THE EFFECTS OF PLASMA BETA AND ANISOTROPY INSTABILITIES ON THE DYNAMICS OF RECONNECTING MAGNETIC FIELDS IN THE HELIOSHEATH

The plasma beta (the ratio of the plasma pressure to the magnetic pressure) of a system can have a large effect on its dynamics as high beta enhances the effects of pressure anisotropies. We investigate the effects of beta in a system of stacked current sheets that break up into magnetic islands due to magnetic reconnection, which is analogous to the compressed heliospheric current sheet in the heliosheath. We find significant differences between systems with low and high initial values of beta. At low beta, growing magnetic islands are modestly elongated and become round as contraction releases magnetic stress and reduces magnetic energy. At high beta, the increase of the parallel pressure in contracting islands causes saturation of modestly elongated islands as island cores approach the marginal firehose condition. Only highly elongated islands reach finite size. The anisotropy within these islands prevents full contraction, leading to a final state of highly elongated islands in which further reconnection is suppressed. The elongation of islands at finite beta is further enhanced by reducing the electron-to-ion mass ratio to more realistic values. The results are directly relevant to reconnection in the sectored region of the heliosheath where there is evidence that elongated islands are present, and possibly to other high-beta systems such as astrophysical accretion flows and the magnetosphere of Saturn.