Radio sounding in space: magnetosphere and topside ionosphere

Abstract

Modern sounding techniques have been developed for the space-borne exploration of Earth's magnetosphere and topside ionosphere. Two new satellite instruments will use the advanced techniques of the ground-based Digisondes. The Radio Plasma Imager (RPI), a low-frequency sounder with 500-m dipole antennas designed to sweep from 3 kHz to 3 MHz, will be part of NASA's IMAGE mission to be launched in February 2000 into an elliptical orbit with an altitude at apogee of 7 R e. While in the magnetospheric cavity, RPI will receive echoes from the magnetopause and the plasmasphere and will measure the direct response of the magnetosphere's configuration to changes in the solar wind. With three orthogonal dipole antennas (two 500-m tip-to-tip antennas in the spin plane used for transmission and reception, one 20-m antenna along the spin axis for reception only) the arrival angle of returning echoes can be determined with high accuracy. The other instrument is the TOPside Automated Sounder (TOPAS), which was originally conceived for the Ukrainian WARNING mission with a launch date in 2001. Using one antenna for transmission and three orthogonal 10-m antennas for reception, TOPAS will be able to determine the arrival angle of ionospheric echoes and their wave polarization. It will then be possible to automatically scale the topside ionograms and calculate the electron density profiles in real time. Operating as a high-frequency radar, TOPAS will for the first time measure topside plasma velocities by tracking the motions of plasma irregularities.