Particle Energy Diffusion in Linear Magnetohydrodynamic Waves

Abstract

In high-energy astronomical phenomena, the stochastic particle acceleration by turbulences is one of the promising processes to generate non-thermal particles. In this paper, we investigate the energydiffusion efficiency of relativistic particles in a temporally evolving wave ensemble that consists of a single mode (Alfvén, fast or slow) of linear magnetohydrodynamic waves. In addition to the gyroresonance with waves, the transit-time damping (TTD) also contributes to the energy-diffusion for fast and slow-mode waves. While the resonance condition with the TTD has been considered to be fulfilled by a very small fraction of particles, our simulations show that a significant fraction of particles are in the TTD resonance owing to the resonance broadening by the mirror force, which non-resonantly diffuses the pitch angle of particles. When the cutoff scale in the turbulence spectrum is smaller than the Larmor radius of a particle, the gyroresonance is the main acceleration mechanism for all the three wave modes. For the fast-mode, the coexistence of the gyroresonance and TTD resonance leads to anomalous energy-diffusion. For a particle with its Larmor radius smaller than the cutoff scale, the gyroresonance is negligible, and the TTD becomes the dominant mechanism to diffuse its energy. The energy-diffusion by the TTD-only resonance with fast-mode waves agrees with the hard-sphere-like acceleration suggested in some high-energy astronomical phenomena.

The Interplay of Magnetically Dominated Turbulence and Magnetic Reconnection in Producing Nonthermal Particles

“…In this case, most of the previous works have used test particle simulations, where turbulence was represented by prescribed fields (e.g. Micha lek & Ostrowsky 1996;Arzner et al 2006;Fraschetti & Melia 2008;O'Sullivan et al 2009;Teraki & Asano 2019) or it was provided by turbulent fields obtained from MHD simulations (e.g. Ambrosiano et al 1988;Dmitruk et al 2004;Kowal et al 2012;Dalena et al 2014;Lynn et al 2014;Kimura et al 2016;Beresnyak & Li 2016;Isliker et al 2017;González et al 2017;Kimura et al 2019).…”

On the narrow spectral feature at $\sim$3 TeV in the MAGIC spectrum of Mrk 501

“…Since the properties of the turbulences are highly uncertain (see Teraki & Asano 2019;Demidem et al 2020, and references therein). In this study, we use = 2 to simulate hard sphere scattering between the MHD waves and the electrons.…”

Numerical Modeling and Physical Interplay of Stochastic Turbulent Acceleration for Nonthermal Emission Processes

“…Recently some research has been devoted to combine the fluid and the PIC approaches (Bai et al 2015) to study the DSA (Mignone et al 2018). The final numerical method uses a Monte Carlo technique to study particle acceleration by shock waves (Achterberg & Krulls 1992;Baring et al 1994;Marcowith & Kirk 1999;Wolff & Tautz 2015) and turbulence (Giacalone & Jokipii 1999;Teraki & Asano 2019). Among all of the numerical techniques available, the PIC method has an advantage (Ostrowski 1988;Ellison et al 1990;Ellison & Double 2002;Lemoine & Pelletier 2003;Baring 2004;Niemiec & Ostrowski 2006) over all other techniques because PIC not only can model the particle acceleration process, it can also determine the self-generated magnetic turbulence, and treat them self-consistently with the cosmic ray particles.…”