Whistler-mode wave-injection experiments in the plasmasphere with a radio sounder

Whistler-mode wave-injection experiments with a high-altitude radio sounder offer an opportunity to greatly extend the observing power of satellites s uch as imager for magnetopause-to-aurora global exploration (IMAGE) when th e satellite is within or near the plasmasphere or at low altitudes over the polar regions. We use as an example the radio plasma imager (RPI) instrume nt on IMAGE, which includes crossed 500-m electric antennas in the spin pla ne and a 20-m antenna along the spin axis (for reception only). The 500-m a ntennas approach a half-wavelength at whistler-mode frequencies in the 3-30 kHz range and should have a radiation efficiency of 1-10%. The wave power within similar to 100 km of the transmitter should be greater than that pro duced by wave injection from the ground-based very low-frequency (VLF) tran smitter at Siple, Antarctica, thus making possible experiments on wave-part icle energy and momentum exchange. We use ray tracings along a sample IMAGE orbit (polar, apogee similar to 8R(E)) to show the conditions under which transmitted signals may return to the satellite as echoes, following reflec tion, or be observed by satellites of opportunity, such as EXOS-D. We find that ground reception of transmitted signals should be possible as a result of a linear mode conversion process in regions of ionospheric irregulariti es. We discuss the determination of wave-normal angles of returning signals , which will aid in identifying the signal path and obtaining information o n plasma boundaries and irregularities. The science topics that may be addr essed include: ( 1) investigation of the nonlinear process by which weak co herent waves excite VLF emissions; (2) probing plasmaspheric density struct ure, including plasmaspheric density cavities, field aligned waveducts, den sity irregularities in the plasmapause region, and the ionospheric density structure where conversion of whistler-mode wave energy to quasi-electrosta tic lower hybrid (LHR) waves land vice versa) can take place. (C) 2001 Else vier Science Ltd. All rights reserved.