Ray-Tracing Modeling of Guided Echoes Detected by Radio Plasma Imager on the IMAGE Satellite

Using the Radio Plasma Imager (RPI) aboard the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite, radio echo signatures with discrete virtual range-frequency characteristics have been observed at different altitudes and magnetic latitudes varying from the polar cap to the equator. Long-range discrete echoes have also been observed to return from conjugate hemispheres. Such observations, being analogous to very similar past observations made in the ionosphere (at much lower altitudes), strongly suggest that the radio signals have been ducted by field-aligned irregularities, similar to the mechanism generally invoked to explain the ionospheric observations. Reinisch et al [2001] recently suggested another possible signal guiding mechanism due to the presence of field-aligned density gradients, such as might be present over the polar cap regions. In this scenario, field-aligned propagating radio signals can naturally follow the magnetic field lines without the trapping action of a density duct. In order to determine the mechanism responsible for guiding of long-range discrete echoes observed in the magnetosphere by the RPI, we have performed ray-tracing modeling of discrete echo propagation to investigate the plasma conditions needed to support ducting and wave-guiding by a field-aligned density gradient. This paper discusses the plasma models used and presents modeling results. Reinisch, B. W., et al., Plasma Density Distribution Along the Magnetospheric Field: RPI Observations From IMAGE, Geophys. Res. Lett., 28, 4521-4524, 2001.