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

The scattering of radio waves by the interplanetary medium (subsequently re ferred to simply as 'scattering') imposes limitations on measurements of fi ne temporal and angular structure at low radio frequencies. The jovian broa d-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. S tructures observed in the bKOM during 1995-6, when the spacecraft was at di stances greater than 5 AU from the planet, are compared with observations m ade in 1991 when the spacecraft was relatively close to Jupiter (similar to 1 AU). Measured values of the e-folding time are compared with theoretical values of temporal broadening and with solar wind electron density measure ments made by the SWOOPS experiment on Ulysses. In general, we find that te mporal broadening increases with source-observer distance and decreasing fr equency, as expected from standard broadening theory, although a few inordi nately low values were also observed at distances of 5 to 7 AU. At frequenc ies close to 45 kHz, an upper limit of about 8 min is clearly visible in ob servations 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 th is as 'saturated scattering,' where all ray paths between source and observ er are explored, giving an,upper limit to the temporal broadening which the medium can introduce. These observations establish limits for scattering i n the interplanetary medium (IPM), which are relevant to all low frequency radio observations.