Heavy-ion Acceleration and Self-generated Waves in Coronal Shocks

TL;DR

This study uses Monte Carlo simulations to investigate how minor ions are accelerated at coronal shocks, revealing the influence of self-generated waves on maximum energies and the importance of trapping dynamics.

Contribution

First simulations of shock-accelerated minor ions that explore trapping dynamics, wave generation, and energy limits in coronal shock environments.

Findings

Minor ions significantly influence wave generation at low wavenumbers.

Maximum ion energy depends on charge-to-mass ratio as approximately (Q/A)^1.5.

Steady-state models are insufficient for describing heavy ion acceleration.

Abstract

Context: Acceleration in coronal mass ejection driven shocks is currently considered the primary source of large solar energetic particle events. Aims: The solar wind, which feeds shock-accelerated particles, includes numerous ion populations, which offer much insight into acceleration processes. We present first simulations of shock-accelerated minor ions, in order to explore trapping dynamics and acceleration timescales in detail. Methods: We have simulated diffusive shock acceleration of minor ions (3He2+, 4He2+, 16O6+ and 56Fe14+) and protons using a Monte Carlo method, where self-generated Alfv\'enic turbulence allows for repeated shock crossings and acceleration to high energies. Results: We present the effect of minor ions on wave generation, especially at low wavenumbers, and show that it is significant. We find that maximum ion energy is determined by the competing effects of particle escape due to focusing in an expanding flux tube and trapping due to the amplified turbulence. We show the dependence of cut-off energy on the particle charge to mass ratio to be approximately (Q/A)^1.5. Conclusions: We suggest that understanding the acceleration of minor ions at coronal shocks requires simulations which allow us to explore trapping dynamics and acceleration timescales in detail, including evolution of the turbulent trapping boundary. We conclude that steady-state models do not adequately describe the acceleration of heavy ions in coronal shocks.