Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.2225h&link_type=abstract
American Astronomical Society, DPS meeting #28, #22.25; Bulletin of the American Astronomical Society, Vol. 28, p.1146
Astronomy and Astrophysics
Astrophysics
Scientific paper
Radio occultation measurements with Pioneer 10 provided the first evidence for the multilayered structure of Jupiter's lower ionosphere (Fjeldbo et al., Astron. Astrophys. 39, 91-96, 1975). However, radio occultation studies of this region are inherently difficult for the following reason. Radio signals propagating from spacecraft to Earth can follow a variety of paths through the ionosphere, refracting off the top or bottom of the various layers, with several distinct signals arriving simultaneously at a terrestrial tracking station. This condition is known as "multipath" propagation. The presence of multiple signals, some of which are very weak and whose total number is unknown a priori, presents a major challenge to basic signal detection and frequency estimation. For this reason profiles of electron number density retrieved from Voyager radio occultation data were never extended downward through the region where layers were evident in the raw data (Eshleman et al., Science 204, 976-978, 1979; Science 206, 959-962, 1979). Motivated by the acquisition of new radio occultation data with Galileo, we have developed a new method of data analysis that can remove the complications caused by multipath propagation. It is analogous to the Fresnel filter developed by Marouf et al. (Icarus 68, 120-166, 1986) to remove diffraction effects from radio occultation profiles of Saturn's rings. The basic idea is to represent the complex signal (i.e., amplitude and phase) received at the terrestrial tracking station as a spectrum of plane electromagnetic waves. Each plane-wave component of the spectrum is then propagated backward (through vacuum) toward Jupiter. The complex field can be reconstructed at an arbitrary location through superposition of the various back-projected Fourier components. When the field is reconstructed at a location close to Jupiter, ray crossing has not yet occurred, so that profiles of electron number density can be retrieved from the back-projected data using standard techniques. We will show new results concerning narrow layers in Jupiter's lower ionosphere obtained by applying this formalism to radio occultation data from Voyager and Galileo.
Hinson David P.
Karayel Tuna E.
Twicken Joseph D.
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