Direct Evidence for Magnetic Reflection of Heavy Ions from High Mach Number Collisionless Shocks

TL;DR

This paper provides the first direct observational evidence that heavy ions, specifically alpha particles, are reflected by high Mach number collisionless shocks due to intense magnetic amplification, influencing their acceleration.

Contribution

It presents the first direct in-situ evidence of magnetic reflection of heavy ions at high Mach number shocks, emphasizing the role of magnetic amplification in ion energization.

Findings

Heavy ions are reflected by shock magnetic fields.

Magnetic amplification modulates ion trajectories.

Reflected ions are further accelerated upstream.

Abstract

Strong shocks in collisionless plasmas, such as supernovae shocks and shocks driven by coronal mass ejections, are known to be a primary source of energetic particles. Due to their different mass per charge ratio, the interaction of heavy ions with the shock layer differs from that of protons, and injection of these ions into acceleration processes is a challenge. Here we show the first direct observational evidence of magnetic reflection of alpha particles from a high Mach number quasi-perpendicular shock using in-situ spacecraft measurements. The intense magnetic amplification at the shock front associated with nonstationarity modulates the trajectory of alpha particles, some of which travel back upstream as they gyrate in the enhanced magnetic field and experience further acceleration in the upstream region. Our results in particular highlight the important role of high magnetic amplification in seeding heavy ions into the energization processes at nonstationary reforming shocks.