An overview of Radio Plasma Imager accomplishments James Green (1), Bodo Reinisch (2) (1) NASA/GSFC, Code 630 Greenbelt, Maryland, 20771, USA Tel: 301-286-7354, Fax: 301-286-1771, Email: green@mail630.gsfc.nasa.gov (2) University of Massachusetts at Lowell, Center for Atmospheric Research 600 Suffolk Street Lowell, Massachusetts, 01854, USA Tel: 978-934-4903, Fax: 978-459-7916, Email: bodo.reinisch@uml.edu The Radio Plasma Imager (RPI) on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft was designed as a long range magnetospheric radio sounder, relaxation sounder, and a passive plasma wave instrument. The RPI is a highly flexible instrument that can be programmed to perform these types of measurements at times when IMAGE is located in key regions of the magnetosphere. RPI is the first radio sounder ever flown to large radial distances into the magnetosphere. The long-range sounder echoes from RPI allows for the remote sensing of a variety of plasmas structures and boundaries in the magnetosphere. A profile inversion technique for RPI echoes has been developed and provides a method for determining the density distribution of the plasma from either direct or field-aligned echoes. This technique has enabled the determination of the evolving density structure of the polar cap and the plasmasphere under a variety of geomagnetic conditions. New results from RPI show that the plasmasphere refills significantly faster (by a factor of ~2) than theories have predicted. In addition plasma resonance observations from RPI at large radial distances over the polar cap have been made allowing for the determination of the plasma density to within an accuracy of a few percent. RPI's long antennas and its very low noise receivers provide excellent observations in the passive receive-only mode when the instrument measures the thermal plasma noise as well as natural emissions such as the continuum radiation and auroral kilometric radiation (AKR). Recent passive measurements of AKR show a new doubled peaked AKR emission spectrum that is predominately observed in the northern hemisphere during times of large dipole tilt. At this time it is not known if the doubled spectrum of AKR is coming from the same source region, or from the same auroral field line or from other auroral field lines at other local times. There are currently no theories that can explain the generation of the observed doubled peaked spectrum of AKR from the same source region. Plasmaspheric measurements from RPI have been compared extensively with the Extreme Ultraviolet (EUV) Imager on IMAGE resulting in a number of new discoveries. For instance, these combined observations show that kilometric continuum is generated at the plasmapause, from sources in or very near the magnetic equator, within a bite-out region of the plasmasphere.The process by which plasmaspheric bite-outs structures are produced is not completely understood at this time. Finally, RPI has been used to successfully test the feasibility of magnetospheric tomography. During perigee passage of the Wind spacecraft, RPI radio transmissions at two frequencies have been observed by Waves instrument. Faraday rotation was measured and occurs when the received electric field is observed to rotate with time due to the changing density of plasma. Many future multi-spacecraft missions propose to use Faraday rotation to obtain global density pictures of the magnetosphere. _______________ To be presented at the 27th General Assembly of International Union of Radio Science (URSI), Maastricht, The Netherlands, August 17-24, 2002.