Stochastic Electron Acceleration by the Whistler Instability in a Growing Magnetic Field

Abstract

We use 2D particle-in-cell (PIC) simulations to study the effect of the saturated whistler instability on the viscous heating and nonthermal acceleration of electrons in a shearing, collisionless plasma with a growing magnetic field, \textbf{B}. In this setup, an electron pressure anisotropy with $p_{\perp ,e}>p_{||,e}$ naturally arises due to the adiabatic invariance of the electron magnetic moment ($p_{||,e}$ and $p_{\perp ,e}$ are the pressures parallel and perpendicular to \textbf{B}). If the anisotropy is large enough, the whistler instability arises, efficiently scattering the electrons and limiting $\Delta p_e$ ($\equiv p_{\perp ,e}-p_{||,e}$). In this context, $\Delta p_e$ taps into the plasma velocity shear, producing electron heating by the so called anisotropic viscosity. In our simulations, we permanently drive the growth of $|\text{B}|$ by externally imposing a plasma shear, allowing us to self-consistently capture the long-term, saturated whistler instability evolution. We find that besides the viscous heating, the scattering by whistler modes can stochastically accelerate electrons to nonthermal energies. This acceleration is most prominent when initially ${\beta }_e\sim 1$, gradually decreasing its efficiency for larger values of ${\beta }_e$ ($\equiv 8\pi p_e/|\text{B}{|}^2$). If initially ${\beta }_e\sim 1$, the final electron energy distribution can be approximately described by a thermal component, plus a power-law tail with spectral index $\sim 3.7$. In these cases, the nonthermal tail accounts for $\sim 5\%$ of the electrons, and for $\sim 15\%$ of their kinetic energy. We discuss the implications of our results for electron heating and acceleration in low-collisionality astrophysical environments, such as low-luminosity accretion flows.

arXiv comment7 pages, 6 figures, accepted in The Astrophysical Journal

References in corpus (19)

• Nonthermal Electrons in Radiatively Inefficient Accretion Flow Models of Sagittarius A*
• Relativistic Reconnection: an Efficient Source of Non-Thermal Particles
• Cosmic rays in galaxy clusters and their non-thermal emission
• The microphysics of collisionless shock waves
• Stochastic Acceleration of Electrons and Protons. I. Acceleration by Parallel Propagating Waves
• On The Nature of the Variable Infrared Emission from Sgr A*
• Simulations of relativistic collisionless shocks: shock structure and particle acceleration
• Hybrid Thermal-Nonthermal Synchrotron Emission from Hot Accretion Flows
• Kinetic turbulence in relativistic plasma: from thermal bath to non-thermal continuum
• A powerful flare from Sgr A* confirms the synchrotron nature of the X-ray emission
• Stochastic Particle Acceleration near Accreting Black Holes
• PIC Simulations of Continuously Driven Mirror and Ion Cyclotron Instabilities in High Beta Astrophysical and Heliospheric Plasmas
• Nonthermally Dominated Electron Acceleration during Magnetic Reconnection in a Low-beta Plasma
• Particle Acceleration and the Origin of X-ray Flares in GRMHD simulations of Sgr A*
• Local 2D Particle-in-cell simulations of the collisionless MRI
• Acceleration of Relativistic Electrons by MHD Turbulence: Implications for Non-thermal Emission from Black Hole Accretion Disks
• PIC Simulations of the Effect of Velocity Space Instabilities on Electron Viscosity and Thermal Conduction
• Possible Origin of Radio Emission from Nonthermal Electrons in Hot Accretion Flows for Low-Luminosity Active Galactic Nuclei
• Whistler instability in a semi-relativistic bi-Maxwellian plasma

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