Jupiter Science Results - Radio and Plasma Waves

Jupiter is a prolific source of natural radio waves, emitting at many wavelengths. The unique direction-finding capability and high sensitivity of the Ulysses radio and plasma wave (URAP) experiment have provided new insights and clues as to the origin of these radio signals (Fig.5). For example, the so-called "narrow-band kilometric" (nKOM) radiation has been found to originate from discrete, long-lived sources that are located in the outer regions of the Io Plasma Torus, and which rotate around Jupiter at slightly different rates. Ulysses observations of the hectometric radiation (HOM) revealed narrow latitudinal beaming along the magnetic equator, and provided additional constraints on existing models for the source of this radio emission. Several bursts of radio emission showing a characteristic rapid drift in frequency, so-called "Jovian type III" events were detected with Voyager. With Ulysses, many events of this type have been recorded and they appear to be a major component of Jupiter's radio spectrum.

Figure 5: Overview of URAP radio and plasma wave data during the flyby displayed as frequency vs. time dynamic spectra, with relative intensity indicated by the colour bar on the right. (A): 16 day overview centred on Closest Approach (CA, day 039). (B): two typical Jovian rotations before closest approach beginning at 05:00 SCET on 5 Feb. (C): 24 hour period centred on CA including passage through the Io Plasma Torus. (D): 24 hour period on the day of the inbound bow shock and magnetopause crossing. (E): 24 hour period containing some of the outbound magnetopause crossings.

At the lowest frequencies, the Jovian continuum emission has been observed by URAP at large distances from the planet. Both the frequency range and intensity of the continuum have been seen to vary with solar wind ram pressure, thus providing a unique long-term remote monitor of solar wind conditions at Jupiter.