Session JM9: Mini-Conference: Plasma Energization - Interactions Between Fluid and Kinetic Scales III

Mechanisms for Electron Acceleration and Heating in Multi-island Magnetic Reconnection

Magnetic reconnection is a significant driver of energetic particles in flares both on the sun and beyond. Single x-line models fail to explain the large number of energetic electrons seen in flares. Reconnection in systems with weak collisions multi-island reconnection spontaneously develops and dominates energy release. There are three basic mechanisms for particle energy gain in multi-island reconnection: motion along parallel electric fields; and the magnetic curvature and gradient B drifts along perpendicular fields. The latter two produce the classical Fermi and betatron acceleration, respectively. Observations in the magnetosphere and solar wind suggest that Fermi reflection drives most ion heating. The observational evidence on the physics of electron heating is not as clear. PIC simulations reveal that Fermi reflection dominates the energy gain of the most energetic electrons. The rate of production of energetic electrons in 3-D systems, where reconnecting fields become stochastic, dramatically increases compared with 2-D reconnection. A challenge is to extend small-scale kinetic simulations to the energy release in large-scale magnetic fields. Extensions of the Parker transport model to describe reconnection-driven particle acceleration are a promising approach.   

Kinetic simulations of particle acceleration in magnetic reconnection with open boundary conditions

Kinetic simulations of magnetically dominated reconnection (plasma beta $\ll$ 1) with closed or periodic simulation domains have shown the formation of hard power-law distribution with spectral index p $\sim$ 1. However, for most of applications observations have inferred significantly softer spectra. Here we present 2D and 3D fully kinetic simulations with open boundary conditions that allow the escape of accelerated particles from the reconnection acceleration region. While the primary acceleration mechanism is still the Fermi acceleration through curvature drift motion of particles, we show that the energy spectrum can be significantly softer than ``-1.'' We further examine the effect of domain size, plasma beta, and magnetization parameters.   

Plasma dynamics and heating/acceleration during driven magnetic reconnection

Highlights of the plasma dynamics and energization during driven anti-parallel magnetic reconnection are presented. The MHD condition breaks down in the entire reconnection layer (the reconnection current layer, the separatrix region and the whole downstream), and the plasma dynamics is significantly different from the results of the Hall-MHD model. In particular, we explain (1) how electron and ion dynamics decouple and how the charge separation and electrostatic electric field are produced in the magnetic field reversal region (reconnection current layer and outflow exhaust) and around the separatrix regions, (2) how electrons and ions gain energy in the reconnection current layer, (3) why the electron outflow velocity in the reconnection exhaust reaches super-Alfvenic speed and the ion outflow velocity reaches Alfvenic speed and how the parallel electric field is produced, (4) how electrons are accelerated by the parallel electric field around the separatrix region, and (5) how ions gain energy when they move across the separatrix region into the downstream. Finally we show that electrons and ions gain energy mainly from the inductive reconnection driven electric field and less from the electrostatic electric field.   

Dynamic of Current Sheets and Their Associated Particle Energization

Large-scale numerical simulations have revealed the critical role of current sheets in regulating the energy conversion processes from fluid scale to kinetic scale. Recently, we have found that efficient particle acceleration can occur in association with these sheets as well. We will present fluid and particle-in-cell simulation results that show the dynamics of current sheets and particle acceleration processes. We discuss the implications of these studies for solar plasma heating and AGN jets/lobes.   

Studying astrophysical particle acceleration mechanisms with colliding magnetized laser-produced plasmas

Significant particle energization is observed to occur in many astrophysical environments, and in the standard models this acceleration occurs as a part of the energy conversion processes associated with collisionless shocks or magnetic reconnection. A recent generation of laboratory experiments conducted using magnetized laser-produced plasmas has opened opportunities to study these particle acceleration processes in the laboratory. Ablated plasma plumes are externally magnetized using an externally-applied magnetic field in combination with a low-density background plasma. Colliding unmagnetized plasmas demonstrated ion-driven Weibel instability [1] while colliding magnetized plasmas drive magnetic reconnection [2]. Both magnetized and unmagnetized colliding plasma are modeled with electromagnetic particle-in-cell simulations which provide an end-to-end model of the experiments. Using particle-in-cell simulations, we provide predictions of particle acceleration driven by reconnection, resulting from both direct x-line acceleration and Fermi-like acceleration at contracting magnetic fields lines near magnetic islands.\\[4pt] [1] W. Fox, G. Fiksel, A. Bhattacharjee, et al, PRL 111, 225002 (2013).\\[0pt] [2] G. Fiksel, W. Fox, A. Bhattacharjee, et al, PRL 113, 105003 (2014).   

Particle Energization via Tearing Instability with Global Self-Organization Constraints

Impulsive tearing magnetic reconnection leads to powerful ion energization in reversed field pinch (RFP) plasmas. Many characteristics of the process are measured, e.g., ion species dependence, anisotropy, and energetic tail formation. While ions are strongly energized, the bulk electrons cool, most likely from increased stochastic transport during the reconnection burst. However, new x-ray spectrum measurements reveal formation of an energetic electron tail. Global self-organization strongly impacts the energization behavior: multiple tearing interactions spanning the core to edge are required, dynamo-like magnetic flux generation drives runaway acceleration, dynamo feedback regulates correlations between electric and magnetic field fluctuations, and transport processes differ for ions and electrons. A mature MHD model for tearing instability has been developed that captures key nonlinear dynamics from the global to intermediate spatial scales, but a turbulent cascade is also present to at least the ion gyroradius scale. These features point to the need for a self-contained framework that spans the global-to-micro scales, including nonlinear self-organization feedback that influences the global structure and inhomogeneity on the system scale.   

On particle acceleration in astrophysical relativistic jets

Relativistic jets, e.g., in active galactic nuclei, are believed to be accelerators of high-energy cosmic rays. This is a lore but no justification of it exists. We investigate this problem from the first principles and present arguments that ``no-jets'' are better accelerators than the jets themselves.