The Dynamics of Thin Current Sheets and the Triggering of Fast Reconnection in Different Plasma Environments

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

This research aims to understand the triggering mechanism for some of the most energetic events and beautiful light shows in the visible universe. Space and astrophysical plasmas - the ionized gases constituting the greatest part of the visible (baryonic) Universe - are responsible for a variety of observed energetic phenomena from pulsar gamma-ray flares, to solar flares, to magnetospheric substorms leading to Aurora Borealis. All of these processes are characterized by a preliminary phase of energy storage, where magnetic field energy is built up due to rotational, gravitational, or convective motions of the plasma, followed by the sudden triggering of rapid energy release. Magnetic reconnection, the splicing and reforming of magnetic field lines is thought to be at the heart of most observed explosive phenomena. The results of this research will allow a better understanding of the transition from stability to the sudden release of magnetic energy, with the ability to predict the critical parameters necessary for the transition in different astrophysical, as well as laboratory plasmas. This research project?s broader impact includes a more advanced understanding of this decades-old plasma physics problem both in laboratory and astrophysical contexts, with applications involving future laboratory experiments and advanced numerical computations, as well as NASA missions. The classic picture of magnetic reconnection involves current sheets, assumed to be planar-like and concentrated very narrowly in the third dimension. Recently, the slow, stationary reconnection scenario was transformed by the discovery that such configuration is unstable to tearing at large values of the Lundquist number, S, leading to one promising picture (a.k.a. the plasmoid instability) of fast reconnection. The plasmoid instability of the planar current sheet has a paradoxical feature, in that the instability growth rate diverges with S. Growth rates which become arbitrarily large at high S therefore beg the question of how a system transitions from stability to instability. This difficulty with diverging growth rates was resolved recently by Pucci and Velli, who showed that a limiting current sheet inverse aspect ratio separates slow and fast reconnecting modes, a property they called "Ideal Tearing", or IT. As a consequence, fast reconnection sets in in relatively thick current sheets and all plasmoid instability scalings (number of islands etc.) require correction. Such a scenario is promising in that it not only can explain observed fast reconnection rates, but might also account for the reconnection trigger mechanism. The present research program builds on the IT reconnection framework and generalizes it to different plasma configurations in different regimes by: 1) extending the linear scaling theory to more general equilibria and two dimensions including flows and kinetic regimes; 2) simulating collapsing current sheet in 3D resistive MHD to study nonlinear evolution in configurations with and without initial flows. The results will impact the understanding of catastrophic energy release in natural and laboratory plasmas.