The Role of Pressure Anisotropy on Particle Acceleration during Magnetic Reconnection

Voyager spacecraft observations have revealed that contrary to expectations, the source of anomalous cosmic rays (ACRs) is not at the local termination shock. A possible mechanism of ACR acceleration is magnetic reconnection in the heliosheath. Using a particle-in-cell code, we investigate the effects of β on reconnection-driven particle acceleration by studying island growth in multiple interacting Harris current sheets. Many islands are generated, and particles are dominantly heated through Fermi reflection in contracting islands during island growth and merging. There is a striking difference between the heating of electrons versus the heating of ions. There is a strong dependence of β on electron heating, while the ion heating is insensitive to β. Anisotropies develop with T_∥ ≠ T_⊥ for both electrons and ions. The electron anisotropies support the development of a Weibel instability that suppresses the Fermi acceleration of the electrons. Since the Weibel instability develops at a larger T_∥/T_⊥ in lower β systems, electrons are able to accelerate more efficiently by the Fermi mechanism at low β. The variance in anisotropy implies less electron acceleration in higher β systems, and thus less heating. This study sheds light on particle acceleration mechanisms within the sectored magnetic field regions of the heliosheath and the dissipation of turbulence such as that produced by the magnetorotational instability in accreting systems.