Simultaneous Observations of Multi-mode Echoes on IMAGE: Propagation, Reflection, and Scattering of Whistler-, Slow Z-, Fast Z-, LO-, and RX-mode Waves at Low Altitude (<5,000 km)

RPI on IMAGE designed to sweep from 3 kHz to 3 MHz permitted sounding in all five cold plasma wave modes including whistler- , slow Z-, fast Z-, LO- and RX-mode. Previous studies presented simultaneous observations of two or three echoes on IMAGE each propagating in distinct plasma wave mode (e.g. Reinisch et al. [2001], Carpenter et al. [2003], Sonwalkar et al. [2004]). We define multimode echoes to be simultaneously occurring echoes that have each propagated in distinctive plasma wave mode. We present here first simultaneous observations of multimode echoes containing four or five echoes. These echoes were typically observed below 5000 km in the middle to high geomagnetic latitude region. From the ~10,000 RPI transmissions in 20 kHz to 1000 kHz frequency range, during 2003-05 period, multimode echoes containing up to two echoes were found in ~2500 cases, up to three echoes in ~1000 cases, up to four echoes in ~200 cases, and up to five in ~20 cases. Because different wave modes have unique propagation characteristics (e.g. frequency, wave length, cut offs, refractive index surface), they propagate along different paths, undergo distinctive reflections, and are affected differently by field aligned irregularities of various scale sizes. With the help of ray tracing analysis of one case, 27 July 2003, when whistler-, fast Z- , slow Z- , LO- and RX-mode echoes were observed, we illustrate propagation of echoes in each mode. At this time IMAGE was at 43°S geomagnetic latitude, 2045 km altitude, L=2.5, and 5 MLT. We found that whistler mode waves propagated in a nonducted mode and reflected at the Earth-ionosphere boundary; fast Z-mode waves propagated in a duct, both above and below the satellite, and reflected at an altitude where fZ=f, where f is the transmission frequency and fZ is the fast Z-mode cutoff; the slow Z-mode waves propagated obliquely with respect to magnetic field and were scattered back by field aligned irregularities; LO mode waves propagated in a ducted mode and reflected from the conjugate hemisphere; RX mode waves propagated in a nonducted mode and reflected below the satellite at an altitude where fX=f, where fX is the RX-mode cutoff frequency. This example demonstrates the potential of multimode echoes for radio sounding and the advantages that it presents over single mode radio sounding. Propagation analysis of multimode echoes should provide us with better understanding of the propagation, reflection, and scattering of naturally occurring plasma waves.