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.

Recent ISEE 1 satellite observations indicate that electrostatic waves are commonly stimulated by coherent VLF electromagnetic whistler mode signals propagating on magnetic field lines in, and near, the inner radiation belt. Evidence for the existence of the electrostatic waves consists of apparent bandwidth increases of up to 300 Hz in the electric field component of 10 to 15 kHz signals from ground‐based VLF transmitters observed in the inner radiation belt. During the period July through December 1983, the effect was observed on 60% of the satellite orbits which penetrated the inner radiation belts (L ≤ 2), and in many cases the effect was observed continuously from the magnetic equator at L ∼ 2 down to perigee. The effect appeared in signals propagating from sources in both the northern and southern hemispheres. In general, the stimulated electrostatic waves were delayed with respect to the input signal by 50 ms to 500 ms, suggesting that the generation region was removed from the satellite location. The total apparent bandwidth Δω of the stimulated electrostatic waves observed on any given satellite pass was found to be roughly proportional to the local magnitude of the Earth's magnetic field, but the constant of proportionality varied over a 4‐to‐1 range for the complete data set. In general at any given time, signals of similar frequency and propagation direction exhibited similar Δω. However, signals of similar frequency propagating in opposite directions occasionally exhibited marked differences in Δω. Characteristics of the stimulated wave structure coincide closely with those reported in a recent study of low altitude satellite VLF wave data (Bell et al., 1983). In consonance with this earlier work it is hypothesized that the electrostatic waves are stimulated as the electromegnetic whistler‐mode input waves scatter from magnetic‐field‐aligned plasma density irregularities which exist within the inner radiation belt. It is further hypothesized that once generated, these short wavelength electrostatic waves may then interact with energetic radiation belt particles, producing particle pitch angle scattering and precipitation. The induced precipitation could result in ionospheric plasma density enhancements, and upward diffusion of cold plasma from these enhancements may produce new magnetic‐field‐aligned density irregularities. This feedback process may provide a mechanism for the creation and maintenance of magnetic‐field‐aligned plasma density irregularities in the inner radiation belts.

In this paper, numerical simulations of the scattering from time‐dependent realizations of one‐dimensional ocean surface waves are described. A new technique is used that allows efficient generation of ocean surface realizations that preserve the dominant nonlinear hydrodynamic characteristics. Thus unique scattering effects of real ocean surface waves can be explored. Until very recently, numerical simulations of rough‐surface scattering were used mainly to test and/or improve theoretical models that predict the average bistatic scatter cross section. We carry the simulations further by generating Doppler spectra from dynamically evolving surface realizations. Doppler spectra of signals scattered from the ocean surface are affected by both hydrodynamic nonlinearities and higher‐order scatter terms. The simulated Doppler spectra from nonlinear surface realizations reproduce the measured characteristics of ocean and wave‐tank data for low and high wind conditions. We also show that the results are essentially reproduced by the second‐order Kirchhoff approximation.

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Numerical simulations of rough surface scattering at near‐grazing incidence require very large surfaces (≳500λ). Conventional methods of exact solutions require the inversion of a very large matrix, which can exceed the memory and speed capabilities of even modern supercomputers. The beam simulation method proposed by Saillard and Maystre circumvents this problem by decomposing the large incident beam into narrower subbeams and then synthesizing the large beam by coherent superposition. The radius of these narrower subbeams is determined by the local interaction distance on the surface, which is found to increase with incidence angle, ultimately forcing a single beam in the limit of strict grazing incidence. This paper demonstrates that this technique gives essentially the same results as can be obtained by the method of moments and can handle surfaces as large as 1000λ for grazing incidence angle as low as 10°.