Role of Collisionless Reconnection in Magnetospheric Substorms and Solar Flares: a Comparative Study

A. Bhattacharjee

Space Science Center and Center for Magnetic Self-Organization
University of New Hampshire, Durham, NH 03824

Magnetic reconnection is widely believed to play an important role in magnetospheric substorms and solar flares. However, observations impose powerful constraints on theories of reconnection. Since the plasmas in the corona and the magnetosphere are characterized by very high values of the Lundquist number, it is likely that the relevant regime of reconnection is collisionless, and described by a generalized Ohm's law. Observations of substorms and flares demonstrate that they are generically not quasi-steady but impulsive phenomena, characterized not only by rapid growth, but a sudden change in the time-derivative of the reconnection rate, which places an additional burden on theory. We will present strong theoretical evidence that such fast and impulsive signatures can be obtained within the framework of two-fluid or Hall MHD theory, subject to the caveat that there are unresolved questions on how the present theoretical results on fast reconnection scale to large systems. We will also discuss the role of secondary instabilities such as ballooning (in the context of substorms) and tearing (in the context of flares) of thin current sheets that are inevitably formed during the dynamics of reconnection, and may play an important role in accounting for important features of observations. Finally, we will also identify processes such as the diamagnetic drift of electron and ion fluids that can quench the reconnection process nonlinearly, inhibiting explosive reconnection dynamics.