Dependence of kinetic plasma waves on ion-to-electron mass ratio and light-to-Alfvén speed ratio

Abstract

The magnetization |Ω e |/ω e is an important parameter in plasma astrophysics, where Ω e and ω e are the electron gyro-frequency and electron plasma frequency, respectively. It only depends on the mass ratio m i /m e and the light-to-Alfvén speed ratio c/v Ai , where m i (m e ) is the ion (electron) mass, c is the speed of light, and v Ai is the ion Alfvén speed. Nonlinear numerical plasma models such as particle-in-cell simulations must often assume unrealistic values for m i /m e and for c/v Ai . Because linear theory yields exact results for parametric scalings of wave properties at small amplitudes, we use linear theory to investigate the dispersion relations of Alfvén/ion-cyclotron and fast-magnetosonic/whistler waves as prime examples for collective plasma behaviour depending on m i /m e and c/v Ai . We analyse their dependence on m i /m e and c/v Ai in quasi-parallel and quasi-perpendicular directions of propagation with respect to the background magnetic field for a plasma with β j ∼ 1, where β j is the ratio of the thermal to magnetic pressure for species j. Although their dispersion relations are largely independent of c/v Ai for c/v Ai 10, the mass ratio m i /m e has a strong effect at scales smaller than the ion inertial length. Moreover, we study the impact of relativistic electron effects on the dispersion relations. Based on our results, we recommend aiming for a more realistic value of m i /m e than for a more realistic value of c/v Ai in nonrelativistic plasma simulations if such a choice is necessary, although relativistic and sub-Debye-length effects may require an additional adjustment of c/v Ai .

Anisotropic Electron Heating in Turbulence-driven Magnetic Reconnection in the Near-Sun Solar Wind

“…In this sense, our parameters are just intermediate between the typical solar wind values mentioned above and those in the upper solar corona, i.e., c/v Ai ∼ 10 2 and c/v Ae ∼ 3. Verscharen et al (2020) demonstrated that plasma models employing m i /m e = 100 and c/v Ai  10 can successfully cover physics on scales 0.2d i for β i ∼ β e ∼ 1, which is the regime we have explored here. Since we employ that exact value of m i /m e , an order of magnitude larger value of c/v Ai , and we also have c/v Ae  10, we are confident that our fully kinetic simulations provide quite a correct modeling of the plasma dynamics down to electron scales.…”

Three-dimensional magnetic reconnection in particle-in-cell simulations of anisotropic plasma turbulence

“…With these parameters, the simulated electrons are mildly relativistic, which they are not in the real solar wind. However, the effect of mildly relativistic electrons on the propagation and damping of kinetic-scale normal modes, including KAWs, Alfvén/ion-cyclotron (known as A/IC) waves and fast-magnetosonic/whistler (known as FM/W) waves, is negligible (Verscharen et al 2020) and not important for the evolution of the turbulent cascade, regardless of the processes that carry the cascade to subproton scales.…”

The Gary Picture of Short-Wavelength Plasma Turbulence—The Legacy of Peter Gary

“…An important difference between the whistler and kinetic Alfvén simulations was that the mass ratio used in the latter was m i / m e = 100. One of the side effects of lower mass ratio, in the linear limit, is that the kinetic Alfvén waves can reach beyond electron scales for smaller mass ratio, but damp critically well before electron scales are reached for realistic mass ratios [91].…”

“…These parameters are varied over a broad range of values. For example, in the linear theory calculations, Verscharen et al [91] studied 1 < m i /m e < 1836 and 10 -4 < v A /c < 1/3. In nonlinear PIC simulations Gary et al [56] studied 25 < m i /m e < 1836 and Hughes et al [93] studied 0.025 < vth , e /c < 0.10.…”

“…In nonlinear PIC simulations Gary et al [56] studied 25 < m i /m e < 1836 and Hughes et al [93] studied 0.025 < vth , e /c < 0.10. Verscharen et al [91] showed that mass ratio had a drastic effect on quasi-parallel magnetosonic/whistler branch, and quasi-perpendicular kinetic Alfvén branch. The kinetic Aflvén waves get critically damped for realistic mass ratio but can reach electron scales for artificially large electron mass.…”