Interpretation of flat energy spectra upstream of fast interplanetary shocks

TL;DR

This paper investigates flat energy spectra of energetic ions upstream of interplanetary shocks, revealing a velocity filter mechanism that explains particle confinement and propagation differences based on energy.

Contribution

It introduces a velocity filter mechanism to interpret flat ion energy spectra upstream of shocks, supported by multi-spacecraft observations.

Findings

High-energy particles can propagate upstream of shocks.

Low-energy particles tend to be confined near the shock front.

The velocity filter mechanism explains the observed flat spectra.

Abstract

Interplanetary shocks are large-scale heliospheric structures often caused by eruptive phenomena at the Sun, and represent one of the main sources of energetic particles. Several interplanetary shock crossings by spacecraft at $1$ AU have revealed enhanced energetic-ion fluxes that extend far upstream of the shock. Surprisingly, in some shock events, ion fluxes with energies between $100$ keV and about $2$ MeV acquire similar values (which we refer to as ``overlapped'' fluxes), corresponding to flat energy spectra in that range. In contrast, closer to the shock, the fluxes are observed to depend on energy. In this work, we analyze three interplanetary shock-related energetic particle events observed by the Advanced Composition Explorer spacecraft where flat ion energy spectra were observed upstream of the shock. We interpret these observations via a velocity filter mechanism for particles in a given energy range. This reveals that low energy particles tend to be confined to the shock front and cannot easily propagate upstream, while high energy particles can. The velocity filter mechanism has been corroborated from observations of particle flux anisotropy by the Solid-State Telescope of Wind/3DP.