Dartmouth Events

Abstract: Magnetic reconnection is a fundamental plasma process that occurs in nearly all magnetized plasmas ranging from space and astrophysical plasmas to laboratory fusion experiments. The detailed physics of reconnection varies significantly depending on the plasma collisionality and system size; and regimes of reconnection have been organized into a “reconnection phase diagram” [1]. The detailed physics of the transition between these regimes is closely related to the onset of fast reconnection in semi-collisional systems, but is not well-understood from first-principles. By examining fully kinetic simulations, it is shown that the transition between collisional and collisionless reconnection may be viewed as a thermodynamic phase transition that occurs when the current sheet reaches a critical temperature, or equivalently a critical thickness [2]. The plasma entropy is calculated and used to derive local, current sheet heat capacities. These heat capacities change discontinuously at the phase transition, and critical behavior is identified. The high-temperature, ordered phase is described by a non-zero viscous electric field, efficient thermal transport across the separatrix, and entropy production due to thermal mixing. Using the critical behavior of the heat capacities, a model for the thermodynamic collapse of an isolated Sweet-Parker current sheet is derived and it is shown that any isolated current sheet will eventually collapse down to kinetic scales. Implications for the onset of kinetic reconnection in both single and multi X-line scenarios are discussed.

[1] Ji and Daughton. Physics of Plasmas 18.11 (2011)
[2] Jara-Almonte and Ji. Physical Review Letters 127.5 (2021)