"Investigation of Shear Driven Electron-Ion Hybrid Scale Instabilities"
by Landry Horimbere
Advisor: Prof. Daniel Lathrop
Abstract: 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. I have analytically calculated 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. Finally, I solve numerically for the electrostatic EIH dispersion relation for the flow in our experimental configuration to determine our fastest growing mode.
Additionally, I perform particle-in-cell simulations of the evolution of unmagnetised and magnetised plasma jets, in an effort to understand the relaxation mechanisms of plasma shear layers. I find that the unmagnetised 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 due to the symmetry breaking effect of the background magnetic field on the induced antisymmetric field. On the misaligned (low field) side of the jet, the electrons develop vortices that dissipate relatively slowly, generate copious waves, and dramatically destabilize the entire plasma, including the ions.
This work is in preparation for gathering three-dimensional data of shear driven waves in the NRL space simulation chamber. This campaign of experiments aims to provide evidence for the generation of, not only electrostatic waves, but electromagnetic whistler waves so as to experimentally validate the electromagnetic EIH theory and numerical modeling for the first time.