Electrostatic lower hybrid waves excited by electromagnetic whistler mode waves scattering from planar magnetic‐field‐aligned plasma density irregularities

Abstract

Recent satellite observations demonstrate that high amplitude, short wavelength (5 m ≤ λ ≤ 100 m) electrostatic waves are commonly excited by electromagnetic whistler mode waves propagating in regions of the magnetosphere and topside ionosphere where small‐scale magnetic‐field‐aligned plasma density irregularities are thought to exist. A new theoretical model of this phenomenon is presented, based upon passive linear scattering in a cold magnetoplasma. In this model the electrostatic waves are excited by linear mode coupling as the incident electromagnetic whistler mode waves scatter from the magnetic‐field‐aligned plasma density irregularities. The excited short wavelength waves are quasi‐electrostatic whistler mode waves, a type of lower hybrid wave, whose wave normal lies near the whistler mode resonance cone where the wave refractive index becomes very large. For simplicity the case of planar irregularities is considered in which the electron density varies in only a single direction, roughly perpendicular to B0, the Earth's magnetic field. The amplitude of the excited electrostatic lower hybrid waves is calculated for a wide range of values of input electromagnetic wave frequency, wave normal direction, electron plasma frequency, gyrofrequency, ion composition, and irregularity scale and density enhancement. Results indicate that high amplitude lower hybrid waves can be excited over a wide range of parameters for irregularity density enhancements as low as 5% whenever the scale of the irregularity is of the same order as the lower hybrid wavelength. It is shown that lower hybrid waves can be excited only when the planar irregularities are aligned with B0 within a small angle 2χc, where χc is equal to the complement of the resonance cone half‐angle. For the frequencies and L shells considered, χc ≤ 8°. Predictions of the theory are shown to be consistent with satellite data. A VLF “radar” method is demonstrated whereby the lower hybrid wave excitation phenomenon can be used as a diagnostic tool to determine the small scale irregularity structure of the medium. The effective damping of the input electromagnetic wave due to the excitation of the lower hybrid waves is also considered. It is found that this form of damping may be a dominant factor for whistler mode waves throughout large regions of the magnetosphere.