Formation of foreshock transients and associated secondary shocks

TL;DR

This study uses particle-in-cell simulations to investigate how secondary shocks form in the foreshock region, revealing the roles of electric fields, ion properties, and shock Mach number in their development and potential for particle acceleration.

Contribution

The paper demonstrates the formation mechanisms of secondary shocks through 1D and 2D simulations, highlighting the influence of hot ion properties and shock conditions, which was not fully understood before.

Findings

Electric fields shape shock magnetic structures.

Larger hot ion thermal speed favors secondary shock formation.

Higher Mach number shocks are more likely to develop secondary shocks.

Abstract

Upstream of shocks, the foreshock is filled with hot ions. When these ions are concentrated and thermalized around a discontinuity, a diamagnetic cavity bounded by compressional boundaries, referred to as a foreshock transient, forms. Sometimes, the upstream compressional boundary can further steepen into a secondary shock, which has been observed to accelerate particles and contribute to the primary shock acceleration. However, secondary shock formation conditions and processes are not fully understood. Using particle-in-cell simulations, we reveal how secondary shocks are formed. From 1D simulations, we show that electric fields play a critical role in shaping the shock's magnetic field structure, as well as in coupling the energy of hot ions to that of the shock. We demonstrate that larger thermal speed and concentration ratio of hot ions favors the formation of a secondary shock. From a more realistic 2D simulation, we examine how a discontinuity interacts with foreshock ions leading to the formation of a foreshock transient and a secondary shock. Our results imply that secondary shocks are more likely to occur at primary shocks with higher Mach number. With the secondary shock's previously proven ability to accelerate particles in cooperation with a planetary bow shock, it is even more appealing to consider them in particle acceleration of high Mach number astrophysical shocks.