Observations of Magnetospherically Reflected (MR) Whistler Mode (WM) Echoes Observed by Radio Plasma Imager (RPI) on the IMAGE Satellite: Diagnostics of Electron Density and Ion Composition

We report first observations of Magnetospherically Reflected (MR) Whistler Mode (WM) echoes, hereafter called MR echoes, by Radio Plasma Imager (RPI) on the IMAGE satellite. MR echoes were observed in 2004 and 2005 when RPI transmitted 3.2-ms pulses in 6-63 kHz band. These echoes occurred at frequencies <12 kHz with time delays ranging from ~40 ms to 160 ms. They often showed a minimum in time delay in the 6-12 kHz frequency range, a feature commonly seen for lightning-generated MR whistlers observed on satellites. MR echoes were typically observed inside the plasmasphere at altitude ranging from ~700 to ~4000 km, L-shell from ~1.5 to ~3.5, and geomagnetic latitude λ_m from -30° to 50°, and MLT from 3 to 17. These echoes were observed during geomagnetically quiet conditions with the maximum K_p index during the preceding 24 hours varying between 1 and 4. In one case, the MR echo was observed in the polar region at an altitude ~2600km and λ_m ~ 82° during geomagnetically disturbed conditions (K_p ~5-6 during the preceding 12 hours). In general, at higher altitudes (>3000-3500 km) these echoes were either discrete, with time delay spread ~3-6ms at each frequency or exhibited multi path propagation with a time delay spread <20ms at each frequency. At lower altitudes (<3000 km) these echoes appeared diffuse with time delay spread >40-100 ms at each frequency. All MR echoes were accompanied by Proton Gyro Harmonic (PGH) echoes and were frequently accompanied by discrete WM echoes resulting from the reflections at the bottom of the ionosphere. Ray tracing simulations show that MR echoes result from the reflection of WM waves at the altitude where f_ {lh} ≈ f , where f_ {lh} is the lower hybrid frequency at the reflection altitude. In this interpretation, the MR echo frequency is limited to the range flh,local ≤ f < flh,max, where flh,local and flh,max are, respectively, the lower hybrid frequency at the satellite and the maximum lower hybrid frequency along the field line passing through the satellite. Only those WM waves which are injected at large initial wave normal angles close to resonance cone angle undergo MR reflection. WM waves injected at low wave normal angles reflect at the Earth-ionosphere boundary and lead to discrete WM echoes. Measured characteristics of MR echoes lead to several new methods of plasma diagnostics: (1) lower cutoff frequency of MR echo, identified as the local flh, provides a measure of the local effective ion mass, (2) the upper cutoff frequency of MR echo, identified as roughly the maximum lower hybrid frequency, provides a measure of effective ion mass at the altitude (~1000 km) where flh,max occurs, (3) measured dispersion of MR and discrete WM echoes coupled with ray tracing simulations lead to the determination of local and remote electron density along the raypaths.