Synoptic observations of Jupiter's radio emissions: Average statistical properties observed by Voyager

Abstract

Observations of Jupiter's low‐frequency radio emissions collected over one‐month intervals before and after each Voyager encounter have been analyzed to provide a synoptic view of the average statistical properties of the emissions. Compilations of occurrence probability, average power flux density, and average sense of circular polarization are presented as a function of central meridian longitude, phase of Io, and frequency. The results are compared with ground‐based observations. The necessary geometrical conditions and preferred polarization sense for Io‐related decametric emission observed by Voyager from above both the dayside and nightside hemispheres are found to be essentially the same as those observed in earth‐based studies. On the other hand, there is a clear local time dependence in the Io‐independent decametric emission. The emission is prevalent at longitudes >200° when observed from over the dayside hemisphere but is dominant at longitudes <200° when observe from over the postmidnight sector. Decametric emission, which comprises the dynamic spectral lesser arcs near 10 MHz, displays a distinct, bimodal polarization pattern that is predominantly in the left‐hand sense at longitudes below 150° and in the right‐hand sense at longitudes above 150°. The central meridian longitude distributions of occurrence probability and average flux density at hectometric wavelengths appear to depend significantly on both the observer's latitude and local time. Io appears to have an influence on average flux density of the emission down to below 2 MHz. The average power flux density sectrum of Jupiter's emission has a broad peak near 9 MHz. Integration of the average spectrum over all frequencies and all longitudes gives a total radiated power for an equivalent isotropic source of 4 × 1011 W.