Automated Technique to Determine the Magnetospheric Electron Density from Passive Dynamic Spectra Phillip A. Webb Nomad Research, Inc., Code 690, NASA/GSFC, Greenbelt, MD 20771 pwebb@lepvax.gsfc.nasa.gov (301) 286 6172 Robert F. Benson Code 692, NASA/GSFC, Greenbelt, MD 20771 Robert.F.Benson@nasa.gov Richard E. Denton Department of Physics, Dartmouth College, Hanover, NH 03755 richard.e.denton@dartmouth.edu James L. Green Code 630, NASA/GSFC, Greenbelt, MD 20771 james.l.green@gsfc.nasa.gov Leonard. Garcia Code 630, QSS/GSFC, Greenbelt, MD 20771 garcia@mail630.gsfc.nasa.gov Bodo W. Reinisch University of Massachusetts at Lowell, Lowell, MA 01854 bodo_reinisch@uml.edu The magnetospheric electron density (Ne) is a fundamental space-physics parameter. It is often difficult to make Ne measurements to an accuracy better than a factor of two in low-density (Ne Å 1 cm-3 or less) space plasmas due to problems associated with spacecraft/plasma interactions which become enhanced in such an environment. Passive or active plasma-wave techniques are the experiments of choice under such conditions because they are based on the reception of plasma waves that sample a region of space around the spacecraft that is large compared to the perturbed spacecraft/plasma interaction region. Also, they are not restricted to energetic electrons as are particle detectors. The frequency of the amplitude-modulated signal received during passive observations is usually displayed as a function of time to form a dynamic spectrum. An automated fitting technique has been developed to extract Ne from dynamic spectra obtained from the Radio Plasma Imager (RPI) on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. This technique is often able to provide in-situ Ne measurements along extended portions of the orbit of IMAGE. While this automatic fitting technique will be demonstrated with RPI data, it promises to have valuable applications to other data sets from planetary magnetospheres. _______________ Presentation, Fall A.G.U. Meeting, San Francisco, CA, 13-17 December 2004