particle Acceleration in Stratified 2D MRI Turbulence: From Reconnection to First- and Second-Order Fermi and Shear Acceleration}},
author = {Astor Sandoval and Mario Riquelme and Jorge Cuadra},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.24735},
eprint = {2609.24735},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.24735},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Particle Acceleration in Stratified 2D MRI Turbulence: From Reconnection to First- and Second-Order Fermi and Shear Acceleration
AU - Astor Sandoval
AU - Mario Riquelme
AU - Jorge Cuadra
PY - 2026
DA - 2026/09/21
PB - arXiv
DO - 10.48550/arxiv.2609.24735
UR - https://arxiv.org/abs/2609.24735
AB - Low-luminosity accretion disks around black holes are weakly collisional, potentially enabling nonthermal particle acceleration. Local two-dimensional particle-in-cell simulations of the stratified magnetorotational instability (MRI) in pair plasmas by Sandoval et al. (2024; hereafter S24) revealed nonthermal populations with maximum energies proportional to the scale-separation ratio ω_c,0/Ω_0, where ω_c,0 and Ω_0 are the initial particle cyclotron and Keplerian frequencies, respectively. We identify the acceleration mechanisms involved and quantify their scale-separation dependence. Acceleration occurs primarily within large-scale current sheets associated with an MRI dynamo. Particles are initially injected by non-ideal electric fields during reconnection driven by plasmoid splitting, attaining energies independently of scale separation. They subsequently undergo rapid first-order Fermi-like acceleration occurring within converging flows between newly formed, separating plasmoids. During this stage, energies grow at a rate proportional to the instantaneous non-relativistic cyclotron frequency, independently of ω_c,0/Ω_0. This fast acceleration rate implies a maximum energy proportional to ω_c,0/Ω_0. Particles then continue interacting repeatedly with plasmoids, undergoing slower, predominantly second-order Fermi acceleration, with a subdominant contribution from shear acceleration. This stage increases particle energies by only a factor of {\it a few} over several orbits, regardless of ω_c,0/Ω_0. The combination of these three stages implies a maximum energy proportional to ω_c,0/Ω_0, explaining the scaling found by S24 and suggesting that the MRI may accelerate particles to ultrarelativistic energies in low-luminosity disks. Testing these results in three dimensions remains an important next step.
ER -