Investigation of Shear-Driven Electron-Ion Hybrid Scale Instabilities at Dipolarization Fronts

Dipolarization fronts are regions of stressed plasma that are suspected to be the source of broadband electrostatic and electromagnetic noise in the Earth's magnetosphere. It has been posited that after highly impulsive reconnection events, they can develop an electric field and shear layer with gradient length scales smaller than the ion gyroradius. Researchers at the Naval Research Laboratory (NRL) have developed a model for the spectrum of electrostatic and electromagnetic waves that are produced by the Electron-Ion Hybrid (EIH) instabilities of such a compressed front. In this work, I analytically calculate the dispersion relations for the Kelvin-Helmholtz (KH) and electrostatic EIH instability for both a single piecewise continuous shear layer and a piecewise continuous symmetric double shear layer (jet). I find that both the KH and EIH modes exhibit instabilities at arbitrarily low velocities but that the wavelength of the fastest-growing EIH mode also diverges at low-velocity shears.

Additionally, I perform particle-in-cell simulations of the evolution of unmagnetized and magnetized plasma jets, to understand the relaxation mechanisms of plasma shear layers. I find that the unmagnetized system evolves symmetrically, develops small electron eddies that dissipate quickly, has minimally perturbed ions, and develops stabilizing antisymmetric electric and magnetic fields that bring it to a non-linear steady state. In contrast, the magnetized KH case evolves asymmetrically. On the misaligned (low field) side of the jet, the electrons develop vortices that dissipate relatively slowly, generate waves as they merge, and dramatically destabilize the entire plasma, including the ions.

Finally, I compare the numerical and analytical results to measurements from the NRL space simulation chamber sheared plasma experiment. I find that, in agreement with theory, the amplitude of the waves detected grows with the applied electric field gradient and resultant velocity shear.