Remote Radio Sounding Science for JIMO 

J. L. Green (1), B. W. Reinisch (2), P. Song (2), S. F. Fung (1), R. F. Benson (1), 
W. W. L. Taylor (1), J. F. Cooper (1), L. Garcia (1), and D. Gallagher (3)

(1) NASA/GSFC Code 630, Greenbelt, MD 20771, 1st Author email: James.Green@nasa.gov 
(2) Center for Atmospheric Research, University of Mass. Lowell, Lowell, MA 01854 
(3) NASA/MSFC, Huntsville, AL, 35812

Introduction: Radio sounding of the EarthÕs top-side ionosphere and magnetosphere is
a proven tech-nique from geospace missions such as the International Satellites for
Ionospheric Studies (ISIS) and the Imager for Magnetopause-to-Aurora Global
Exploration (IMAGE). Application of this technique to the Jupiter Icy Moons Orbiter
(JIMO) mission will provide unique remote sensing observations of the plasma and
mag-netic field environments, and the subsurface conduc-tivities, of Europa,
Ganymede, and Callisto. Spatial structures of ionospheric plasma above the moon
sur-faces vary in response to magnetic field perturbations from (1) magnetospheric
plasma flows, (2) ionospheric currents from ionization of sputtered surface
material, and (3) induced electric currents in salty subsurface oceans. Radio
sounding at 3 kHz to 10 MHz can pro-vide globally-determined electron densities
necessary for the extraction of the oceanic current signals and supplements in-situ
plasma and magnetic field meas-urements. Subsurface variations in conductivity, can
be investigated by radio sounding from 10 MHz to 40 MHz allowing the determination
of the presence of dense and solid-liquid phase boundaries associated with oceans
and related structures in overlying ice crusts. 

Magnetosphere-Ionosphere Science: From the Galileo results it is clear that a wide
variety of magne-tospheric interactions occur for each of the moons since they are
located in different regions of the Jovian magnetosphere and have time-varying
positions with respect to the local plasma sheet during Jupiter's rota-tion and
their orbital motion around Jupiter. Magneto-spheric plasma ions sputter atoms of
various composi-tion from the moon surfaces and produce extremely thin (~10^15/cm2)
atmospheres. Ionization of the atmos-pheric neutrals by solar UV photons, and by
interac-tions with energetic magnetospheric ions and electrons, creates the local
ionospheric plasma environment. Complex magnetic field and plasma structures (e.g.,
Alfven wings, polar aurora) arise from magnetosphere-ionosphere interactions.
Field-aligned currents connect the ionosphere of Europa and magnetosphere of
Ga-nymede to visibly glowing auroral spots in Jupiter's upper atmosphere. In
addition, GanymedeÕs magneto-sphere generates its own non-thermal continuum
ra-diation. Existing plasma and magnetic field data sets from many Galileo orbiter
flybys of the moons are still not complete enough to answer many of the questions
concerning these interactions. 

Long-range magnetospheric sounding, pioneered by the radio plasma imager (RPI)
instrument on IMAGE, has provided electron density distributions along magnetic
field lines and in radial directions on time scales of minutes [1]. RPI has also
been able to measure the entire electron plasma density distribu-tions (in the orbit
plane) of the EarthÕs polar cap and the plasmasphere within one pass of the
spacecraft. These results have enabled the testing of theories on how the EarthÕs
ionosphere reacts to magnetospheric changes under a variety of geomagnetic storm
condi-tions [2]. In a similar manner, a radio sounder orbiting the icy moons would
be able to measure the electron density along the magnetic field into each
hemisphere and provide information on the Jovian magnetospheric background and
influence on the moonÕs ionospheres. Extension of JIMO to Io would illuminate an
even more complex environment.

Subsurface Science: Radio wave sounding will add important new capabilities to JIMO
for geophysi-cal remote sensing of icy moon crusts and oceans. The radio sounder on
ISIS showed that at frequencies higher than the peak ionospheric plasma frequency,
radio sounder echoes could be used for subsurface mapping [3]. Layering in the
surfaces of the icy moons produce changes in permittivity leading to reflection of
radio waves incident normal to the layering structure. A large variety of rocks and
soil along with water have been investigated on Earth for a range of permittivity
and absorption at different radio sounding frequencies. Most of these measurements
have shown that absorp-tion of radio waves increases with frequency in the range
from one to a few hundred MHz. Typically, greater depths can be probed at lower
frequencies. The performance of a ground-penetrating radar is generally expressed in
terms of its "radar potential" which is the ratio between transmitted power and the
smallest de-tectable signal. Synthetic aperture radar (SAR) and optimized coherent
detection techniques, which have already been developed, may be required to achieve
acceptable radar potential for probing icy moon sub-surface regions. The higher
power source available from JIMO would allow radio sounding transmissions at much
higher powers than those used on ISIS or IMAGE making subsurface sounding of the
Jovian icy moons possible. Key science objectives include global measurements of ice
crust conductivity, boundary lay-ers for subsurface geologic structures (e.g.,
linear and cycloidal cracks, diapiric plumes, buried impact craters, and
small-scale brine cavities), and the search for crust-ocean boundaries.

References: [1] Reinisch, B. W., et al., (2001) Geophys. Res. Lett., 28, 1167-1170.
[2] Green, J.L. and B.W. Reinisch, (2003) Space Sci. Rev., in press. [3] Hagg, E.
L., et al., (1969) Proc. IEEE, 57, 6, 949-960, 1969.

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Presented at the Jupiter Icy Moons Orbiter Community Science Workshop  
Houston, Texas, June, 2003