Abstract
Radio sounding is a well-established technique that was first deployed in the 1920's for ionospheric sounding from the ground. In recent years, advanced digital sounders were developed that use multi-antenna Doppler imaging for ground-based and satellite observations of geospace plasmas providing detailed information about its structure and dynamics. These modern sounders measure more than just the time of flight and amplitude of the reflected echo pulses, they also determine the arrival angle, wave polarization, and Doppler frequency. Radio sounding relies on total (specular) reflection of radio waves from plasma structures that have plasma frequencies fN equal to the radio frequencies f for the ordinary wave mode. It is, therefore, not possible on the ground to receive echoes reflected from the topside ionosphere -unlike the incoherent scatter radar echoes- or the magnetosphere/plasmasphere since the F2 layer prevents all transmitted waves with frequencies f < foF2 (the maximum plasma frequency) from propagating beyond the height of the F2 layer peak. This presentation will review the state of the art in interferometric Doppler imaging on the ground for routine measurement of F region plasma drifts and show observational results from the polar and equatorial ionosphere. Recently, the radio plasma imager (RPI) on NASA's IMAGE satellite started using modern radio sounding techniques for measurements in the magnetosphere, sounding with frequencies from 3 kHz to 3 MHz, corresponding to electron densities of Ne " 105 to 1011 m-3. This talk will show how the plasma density distribution in the plasmasphere is derived from echo signals with f > fpe that propagate along magnetic field lines.