Radio-Frequency Sounders in Space

Robert. F. Benson
Laboratory for Extraterrestrial Physics
NASA/Goddard Space Flight Center
Greenbelt, MD 20771, USA
Phone: (301) 286-4037, Fax: (301) 286-1683
e.mail: u2rfb@lepvax.gsfc.nasa.gov


Radio-frequency sounders in space are most often referred to as
topside sounders because their original purpose was to investigate
the global distribution of electron density in the ionosphere at
altitudes above the F layer peak density.  They have been used
predominately for four types of investigations: (1) local and remote
topside electron densities and electron density gradients, (2) plasma
wave propagation, (3) natural emissions and (4) stimulated plasma
emissions.  This paper will give a brief overview of some of the
results that have been obtained in these areas from investigations
of ionograms as recorded on 35 mm film from the Alouette/ISIS program
(International Satellites for Ionospheric Studies). Topics will 
include (1) electron density contours and spread F, (2) electrostatic
plasma waves near resonant frequencies in the ambient plasma,
(3) auroral kilometric radiation (AKR) and (4) stimulated emissions
at frequencies below the electron plasma frequency (known as diffuse
resonances).

The lifetimes of the Alouette/ISIS satellites span approximately
60 satellite-years of pole-to-pole operation. Unfortunately, not
all of the sounder data have been extracted from the original
telemetry tapes.  In particular, only about 15% of the ISIS 1
and 2 tapes have been processed to produce topside-sounder
ionograms on 35 mm film.  This paper will briefly describe an
effort underway at the Goddard Space Flight Center to transform
selected samples of several decades of ISIS 1 and 2 ionospheric
topside sounder data to digital ionograms.  The digital ionograms
are being archived at the National Space Science Data Center (NSSDC)
at Goddard for computer-retrieval via the internet (see the NSSDC
ISIS WWW home page at http://nssdc.gsfc.nasa.gov/space/isis/isis-status.html).

Some results from the recent OEDIPUS C rocket bistatic wave
propagation experiment will be discussed along with plans to
orbit a radio sounder called the Radio Plasma Imager (RPI) on
the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE)
which was selected as NASA's first Medium Class Explorer (MIDEX)
satellite. IMAGE is scheduled for launch in January, 2000.