Electrostatic waves stimulated by coherent VLF signals propagating in and near the inner radiation belt

Abstract

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.