Magnetic structures and reformation at quasi-parallel shocks

Shock waves in collisionless plasmas are common in heliospheric and astrophysical settings and are some of the most efficient particle accelerators in space. Shocks can undergo self-reformation where a new shock front appears in front of the previous shock front. Shock reformation has been observed previously in both spacecraft observations and simulations, but the process is not yet fully understood. We here study self-reformation of Earth's quasi-parallel bow shock with observations from the four MMS spacecraft and simulation results from the hybrid-Vlasov simulation Vlasiator. We find, in both observations and simulation, that short large amplitude magnetic structures (SLAMS) can constitute shock reformation. The SLAMS form upstream of the shock and grow in amplitude while being convected towards the shock and eventually forming the new shock front. Using MMS's and Vlasiator's high-cadence field and ion measurements, we study how the shock reformation process influences the dynamics and acceleration of ions at the quasi-parallel shock.