Note on the beaming of Jupiter's decameter-wave radiation and its effect on radio rotation period determinations

Comparisons of dynamic spectra of Jupiter's decameter wave radiation for events recorded up to 12 years apart display a high degree of repeatability in that certain spectral features occur at the same central meridian longitude to within + 10. The variation in the longitude of the spectral landmarks, which is attributable to a +10 beaming envelope into which the radiation escapes, places a constraint on the precision to which radio rotation-period determinations can be made from measurement of the apparent longitude drift of dynamic spectra. Subject headings: Jupiter - radio radiation, planetary - spectra, radio