Plasma turbulence at ion scales: a comparison between particle in cell and Eulerian hybrid-kinetic approaches

Abstract

Kinetic-range turbulence in magnetized plasmas and, in particular, in the context of solar-wind turbulence has been extensively investigated over the past decades via numerical simulations. Among others, one of the widely adopted reduced plasma model is the so-called hybrid-kinetic model, where the ions are fully kinetic and the electrons are treated as a neutralizing (inertial or massless) fluid. Within the same model, different numerical methods and/or approaches to turbulence development have been employed. In the present work, we present a comparison between two-dimensional hybrid-kinetic simulations of plasma turbulence obtained with two complementary approaches spanning about two decades in wavenumber -from MHD inertial range to scales well below the ion gyroradius -with a state-of-the-art accuracy. One approach employs hybrid particlein-cell (HPIC) simulations of freely-decaying Alfvénic turbulence, whereas the other consists of Eulerian hybrid Vlasov-Maxwell (HVM) simulations of turbulence continuously driven with partially-compressible large-scale fluctuations. Despite the completely different initialization and injection/drive at large scales, the same properties of turbulent fluctuations at k ⊥ ρ i 1 are observed. The system indeed self-consistently "reprocesses" the turbulent fluctuations while they are cascading towards smaller and smaller scales, in a way which actually depends on the plasma beta parameter. Small-scale turbulence has been found to be mainly populated by kinetic Alfvén wave (KAW) fluctuations for β 1, whereas KAW fluctuations are only sub-dominant for low-β.

Electron-Only Reconnection in Plasma Turbulence

“…Accordingly, we let the (kinetic) response of the plasma to the magnetic-field perturbations to develop velocity and density fluctuations self-consistently. This particular choice has been indeed shown to not affect the development of the turbulent cascade at (and below) the ion scales [23]. However, a study about the precise dependence on a much wider variety of injection properties is beyond the scope of this work-which itself is meant to be a first proof-of-concept-and will be left for future investigation.…”

“…Decaying turbulence is initialized by imposing random, isotropic magnetic-field perturbations, δB = δB ê z + δB ⊥ with ∇ • δB = 0. Such perturbations self-consistently excite "compressive" fluctuations in the velocity and density fields [23]. The choice of injecting compressive magnetic perturbations (rather than, for instance, purely Alfvénic fluctuations having δB = 0) is justified by the fact that inside the bow shock (i.e., in the magnetosheath) the majority of the fluctuations are observed to be compressive (see, e.g., [39,40]).…”

“…Those authors dubbed these events "electron-only reconnection" (hereafter, e-rec), to differentiate them from the usual (fast) collisionless reconnection process (e.g., [7][8][9][10]) in which reconnection takes place in an "electron diffusion region" embedded within a larger "ion diffusion region" and in which both species are expelled in a collimated outflow named the "exhaust" (e.g., [11]). While e-rec has been recently shown to occur in kinetic simulations of magnetic reconnection when a single electron-scale CS is ad-hoc initialized [12] or after a quasi-parallel shock [13], it is difficult to explain e-rec in the context of a classical turbulent cascade, in which turbulent energy is transferred conservatively from magneto-fluid scales down to ion-kinetic scales, thereby coupling to the ion dynamics in the usual way [4,[14][15][16][17][18][19][20][21][22][23][24]. This difficulty also holds in scenarios in which the turbulent cascade of magnetic fluctuations is shown to be mediated by the reconnection of ion-scale current sheets (e.g., [20,22,[25][26][27]).…”

Vlasov methods in space physics and astrophysics

“…( 2015 ) FD 2D–3V hybrid-Vlasov (H + ) Cerri et al. ( 2016 , 2017a ), Leonardis et al. ( 2016 ), Pucci et al.…”

A hybrid gyrokinetic ion and isothermal electron fluid code for astrophysical plasma

“…We note that the mass ratio expansion approach may be also applied for the full Vlasov-Maxwell system to derive fully kinetic ions and an electron fluid hybrid model, which is capable of capturing the high frequency (faster than ion cyclotron frequency) dynamics for ions. This model has a long history both in fusion[20,21,22,23,24,25] and astrophysical contexts[26,27,28,29,30,31,32]. In terms of the computational algorithm, the particle-in-cell method is employed to solve ion motion in most of the cases (except for Ref [33].…”