Observations of magnetospheric plasmas by the radio plasma imager (RPI) on the image mission

The Imager for Magnetopause-to-Aurora Global Exploration satellite (IMAGE, see http://image.gsfc.nasa.gov), launched on March 25, 2000, is the first mission dedicated to observing the global-scale structure and dynamics of the magnetosphere. Remote-sensing instruments on IMAGE are designed to observe the magnetopause, ring current, plasmasphere, polar cusp, and the auroral region, in order to reveal the morphologies and interactions among these inter-connected regions. In particular, the RPI, a digital radio sounder with direction-finding capabilities, can transmit radio pulses from 3 kHz to 3 MHz (corresponding to plasma densities of 0.1-10 ⁵ cm ^-3 ) to probe different magnetospheric plasma. Using two 500-m tip-to-tip orthogonal dipole antennas in the spin plane and a 20-m tip-to-tip spin-axis dipole antenna, signals reflected at remote plasma regions are received as echoes. The RPI measures the echo amplitude, phase, range, polarization, Doppler shift, and angle of arrival as a function of sounding frequency. Advanced digital techniques allow echoes of low-power transmitted signals to be detected over magnetospheric distances. With a nominal 2-minute IMAGE spin period in a highly inclined (90°) elliptical orbit (apogee of 8 R _E ), the RPI (with a nominal 1-min resolution) situated in the magnetospheric cavity observes the inner and outer magnetospheric boundaries. Its observations can reveal the electron density profiles and dynamics of key magnetospheric plasma regions. This paper highlights the science objectives of the RPI on IMAGE and presents some early RPI observations. Published by Elsevier Science Ltd on behalf of COSPAR.