Jovian radio “bullseyes” observed by Ulysses

Abstract

We report the observation of extremely rare semi-bullseye or target-like features in the dynamic spectra of Jupiter obtained in 1991-1994 by the Ulysses Unified Radio and Plasma wave instrument. The emissions lie in the 20-50 kHz band and consist of narrow band (∼1-2 kHz) semi-concentric wisps on frequency-time dynamic spectra. The individual wisps correspond to a 4π steradian radiated power in the range of tens of kilowatts to more than ten megawatts. The emissions show a tendency to occur in groups every 25-27 days with a spacing between some individual episodes of about 10.5 hours. The 25-27 day periodicity is strongly correlated with solar wind ram pressure at Jupiter. Speculation on the cause of these events includes constructive-destructive interference or diffraction of a previously known radio source viewed through the Io plasma torus, or perhaps radio emission from Ganymede or from the magnetic flux tube threading through Ganymede.