Remote Sensing of Magnetospheric Plasma Density from the Analysis of Discrete Whistler Mode Echoes Received by RPI on IMAGE

Whistler mode wave injection and reception using the RPI Instrument on IMAGE satellite has led to a new remote sensing method to measure the plasma density. During 2000-2002 period, RPI frequently recorded discrete whistler mode echoes in ∼10-400 kHz frequency range when IMAGE was at low altitudes (<5000 km) in the inner plasmasphere or near its perigee in the southern hemisphere. In most cases, the whistler mode echoes were accompanied by Z-mode echoes. About 85% of the discrete echoes were observed during the winter time with an occurrence rate of ∼ 5% to 10% of the total number of transmissions at low altitude. No discrete echoes were observed during the summer time, which could possibly be due to the D region absorbtion. Ray tracing simulations indicate that the discrete echoes may result from reflections of RPI signals from the Earth-ionosphere boundary. By comparing the measured dispersion of discrete echoes with that from ray tracing simulations, it is possible to determine the plasma density along the ray path as well as the nonducted or ducted modes of propagation. When the discrete echoes were accompanied by Z-mode echoes, it was possible to determine the local electron density at the satellite from the Z mode upper hybrid frequency and the local gyrofrequency. This local electron density is used as the reference in the ray tracing model, which includes a diffusive equilibrium model for electron density inside the plasmapause and a R^-4.5 dependence with altitude outside the plasmapause. The altitude of the base of the diffusive equilibrium model is chosen as 1000 km based on the previous measurements of electron densities from ISIS-A, DE-1 and S3-3 satellites. The ray tracing simulations were carried out for 8 out of 68 cases observed in the year 2002 when discrete echoes were accompanied by Z mode echoes. The 8 cases were chosen to cover the maximum and minimum local electron densities, which varied from ∼ 300 - 4000 el/cc at the satellite location as measured from Z mode echoes. For four cases, IMAGE was at relatively low invariant latitudes (47^o - 59^o) and altitude between ∼1200 - 1500 km; for other four cases, IMAGE was at higher invariant latitudes ranging from 65^o - 82^o and altitude from ∼1200 - 2300 km, corresponding to the auroral and polar regions. The simulations showed that the electron densities at the F2 layer peak ( ∼ 250 km altitude)varied from 10⁵ to 7 × 10⁵ el/cc for all the cases. The interpolated electron density at ∼ 4000 km altitude varied from ∼ 200 to 2000 el/cc for the cases at lower invariant latitudes and ∼ 60 to 1000 el/cc for the cases in the auroral and polar regions. The interpolated electron density at ∼ 8000 km altitude ranged from 100 - 1000 el/cc for the cases at lower invariant latitudes and from 20 - 60 el/cc for the cases in the auroral and polar regions. These results are consistent with previous observations of plasma density.