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.