Energetic Particle Anisotropies at the Heliospheric Boundary

In 2012 August the Voyager 1 space probe entered a distinctly new region of space characterized by a virtual absence of heliospheric energetic ions and magnetic fluctuations, now interpreted as a part of the local interstellar cloud. Prior to their disappearance, the ion distributions strongly peaked at a 90° pitch angle, implying rapid escape of streaming particles along the magnetic field lines. Here we investigate the process of particle crossing from the heliosheath into the interstellar space, using a kinetic approach that resolves scales of the particle's cyclotron radius and smaller. It is demonstrated that a "pancake" pitch-angle distribution naturally arises at a tangential discontinuity separating a weakly turbulent plasma from a laminar region with a very low pitch-angle scattering rate. The relatively long persistence of gyrating ions is interpreted in terms of field line meandering facilitating their cross-field diffusion within the depletion region.