Magnetic islands in the heliosheath: Properties and implications

Abstract

In the heliosheath there are sectors of magnetic fields separated by current sheets thinner than the ion inertial length and thus subject to the tearing instability. This instability allows the development of magnetic islands that grow due to magnetic reconnection. Using PIC (particle-in-cell) simulations, we show that these islands are relevant because they quickly grow to fill up the space between the sectors and in the meanwhile generate temperature anisotropies, accelerate particles, and form instabilities based on the anisotropies. The plasma β (the ratio of the plasma pressure to the magnetic pressure) of a system can have a large effect on its dynamics since high β enhances the effects of pressure anisotropies. In our PIC simulations, we investigate 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 that for high β, and for realistic ion-to-electron mass ratios, 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. In the heliosheath there is evidence that these elongated islands are present. We performing a scaling of the growth of magnetic islands versus the system size. We thus determine that the islands, although reaching a final elongated state, can continue growing via the merging process until they reach the sector width. The islands achieve this size in much less time than it takes for the islands to convect through the heliosheath. We also find that the electron heating in our simulations has a strong β dependence. Particles are dominantly heated through Fermi reflection in contracting islands during island growth and merging. However, electron anisotropies support the development of a Weibel instability which impedes the Fermi acceleration of the electrons. In the heliosheath, we predict that energization of particles in general is limited by interaction with anisotropy instabilities such as the firehose instability, and by the the Weibel instability for electrons in particular.