Physics Colloquium

Prof. Amitava Bhattacharjee, Princeton University

"Fast Magnetic Reconnection in Astrophysical and Laboratory Plasmas"

4:15pm, The John A. Wheeler Lecture Hall (RLM 4.102). Coffee and cookies will be served at 4:00pm in RLM 4.102

Abstract: The problem of fast magnetic reconnection in high-Lundquist-number plasmas has been an active area of research for several decades. The main challenge is to explain why reconnection in natural or laboratory plasmas (including fusion devices) can proceed rapidly from a relatively quiescent state characterized by high values of the Lundquist number. Recent work has demonstrated that there is a fundamental shortcoming in the classical Sweet-Parker theory even within the framework of resistive magnetohydrodynamics (MHD). When the Lundquist number exceeds a critical value, the Sweet-Parker layer is unstable to a rapid tearing instability, hereafter referred to as the plasmoid instability. Numerical simulations, supported by heuristic scaling arguments, strongly suggest that within the framework of resistive MHD, the nonlinear reconnection rate mediated by the plasmoid instability becomes insensitive to the Lundquist number. Because the plasmoid instability can initiate a cascade to thin current sheets that are much thinner than the original Sweet-Parker layer, the so-called Hall current terms in the generalized Ohm’s law become important, triggering the onset of Hall reconnection, which lead to higher reconnection rates. Recent results from the largest two-dimensional Hall MHD simulations to date will be presented that demonstrate the rich dynamics enabled by the interplay between the plasmoid instability and the Hall current. In three dimensions, the plasmoids evolve to form flux ropes that produce turbulence spontaneously, but the reconnection rate remains robustly at the two-dimensional value. Examples of applications will be drawn from magnetically confined laboratory plasmas, high-energy density plasmas, and space plasmas.