Particle energization in relativistic plasma turbulence: solenoidal versus compressive driving

Abstract

Many high-energy astrophysical systems contain magnetized collisionless plasmas with relativistic particles, in which turbulence can be driven by an arbitrary mixture of solenoidal and compressive motions. For example, turbulence in hot accretion flows may be driven solenoidally by the magnetorotational instability or compressively by spiral shock waves. It is important to understand the role of the driving mechanism on kinetic turbulence and the associated particle energization. In this work, we compare particle-in-cell simulations of solenoidally driven turbulence with similar simulations of compressively driven turbulence. We focus on plasma that has an initial beta of unity, relativistically hot electrons, and varying ion temperature. Apart from strong large-scale density fluctuations in the compressive case, the turbulence statistics are similar for both drives, and the bulk plasma is described reasonably well by an isothermal equation of state. We find that nonthermal particle acceleration is more efficient when turbulence is driven compressively. In the case of relativistically hot ions, both driving mechanisms ultimately lead to similar power-law particle energy distributions, but over a different duration. In the case of non-relativistic ions, there is significant nonthermal particle acceleration only for compressive driving. Additionally, we find that the electron-to-ion heating ratio is less than unity for both drives, but takes a smaller value for compressive driving. We demonstrate that this additional ion energization is associated with the collisionless damping of large-scale compressive modes via perpendicular electric fields.

Heating of Magnetically Dominated Plasma by Alfvén-Wave Turbulence

“…There is no extended high-energy tail beyond the expected mean Lorentz factor γ ≈ (δB/B 0 ) 2 σ 0 . This is in stark contrast with simulations of turbulence with violent driving [9][10][11][12][13][14][15]. Another big difference from [9][10][11][12][13][14][15].…”

“…This is in stark contrast with simulations of turbulence with violent driving [9][10][11][12][13][14][15]. Another big difference from [9][10][11][12][13][14][15]. is that the heated particles move mainly along B.…”

“…One possible mechanism for magnetic energy release is the dissipation of Alfvén wave turbulence in the magnetically dominated plasma, also known as relativistic turbulence [5][6][7][8]. Recent numerical experiments demonstrated that when an initial magnetic field B 0 is stirred by a strong external perturbation, a turbulence cascade develops that can accelerate nonthermal particles [9][10][11][12][13][14][15]. These works used rather violent excitation of the turbulence by driving external electric currents into the plasma or by starting with strongly deformed magnetic fields, which both result in δB ∼ B 0 .…”

Kinetic Simulations of Imbalanced Turbulence in a Relativistic Plasma: Net Flow and Particle Acceleration

“…The higher density fluctuations are expected in the relativistic regime due to the coupling of the Alfvénic and fast modes (Takamoto & Lazarian 2016). Solenoidal driving (Zhdankin 2021) could help further test whether these compressible modes affect the Alfvén ratio measured in our simulations. For now, we conclude that the turbulence in our simulations can be considered predominately Alfvénic.…”

Generalized entropy production in collisionless plasma flows and turbulence

“…17) but with amplitude F 0 = 2.4T 0 k ⊥ for each mode. The driving mechanism is described in more detail in our previous work [42].…”