Magnetic islands in the heliosheath: Properties and implications

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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.