Injection to rapid diffusive shock acceleration at perpendicular shocks in partially ionized plasmas

TL;DR

This study uses a 3D hybrid simulation to demonstrate how hydrogen atoms leak and are accelerated at perpendicular shocks in partially ionized plasmas, relevant for cosmic ray acceleration in supernova remnants.

Contribution

First 3D hybrid simulation of perpendicular collisionless shock in partially ionized plasma showing detailed particle acceleration mechanisms.

Findings

Hydrogen atoms leak into upstream and are accelerated via pickup process.

Magnetic field fluctuations exhibit 3D structures.

Particles reach relativistic speeds faster than in parallel shocks.

Abstract

We present a three-dimensional hybrid simulation of a collisionless perpendicular shock in a partially ionized plasma for the first time. In this simulation, the shock velocity and the upstream ionization fraction are Vsh ~ 1333 km/s and fi ~ 0.5, that are typical values for isolated young supernova remnants in the interstellar medium. We confirm previous two-dimensional simulation results that downstream hydrogen atoms leak into the upstream region, they are accelerated by the pickup process in the upstream region, and large magnetic field fluctuations are generated both in the upstream and downstream regions. In addition, we find that the magnetic field fluctuations have three-dimensional structures and the leaking hydrogen atoms are injected to the diffusive shock acceleration at the perpendicular shock after the pickup process. The observed diffusive shock acceleration can be interpreted as the shock drift acceleration with scattering. Particles are accelerated to v ~ 100 Vsh ~ 0.3c within ~ 100 gyroperiods in this simulation. The acceleration time scale is faster than that of the diffusive shock acceleration in parallel shocks. Our simulation results suggest that supernova remnants can accelerate cosmic rays to 10^{15.5} eV (the knee) during the Sedov phase.