Compression Acceleration of Protons and Heavier Ions at the Heliospheric Current Sheet

Abstract: Recent observations by Parker Solar Probe (PSP) suggest that protons and heavier ions are accelerated to high energies by magnetic reconnection at the heliospheric current sheet (HCS). By solving the energetic particle transport equation in large-scale MHD simulations, we study the compression acceleration of protons and heavier ions in the reconnecting HCS. We find that the acceleration of multi-species ions results in nonthermal power-law distributions with spectral index consistent with the PSP observations. Our study shows that the high-energy cutoff of protons can reach $E_{max} \sim 0.1$ - $1$ MeV depending on the particle diffusion coefficients. We also study how the high-energy cutoff of different ion species scales with the charge-to-mass ratio $E_{max} \propto (Q/M)^\alpha$. When determining the diffusion coefficients from the quasilinear theory with a Kolmogorov magnetic power spectrum, we find that $\alpha \sim 0.4$, which is somewhat smaller than $\alpha \sim 0.7$ observed by PSP.