Radio sounding from IMAGE: New perspectives on field aligned density irregularities, Z and whistler mode diagnostics, and proton cyclotron harmonic echoes

The operating frequency of the Radio Plasma Imager (RPI) instrument on the IMAGE satellite extended from 3 kHz to 3 MHz. This wide range made possible free-space O and X mode sounding from altitudes on both sides of the plasmasphere boundary layer (PBL) while also making possible wave injection in the whistler-mode and Z-mode domains at altitudes less than ≈ 10,000 km. We briefly review new findings in four areas: (i) density irregularities in the PBL and within the plasmasphere; (ii) upward Z-mode probing along geomagnetic field lines from within a Z-mode propagation `cavity´ (iii) downward probing from 4500-7000 km altitude using whistler-mode waves that undergo two fundamentally different types of reflection; (iv) strong coupling of RPI pulses to the local proton plasma