PIC simulations of microinstabilities and waves at near-Sun solar wind perpendicular shocks: Predictions for Parker Solar Probe and Solar Orbiter

TL;DR

This study uses PIC simulations to analyze microinstabilities and wave excitations at near-Sun perpendicular shocks, providing insights relevant for Parker Solar Probe and Solar Orbiter observations.

Contribution

It offers the first detailed PIC simulation analysis of wave excitations and microinstabilities at near-Sun solar wind shocks, revealing mechanisms for ES and EM wave generation.

Findings

Identification of two types of electrostatic waves triggered by electron cyclotron drift instability.

Observation of electromagnetic X mode waves likely caused by relativistic electrons.

Insights into wave coupling and acceleration processes at young CME-driven shocks.

Abstract

Microinstabilities and waves excited at moderate-Mach-number perpendicular shocks in the near-Sun solar wind are investigated by full particle-in-cell (PIC) simulations. By analyzing the dispersion relation of fluctuating field components directly issued from the shock simulation, we obtain key findings concerning wave excitations at the shock front: (1) at the leading edge of the foot, two types of electrostatic (ES) waves are observed. The relative drift of the reflected ions versus the electrons triggers an electron cyclotron drift instability (ECDI) which excites the first ES wave. Because the bulk velocity of gyro-reflected ions shifts to the direction of the shock front, the resulting ES wave propagates oblique to the shock normal. Immediately, a fraction of incident electrons are accelerated by this ES wave and a ring-like velocity distribution is generated. They can couple with the hot Maxwellian core and excite the second ES wave around the upper hybrid frequency. (2) from the middle of the foot all the way to the ramp, electrons can couple with both incident and reflected ions. ES waves excited by ECDI in different directions propagate across each other. Electromagnetic (EM) waves (X mode) emitted toward upstream are observed in both regions. They are probably induced by a small fraction of relativistic electrons. Results shed new insight on the mechanism for the occurrence of ES wave excitations and possible EM wave emissions at young CME-driven shocks in the near-Sun solar wind.