Remote Sensing of Magnetospheric Plasma Density from the Analysis of 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 and density structures in the magnetosphere. During 2000-2002 period, RPI frequently recorded discrete (characterized by discrete spectral form) and diffuse (characterized by multiple receptions at each frequency) whistler mode echoes in ∼10-400 kHz frequency range when IMAGE was at low altitudes (<6000 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. The discrete and diffuse echoes, were observed ∼ 5% to 10% and ∼ 10% to 20%, respectively, of the total number of transmissions at low altitude. Ray tracing simulations indicate that the discrete echoes may result from reflections of RPI signals from the Earth-ionosphere boundary and that the diffuse echoes may result from scattering of RPI signals from small scale ( ∼10-100 m) plasma density irregularities. 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 to determine the nonducted or ducted modes of propagation. The ray tracing simulations carried out for 10 cases from the period 21 April to 28 August 2000, when the satellite was at >50^o latitude, indicated that the electron density varied between 500 to 1000 el/cc at ∼ 4000 km altitude and showed R^-4.5 dependence with altitude. Two cases from the period of 20 May to 31 May 2002, when discrete echoes were accompanied by Z-mode echoes, were analyzed to determine the electron density. In these cases, it was possible to determine the local electron density at the satellite from the Z-mode cutoffs and the local gyrofrequency. This allowed determination of electron density at the F2 layer peak. The simulations showed that the electron density at the F2 layer peak ( ∼ 250 km) was about ∼ 3-4 e5 el/cc in either cases, and the electron density at ∼ 4000 km altitude was about 500 el/cc in one case and about 75 in the other case. By comparing measured dispersion of diffuse echoes with that from ray tracing simulations, it is possible to determine the locations and spatial sizes of density irregularities responsible for diffuse echoes. One diffuse case, 06 August 2000, was analyzed. Simulation result indicated the presence of ∼ 10-100 m scale plasma irregularities within ∼1000 km of the IMAGE satellite (Altitude ∼2500 km, λ _m ∼ 65^o) at the time of observation. These results are in general consistent with previous observations of plasma density in the low altitude magnetosphere.