Dissipation of the sectored heliospheric magnetic field near the heliopause: a mechanism for the generation of anomalous cosmic rays

TL;DR

This paper proposes a new mechanism involving magnetic reconnection near the heliopause that explains the origin and energy spectrum of anomalous cosmic rays observed by Voyager spacecraft.

Contribution

It introduces a model where magnetic reconnection driven by sectored magnetic fields produces energetic ions, challenging previous shock-based theories for ACR origins.

Findings

ACR spectrum modeled as a power law with index ~1.5

Magnetic reconnection accelerates ions via Fermi reflection

Model explains Voyager observations of ion spectra

Abstract

The recent observations of the anomalous cosmic ray (ACR) energy spectrum as Voyagers 1 and 2 crossed the heliospheric termination shock have called into question the conventional shock source of these energetic particles. We suggest that the sectored heliospheric magnetic field, which results from the flapping of the heliospheric current sheet, piles up as it approaches the heliopause, narrowing the current sheets that separate the sectors and triggering the onset of collisionless magnetic reconnection. Particle-in-cell simulations reveal that most of the magnetic energy is released and most of this energy goes into energetic ions with significant but smaller amounts of energy going into electrons. The energy gain of the most energetic ions results from their reflection from the ends of contracting magnetic islands, a first order Fermi process. The energy gain of the ions in contracting islands increases their parallel (to the magnetic field ${\bf B}$) pressure $p_\parallel$ until the marginal firehose condition is reached, causing magnetic reconnection and associated particle acceleration to shut down. The model calls into question the strong scattering assumption used to derive the Parker transport equation and therefore the absence of first order Fermi acceleration in incompressible flows. A simple 1-D model for particle energy gain and loss is presented in which the feedback of the energetic particles on the reconnection drive is included. The ACR differential energy spectrum takes the form of a power law with a spectral index slightly above 1.5. The model has the potential to explain several key Voyager observations, including the similarities in the spectra of different ion species.