Measurement of electron density in the earth's magnetosphere using discrete whistler mode echoes observed on the image satellite

Abstract

In this thesis, a new whistler mode sounding method has been developed to measure magnetospheric electron density using the RPI instrument on the IMAGE satellite. During the 2000-2002 period, RPI frequently recorded discrete whistler mode (WM) echoes in the ~10-400 kHz frequency range when IMAGE was at low altitudes (<7000 km) in the inner plasmasphere or near its perigee in the southern hemisphere. Most discrete cases were observed in the wintertime in the southern hemisphere during the local morning or nighttime. Ray tracing simulations indicate that the discrete echoes may result from reflections of RPI signals from the earth-ionosphere boundary. By comparing the measured and calculated time dispersion of discrete WM echoes, it is possible to determine the plasma density along the ray path as well as the nonducted or ducted modes of propagation. The ray tracing simulations were carried out for 8 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. The simulations showed that the electron densities at the F2 layer peak (~250 km altitude) varied from 1 x 10⁵ to 7 X 10⁵ el/ cc for all the cases. The extrapolated electron density of the calculations at ~4000 km altitude varied from ~200 - 2000 el/cc (L <4) and ~60 - 1000 el/cc (L> 4). The extrapolated electron density at ~8000 km altitude ranged from ~100 -1000 el/cc (L <4) and from 20 - 60 el/cc (L> 4). These results are in general consistent with previous observations of plasma density in the low altitude magnetosphere. The ray tracing simulation results also indicate that in each of the 8 cases studied whistler mode waves incident on the Earth-ionosphere boundary have incident angles that fall within the transmission cone angle and hence can be observed at the Earth's surface.