Spontaneous Electromagnetic Emission from a Strongly Localized Plasma Flow

Abstract

The laboratory experiments described in this dissertation establish that strongly localized DC electric fields perpendicular to the ambient magnetic field can behave as a radiation source for electromagnetic ion cyclotron waves, transporting energy away from the generation region. This investigation is motivated by numerous space observations of electromagnetic ion cyclotron waves. Ion cyclotron waves are important to space weather dynamics due to their ability to accelerate ions transverse to the background magnetic field, leading to ion outflows in the auroral regions. Many different theoretical mechanisms have been presented to account for these waves. Sheared flows produced by localized electric fields coupled with a perpendicular magnetic field are a potentially important energy source that can create waves of this type. In situ observations of sheared plasma flows collocated with electromagnetic wave activity have led to this laboratory effort to investigate the impact of electromagnetic, velocity shear-driven instabilities on the near-Earth space plasma dynamics. Under scaled ionospheric conditions in the Space Physics Simulation Chamber at the Naval Research Laboratory (NRL), the transition from electrostatic to electromagnetic ion cyclotron (EMIC) wave propagation has been investigated. Previous experiments at West Virginia University, NRL, and Auburn University demonstrated that transverse sheared plasma flows can drive electrostatic ion cyclotron waves. It was observed that these waves were capable of heating the ions in the direction transverse to the magnetic field. The general wave characteristics and wave dispersion experimentally observed are in agreement with the current theoretical models. The electrostatic waves generated in the experiments described in this dissertation were consistent with the previous electrostatic experiments. In addition, the electromagnetic component of these waves increase by two orders of magnitude as the plasma β was increased. The EMIC waves exhibited an electric field threshold of 60.5 V/m and their frequency increased as the applied electric field increased. The observed EMIC waves are predominantly azimuthally propagating m=1 cylindrical waves, which propagate in the direction of the E×B drift. A velocity shear modified dispersion relation was derived from the Peñano and Ganguli model for electromagnetic waves in the presence of sheared flows and compared with experimental observations.