Magnetic Reconnection in Astrophysical Systems

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.