Green, J. L., and S. F. Fung, Magnetospheric Radio Imaging, presented
at the URSI North American Radio Science Meeting, Montreal, Canada,
July 13-18. 1997.

MAGNETOSPHERIC RADIO IMAGING

James L. Green and Shing F. Fung, NASA/Goddard Space Flight
Center Greenbelt, MD  20771

NASA has recently selected a new mission called the Imager for
Magnetopause-to-Aurora Global Exploration (IMAGE) as the first
mid-sized explorer in magnetospheric physics to be launched in
January 2000. The IMAGE mission will have an impressive array of
remote sensing instruments which will image a number of
important phenomena such as the auroral zone, the geocorona, the
ring current, the plasmasphere, auroral ion fountain, and the
magnetopause on a time scale of minutes.  IMAGE will be placed in a
polar orbit with apogee altitude of 7 Earth radii (RE) where it will be
well situated to observe the structure and dynamics of the
magnetospheric boundaries during geomagnetic storms.

One of the key instruments on IMAGE is the Radio Plasma Imager or
RPI. This instrument utilizes many of the recent advances in radio
transmitter and receiver design, and modern digital processing
techniques that have been perfected for ionospheric sounding. Like
ionospheric sounding, free-space electromagnetic waves, launched
within a lower density region will reflect at the plasma cutoffs. The
location and characteristics of the plasma at a remote reflection point
can then be derived from measurements of the delay time, frequency,
and direction of an echo. The characteristic of large scale ionospheric
disturbances can be displayed in skymaps, Doppler maps, and other
useful images from the hundreds of echoes which are detected by this
technique. In an analogous way, similar images of the magnetosphere
at radio frequencies could be obtained RPI. The RPI will be operating
at frequencies between 3 kHz to 3 MHz and will provide quantitative
electron density profiles simultaneously in several different
directions on a time scale of minutes or less. The IMAGE orbit is well
situated in the magnetospheric density cavity, providing an excellent
opportunity for RPI to observe the structure and dynamics of many
different magnetospheric boundaries at the same time. Simulations
of these observations have been done utilizing RPI characteristics and
extensive ray tracing calculations within a three-dimensional
magnetospheric model.

This paper will review the techniques used in magnetospheric radio
imaging as will be flown on the IMAGE spacecraft . In addition, a
variety of simulations will also be presented to illustrate many of the
expected radio imaging results. For more information on the IMAGE
mission and on the RPI via the world wide web see:
http://image.gsfc.nasa.gov/.