Cyclic reformation of subcritical perpendicular fast magnetosonic shocks due to oblique Whistler waves

TL;DR

This study uses 2D PIC simulations to investigate the stability and cyclic reformation of subcritical perpendicular fast magnetosonic shocks driven by oblique Whistler waves and associated instabilities.

Contribution

It reveals that oblique Whistler waves induce shock reformation through a reactive coupling with ion acoustic waves, a process not externally forced.

Findings

Shock reformation occurs cyclically due to oblique Whistler waves.

The magnetic component modulates the shock's magnetic field.

Electrostatic ion density modulation causes shock collapse and reformation.

Abstract

The stability of subcritical perpendicular fast magnetosonic shocks, which are propagating at 1.7 times the fast magnetosonic speed, is investigated using two-dimensional PIC simulations. The plasma, composed of electrons and fully ionized nitrogen, is permeated by a uniform magnetic field oriented at 45 degrees to the simulation plane normal. This configuration results in a diamagnetic current that sustains the shocks magnetic ramp and is partially resolved within the simulation plane. The diamagnetic current drives an oblique lower-hybrid gradient drift instability within the ramp. This instability has been observed in magnetic reconnection experiments and studied in the framework of a Harris-type sheath in previous studies. It arises from a reactive coupling between the oblique Whistler wave, which is propagating backward in the electron rest frame, and the forward-propagating ion acoustic wave. Our simulations show that the magnetic component of this wave modulates the shocks magnetic field, while the electrostatic ion density modulation forces the shock to collapse into a magnetic piston and then reform. The reformation is not forced by an external perturbation as in previous simulations but by the oblique Whistler wave.