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Astron. Astrophys. 344, 1001-1013 (1999)


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Interplanetary scattering effects in the jovian bKOM radio emission observed by Ulysses

C.H. Barrow 1, G. Woan 2 and R.J. MacDowall 3

1 Max-Planck-Institut für Aeronomie, D-37189 Katlenburg-Lindau, Germany
2 Department of Physics and Astronomy, University of Glasgow, UK
3 NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA

Received 14 May 1998 / Accepted 4 January 1999

Abstract

The scattering of radio waves by the interplanetary medium (subsequently referred to simply as `scattering') imposes limitations on measurements of fine temporal and angular structure at low radio frequencies. The jovian broad-band kilometric radiation (bKOM) is highly structured and observations of this emission, by the Unified Radio and Plasma Experiment (URAP) on board ULYSSES, have been used to study scattering at extremely low frequencies. Structures observed in the bKOM during 1995-6, when the spacecraft was at distances greater than 5 AU from the planet, are compared with observations made in 1991 when the spacecraft was relatively close to Jupiter ([FORMULA] AU). Measured values of the e-folding time are compared with theoretical values of temporal broadening and with solar wind electron density measurements made by the SWOOPS experiment on Ulysses. In general, we find that temporal broadening increases with source-observer distance and decreasing frequency, as expected from standard broadening theory, although a few inordinately low values were also observed at distances of 5 to 7 AU. At frequencies close to 45 kHz, an upper limit of about 8 min is clearly visible in observations of the temporal broadening made about 1 AU from Jupiter. Average values of the e-folding time at these same frequencies show a well-defined increase with distance from Jupiter of about 8 min AU-1. We interpret this as `saturated scattering,' where all ray paths between source and observer are explored, giving an upper limit to the temporal broadening which the medium can introduce. These observations establish limits for scattering in the interplanetary medium (IPM), which are relevant to all low frequency radio observations.

Key words: inteplanetary medium – planets and satellites: individual: Jupiter – radio continuum: solar system

Send offprint requests to: C.H. Barrow

© European Southern Observatory (ESO) 1999

Online publication: March 29, 1999

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