The radio plasma imager investigation on the IMAGE spacecraft

Radio plasma imaging uses total reflection of electromagnetic waves from pl asmas whose plasma frequencies equal the radio sounding frequency and whose electron density gradients are parallel to the wave normals. The Radio Pla sma Imager (RPI) has two orthogonal 500-m long dipole antennas in the spin plane for near omni-directional transmission. The third antenna is a 20-m d ipole along the spin axis. Echoes from the magnetopause, plasmasphere and c usp will be received with the three orthogonal antennas, allowing the deter mination of their angle-of-arrival. Thus it will be possible to create imag e fragments of the reflecting density structures. The instrument can execut e a large variety of programmable measuring options at frequencies between 3 kHz and 3 MHz. Tuning of the transmit antennas provides optimum power tra nsfer from the 10 W transmitter to the antennas. The instrument can operate in three active sounding modes: (1) remote sounding to probe magnetospheri c boundaries, (2) local (relaxation) sounding to probe the local plasma fre quency and scalar magnetic field, and (3) whistler stimulation sounding. In addition, there is a passive mode to record natural emissions, and to dete rmine the local electron density, the scalar magnetic field, and temperatur e by using a thermal noise spectroscopy technique.