PLASMA BETA-LIMITS FOR MAGNETIC ANNIHILATION MODELS

Magnetic annihilation occurs when two oppositel!y directed magnetic fi elds are brought together by a plasma flow. Several exact nonlinear so lutions exist which typically depend on the ratios of plasma pressure to magnetic pressure (the plasma beta), inflow speed to global Alfven speed (the Alfven Mach number) and of the advective to diffusive terms of the induction equation (the Lundquist number). Ensuring that the p lasma pressure is everywhere positive restricts the freedom of choice of these parameters, however. Restrictions on the plasma beta are deri ved for the cases of two- and three-dimensional annihilation and two-d imensional reconnective annihilation At the inflow speeds typically re quired for fast reconnection in diffuse astrophysical plasmas the mini mum plasma beta is several orders of magnitude larger than the observe d values of unity or less. In other words, at the observed plasma beta the models are only valid for extremely small annihilation rates. (C) 1996 American Institute of Physics.