Field-Aligned Radio Wave Propagation in the Plasmasphere Observed with IMAGE/RPI

The IMAGE satellite is in an eccentric orbit that allows the satellite to move from the magnetospheric cavity into the plasmasphere on each orbit. RPI radio sounding made from the IMAGE satellite, as it approaches and recedes from the plasmasphere, suggests that the predominantly observed echoes correspond to waves propagating along the geomagnetic field lines. These echoes are frequently observed on successive plasmagrams covering a period, on the average, of 8 to 10 minutes while the spacecraft moves a few thousand kilometers across the geomagnetic field lines. Multiple echoes are often observed with time delays corresponding to guided paths in both directions along the magnetic field in the vicinity of L = 2 to 4. They appear to be a relatively consistent feature of the plasmasphere/plasmapause region. The RPI three orthogonal-axis antenna systems make it possible to determine the arrival angles of reflected signals. These arrival angle calculations are made at each frequency for each echo within a plasmagram using the received signal amplitude and phase on the three orthogonal antennas. The measured angle-of-arrival is compared with the geomagnetic field direction at the satellite using a dipole model of the earth's magnetic field. The result of this comparison indicates that the echoes arrive from a direction that is close to the model magnetic field direction at the satellite and are thus consistent with the field-aligned propagation interpretation. From the available RPI data for approximately 500 orbits acquired over a nine-month period, statistics on the frequency of occurrence, duration, the location of these events and length of the field-aligned propagation paths are presented.