Deterministic propagation of energetic ions through the outer heliosphere

The PEPSSI instrument onboard New Horizons is providing unprecedented measurements of He+ pickup, suprathermal, and energetic ions in the outer heliosphere over an energy range that was not covered by the Voyagers. The latest updates in the flight software will provide similarly unprecedented high-resolution measurements near shocks.

A data set of particular interest is a period throughout 2015 and 2016 taken beyond 30AU where the shocks of corotating interaction regions regularly passed the spacecraft. After each shock, particle intensities were decreasing. The spectral exponent in the plasma frame for intensity over energy systematically changed with distance to the shocks, instead of remaining at the canonical value of -3/2. To our knowledge, there has been no physical model to quantitatively explain any of these patterns.

Here we are suggesting that keV to at least 100keV ions propagate deterministically and scatter free away from the shocks. The scatter-free assumption is supported measurements that organize well with the nominal magnetic field and show the highest intensities in the field-aligned direction. The ions cool during their propagation along the field lines. We find that the cooling can be so significant that originally sunward moving gyro centers are turning around and convecting outward. We quantitatively compare modeled distributions to the observations and preliminary results suggest a good match.