Magnetic Reconnection in Astrophysical Systems

TL;DR

This paper discusses the conditions under which magnetic reconnection occurs in astrophysical systems, emphasizing criteria like fast, energetic, and observable reconnection, and explores specific environments such as stellar coronae and star-disk systems.

Contribution

It introduces criteria for identifying astrophysical systems where magnetic reconnection is relevant and proposes the Y-point helmet streamer as a key magnetic configuration.

Findings

Reconnection is fast in collisionless, force-free, and optically thin plasmas.

Magnetic reconnection is significant in stellar coronae, star-disk systems, and accretion disk coronae.

The Y-point helmet streamer is a generic magnetic configuration for large-scale reconnection.

Abstract

The main subject of my talk is the question: in what kind of astrophysical systems magnetic reconnection is interesting and/or important? To address this question, I first put forward three general criteria for selecting the relevant astrophysical environments. Namely, reconnection should be: fast; energetically important; and observable. From this, I deduce that the gas density should be low, so that the plasma is: collisionless; force-free; and optically thin. Thus, for example, the requirement that reconnection is fast implies that Petschek's reconnection mechanism must be operating, which is possible, apparently, only in the collisionless regime. Next, I argue that the force-free condition implies that the magnetic field be produced in, and anchored by, a nearby dense massive object, e.g., a star or a disk, strongly stratified by gravity. I then stress the importance of field-line opening (e.g., by differential rotation) as a means to form a reconnecting current sheet. Correspondingly, I suggest the Y-point helmet streamer as a generic prototypical magnetic configuration relevant to large-scale reconnection in astrophysics. Finally, I discuss several specific astrophysical systems where the above criteria are met: stellar coronae, magnetically-interacting star--disk systems, and magnetized coronae above turbulent accretion disks. In the Appendix, I apply the ideas put forward in this talk to the solar coronal heating problem.