Vladimir Sotnikov, Air Force Research Laboratory; James Caplinger, Air Force Research Laboratory; Tony Kim, Air Force Research Laboratory; Evgeny Mishin, Air Force Research Laboratory; Naum Gershenzon, Wright State University; Amit Sharma, Wright State University; Daniel Main, Tech-X Inc.
Keywords: Ionosphere, Radiation Belt, Plasmapause, Space Weather
Abstract:
Generation of Very Low Frequency (VLF) electromagnetic whistler waves due to the parametric interaction of quasi-electrostatic VLF waves known as Lower Oblique Resonance (LOR) waves and Extremely Low Frequency (ELF) waves as well as Ion Acoustic (IA) are analyzed in the framework of two possible scenarios. In the first scenario, quasi-electrostatic LOR waves and ELF waves are excited by conventional loop and dipole antennas. The generation of very low frequency (VLF) waves by antennas in plasma is an important topic because of the wide use of antennas in space and laboratory applications, both military and civilian. There have been a considerable amount of publications related to the excitation of VLF waves by a loop antenna. It is known that when a loop radius is small in comparison with the wavelength of an excited wave, the portion of the radiation field that goes directly into the electromagnetic spectrum of a VLF wave – the whistler mode, is small (less than 3%) in comparison with the wave energy going into the quasi-electrostatic whistler wave component – low oblique resonance (LOR) mode. For this reason, the efficiency of VLF antennas for generation of electromagnetic waves, which can propagate large distance from the source region, is very limited. For many ionosphere applications it is important to increase the level of the radiated power exciting the electromagnetic part of the wave spectrumwhistler waves. The efficiency of antenna may be increase by parametric mechanism of transformation of quasi-electrostatic whistler waves excited by a loop antenna operating at frequency ? on density perturbations produced by a dipole antenna (low frequency source) with frequency ? which excites ion-acoustic waves with frequency above the ion cyclotron frequency but well below the lower hybrid frequency. In this case, whistlers will be excited on combination frequencies. Such an arrangement may be regarded as a parametric antenna for enhanced excitation of whistlers. In the second scenario VLF and ELF waves are naturally excited by plasma instabilities in the plasmasphere boundary layer. We present the results of numerical simulations of nonlinear coupling between LOR and Fast Magnetosonic (MS) waves, which was suggested to explain the observations of VLF electromagnetic emissions at frequencies well above the lower hybrid resonance frequency in the Turbulent Plasmasphere Boundary Layer (TPBL). These emissions represent a distinctive subset of the substorm/storm-related VLF whistler activity contributing to the alteration of the outer radiation belt boundary. As the TPBL is interior to the plasma sheet inner boundary and thus devoid of substorm-injected kiloelectronvolt electrons, the standard whistler generation mechanism must be excluded. Numerical solution of the derived nonlinear equations show that parametric coupling of LOR and ELF waves creates VLF electromagnetic emissions with the spectral characteristics consistent with the observations.
Approved for Public Release: 88ABW-2019-3589
Date of Conference: September 17-20, 2019
Track: Optical Systems & Instrumentation