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

Abstract The stability of subcritical perpendicular fast magnetosonic (FMS) shocks, which are propagating at 1.7 times the FMS speed, is investigated using two-dimensional particle-in-cell simulations. The plasma, composed of electrons and fully ionized nitrogen, is permeated by a uniform magnetic field oriented at 45 ∘ to the simulation plane normal. This configuration results in a diamagnetic current that sustains the shock’s 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 shock’s 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.