Reconnection and Electron Temperature Anisotropy in Sub-Proton Scale Plasma Turbulence

Abstract

Turbulent behavior at sub-proton scales in magnetized plasmas is important for a full understanding of the energetics of astrophysical flows such as the solar wind. We study the formation of electron temperature anisotropy due to reconnection in the turbulent decay of sub-proton scale fluctuations using two dimensional, particle-in-cell (PIC) plasma simulations with realistic electron-proton mass ratio and a guide field out of the simulation plane. A fluctuation power spectrum with approximately power law form is created down to scales of order the electron gyroradius. In the dynamic magnetic field topology, which gradually relaxes in complexity, we identify the signatures of collisionless reconnection at sites of X-point field geometry. The reconnection sites are generally associated with regions of strong parallel electron temperature anisotropy. The evolving topology of magnetic field lines connected to a reconnection site allows spatial mixing of electrons accelerated at multiple, spatially separated reconnection regions. This leads to the formation of multi-peaked velocity distribution functions with a strong parallel temperature anisotropy. In a three-dimensional system, supporting the appropriate wave vectors, the multi-peaked distribution functions would be expected to be unstable to kinetic instabilities, contributing to dissipation. The proposed mechanism of anisotropy formation is also relevant to space and astrophysical systems where the evolution of the plasma is constrained by linear temperature anisotropy instability thresholds. The presence of reconnection sites leads to electron energy gain, nonlocal velocity space mixing and the formation of strong temperature anisotropy; this is evidence of an important role for reconnection in the dissipation of turbulent fluctuations.

On the velocity space discretization for the Vlasov–Poisson system: Comparison between implicit Hermite spectral and Particle-in-Cell methods

“…This is an important problem that can impact PIC applications in areas such as plasma turbulence (see, e.g. [6,80]), suggesting that denoising techniques are mandatory for PIC there.…”

Electron vortex magnetic holes: A nonlinear coherent plasma structure

“…We initialize the simulation with a magnetic guide field B 0 perpendicular to the simulation plane, in the z-direction, and add random long wavelength magnetic field fluctuations, using the same parameters as previous work. 23,24 No spectral slope is imposed on the fluctuations, which comprise of all three components for wave vectors k x ¼ 2pm=L x and k y ¼ 2pn=L y for m ¼ À3,…, 3 and n ¼ À3,…, 3. The initial electric field is zero, but the abrupt perturbation of the magnetic field acts to initialize the self-consistent evolution of the turbulent decay after a short period at the start of the simulation.…”

“…The development of turbulence has been discussed in earlier work. 24 In Fig. 1, we plot the parameter jE þ v e  Bj which in ideal, two-fluid MHD should be equal to zero.…”

“…The role of reconnection in the evolution of turbulence in these simulations has been discussed in a previous work. 23,24 We develop a model for the coherent magnetic hole structures and demonstrate that they can exist in a uniform, stable plasma, given certain criteria for their formation. This indicates that they are a new type of coherent plasma structure that may exist in a variety of plasma regimes.…”

Three-dimensional density and compressible magnetic structure in solar wind turbulence

“…Verdini and Grappin, 2015;Mallet et al, 2016;Pezzi et al, 2017a;Pezzi et al, 2017b), Hall MHD for scales at the transition between proton and electron scales (Pucci et al, 2016;Pezzi et al, 2017a;Pezzi et al, 2017b), hybrid Vlasov (e.g. Perrone et al, 2013;Franci et al, 2015a, b;Valentini et al, 2016;Cerri et al, 2016Cerri et al, , 2017Pezzi et al, 2017a;Pezzi et al, 2017b) or hybrid particle in cell for proton kinetic scales, finally full particle in cell descriptions can be used to investigate sub-ion scales (Camporeale and Burgess, 2011;Haynes et al, 2014; see also the review by Servidio et al, 2014). Recently the same structure function analysis in Chen et al (2012c) were applied to simulated magnetohydrodynamic data by Verdini and Grappin (2015) and Mallet et al (2016) yielding a similar structure of the turbulence.…”