Electron and proton heating in trans-relativistic magnetic reconnection [HEAP]


The coronae of collisionless accretion flows, such as Sgr A* at our Galactic center, provide a unique setting for the investigation of magnetic reconnection. Here, protons are non-relativistic while electrons can be ultra-relativistic. By means of 2D PIC simulations, we study electron and proton heating in the outflows of trans-relativistic ($\sigma_w$~0.1, where the magnetization $\sigma_w$ is the ratio of magnetic energy density to enthalpy density) anti-parallel reconnection. We explore the dependence of the heating efficiency on mass ratio (up to the realistic value), magnetization $\sigma_w$, proton plasma $\beta_i$ (the ratio of proton thermal pressure to magnetic pressure), and electron-to-proton temperature ratio $T_e/T_i$. For both electrons and protons, heating at high $\beta_i$ is dominated by adiabatic compression (adiabatic heating), while at low $\beta_i$ it is accompanied by a genuine increase in entropy (irreversible heating). For our fiducial $\sigma_w=0.1$, we find that at $\beta_i<1$ the irreversible heating efficiency is nearly independent of $T_e/T_i$ (which we vary from 0.1 up to 1). If $T_e/T_i=1$, the fraction of inflowing magnetic energy converted to electron irreversible heating decreases from ~0.016 down to ~0.002 as $\beta_i$ ranges from ~0.01 up to ~0.5, but then it increases up to ~0.03 as $\beta_i$ approaches ~2. Protons are heated more efficiently than electrons at low and moderate $\beta_i$ (by a factor of ~7), whereas the electron and proton heating efficiencies become comparable at $beta_i$~2 if $T_e/T_i=1$, when both species start already relativistically hot. We find comparable heating efficiencies between the two species also in the limit of relativistic reconnection, when the magnetization exceeds unity. Our results have important implications for the two-temperature nature of collisionless accretion flows, and may provide the sub-grid physics needed in general relativistic MHD simulations.

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M. Rowan, L. Sironi and R. Narayan
Thu, 17 Aug 17

Comments: 26 pages, 25 figures, 6 appendices; submitted to ApJ

PIC Simulations of Velocity-Space Instabilities in a Decreasing Magnetic Field: Viscosity and Thermal Conduction [HEAP]


We use particle-in-cell (PIC) simulations of a collisionless, electron-ion plasma with a decreasing background magnetic field, $B$, to study the effect of velocity-space instabilities on the viscous heating and thermal conduction of the plasma. If $B$ decreases, the adiabatic invariance of the magnetic moment gives rise to pressure anisotropies with $p_{||,j} > p_{\perp,j}$ ($p_{||,j}$ and $p_{\perp,j}$ represent the pressure of species $j$ ($=i$ or $e$) parallel and perpendicular to the magnetic field). Linear theory indicates that, for sufficiently large anisotropies, different velocity-space instabilities can be triggered. These instabilities, which grow on scales comparable to the electron and ion Larmor radii, in principle have the ability to pitch-angle scatter the particles, limiting the growth of the anisotropies. Our PIC simulations focus on the nonlinear, saturated regime of the instabilities. This is done through the permanent decrease of the magnetic field by an imposed shear in the plasma. Our results show that, in the regime $2 \lesssim \beta_j \lesssim 20$ ($\beta_j \equiv 8\pi p_j/B^2$), the saturated ion and electron pressure anisotropies are controlled by the combined effect of the oblique ion firehose (OIF) and the fast magnetosonic/whistler (FM/W) instabilities. These instabilities grow preferentially on the ion Larmor radius scale, and make the ion and electron pressure anisotropies nearly equal: $\Delta p_e/p_{||,e} \approx \Delta p_i/p_{||,i}$ (where $\Delta p_j=p_{\perp,j} – p_{||,j}$). We also quantify the thermal conduction of the plasma by directly calculating the mean free path of electrons along the mean magnetic field, which we find strongly depends on whether $B$ decreases or increases. Our results can be applied in studies of low collisionality plasmas such as the solar wind, the intracluster medium, and some accretion disks around black holes.

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M. Riquelme, E. Quataert and D. Verscharen
Tue, 15 Aug 17

Comments: N/A

A Review of the 0.1 Reconnection Rate Problem [CL]


A long-standing problem in magnetic reconnection is to explain why it tends to proceed at or below a normalized rate of 0.1. This article gives a review of observational and numerical evidence for this rate and discusses recent theoretical work addressing this problem. Some remaining open questions are summarized.

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P. Cassak, Y. Liu and M. Shay
Mon, 14 Aug 17

Comments: 16 pages, 4 figures, the manuscript was prepared for the first Journal of Plasma Physics Frontiers in Plasma Physics Conference, accepted to Journal of Plasma Physics, August 2017

Laboratory unravelling of matter accretion in young stars [SSA]


Accretion dynamics in the forming of young stars is still object of debate because of limitations in observations and modelling. Through scaled laboratory experiments of collimated plasma accretion onto a solid in the presence of a magnetic field, we open first window on this phenomenon by tracking, with spatial and temporal resolution, the dynamics of the system and simultaneously measuring multiband emissions. We observe in these experiments that matter, upon impact, is laterally ejected from the solid surface, then refocused by the magnetic field toward the incoming stream. Such ejected matter forms a plasma shell that envelops the shocked core, reducing escaped X-ray emission. This demonstrates one possible structure reconciling current discrepancies between mass accretion rates derived from X-ray and optical observations.

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G. Revet, S. Chen, R. Bonito, et. al.
Wed, 9 Aug 17

Comments: N/A

Force-free collisionless current sheet models with non-uniform temperature and density profiles [CL]


We present a class of one-dimensional, strictly neutral, Vlasov-Maxwell equilibrium distribution functions for force-free current sheets, with magnetic fields defined in terms of Jacobian elliptic functions, extending the results of Abraham-Shrauner (Phys. Plasmas 20, 102117, 2013) to allow for non-uniform density and temperature profiles. To achieve this, we use an approach previously applied to the force-free Harris sheet by Kolotkov et al. (Phys. Plasmas 22, 112902, 2015). In one limit of the parameters, we recover the model of Kolotkov et al., while another limit gives a linear force-free field. We discuss conditions on the parameters such that the distribution functions are always positive, and give expressions for the pressure, density, temperature and bulk-flow velocities of the equilibrium, discussing differences from previous models. We also present some illustrative plots of the distribution function in velocity space.

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F. Wilson, T. Neukirch and O. Allanson
Fri, 4 Aug 17

Comments: 29 pages, 7 figures

Spatially Localized Particle Energization by Landau Damping in Current Sheets Produced by Strong Alfven Wave Collisions [CL]


Understanding the removal of energy from turbulent fluctuations in a magnetized plasma and the consequent energization of the constituent plasma particles is a major goal of heliophysics and astrophysics. Previous work has shown that nonlinear interactions among counterpropagating Alfven waves—or Alfven wave collisions—are the fundamental building block of astrophysical plasma turbulence and naturally generate current sheets in the strongly nonlinear limit. A nonlinear gyrokinetic simulation of a strong Alfven wave collision is used to examine the damping of the electromagnetic fluctuations and the associated energization of particles that occurs in self-consistently generated current sheets. A simple model explains the flow of energy due to the collisionless damping and the associated particle energization, as well as the subsequent thermalization of the particle energy by collisions. The net particle energization by the parallel electric field is shown to be spatially intermittent, and the nonlinear evolution is essential in enabling that spatial non-uniformity. Using the recently developed field-particle correlation technique, we show that particles resonant with the Alfven waves in the simulation dominate the energy transfer, demonstrating conclusively that Landau damping plays a key role in the spatially intermittent damping of the electromagnetic fluctuations and consequent energization of the particles in this strongly nonlinear simulation.

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G. Howes, A. McCubbin and K. Klein
Thu, 3 Aug 17

Comments: 34 pages, 17 figures, submitted to Journal of Plasma Physics

Mechanism for flow generation/acceleration in dense degenerate stellar atmospheres [SSA]


The mechanism for flow generation in dense degenerate stellar atmospheres is suggested when the electron gas is degenerate and ions are assumed to be classical. It is shown, that there is a catastrophe in such system — fast flows are generated due to magneto-fluid coupling near the surface. Distance over which acceleration appears is determined by the strength of gravity and degeneracy parameter. Application of this mechanism for White Dwarfs’ atmospheres is examined and appropriate physical parameter range for flow generation/acceleration is found; possibility of the super-Alfv\’enic flow generation is shown; the simultaneous possibility of flow acceleration and magnetic field amplification for specific boundary conditions is explored; in some cases initial background flow can be accelerated 100 and more times leading to transient jet formation while the Magnetic field amplification is less strong.

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A. Barnaveli and N. Shatashvili
Mon, 31 Jul 17

Comments: 11 pages, Accepted for publication in Astrophysics and Space Science