IMPULSIVE BURSTS OF ENERGETIC PARTICLES IN THE HIGH-LATITUDE DUSKSIDEMAGNETOSPHERE OF JUPITER

Recent studies using the entire set of proton and electron measurement s of the cosmic and solar particle investigations (COSPIN) on Ulysses revealed that the impulsive, sometimes quasi-periodic bursts discovere d during the Ulysses' pass through Jupiter's duskside magnetosphere co nsisted not only of relativistic electrons and radio wave emissions bu t also protons and helium nuclei of energies from similar to 0.7 to si milar to 10 MeV/nucleon. In this paper we present a detailed analysis of observations of the proton and helium bursts and of their correlati on with relativistic electron bursts. Using an automated computer algo rithm, we found 37 nucleonic bursts, all of which occurred in the dusk side magnetosphere. Typically, the nucleonic bursts lasted only simila r to 20 s to similar to 2 min and had a very rapid onset and shutoff, showing a spike-like profile. The flux during the bursts often increas ed by a factor > 10 above the background and had a very strong unidire ctional anisotropy directed along the magnetic fields outward from the southern polar region of the planet. The half width of the pitch angl e distributions for protons in the bursts was usually less than a few degrees. In the bursts, particles with highest energies were detected first, followed by lower-energy particles, consistent with the velocit y dispersion expected for particles arriving from a source at a signif icant distance. From this velocity dispersion relation and the O4CS ma gnetic field model we found that the source of the bursts was located a few Jovian radii above the surface over the southern high-latitude p olar region. Of the 37 nucleonic bursts, about half were associated wi th an electron burst originating at the same source and at the same ti me. Energy spectra for protons in the bursts often showed a peak at si milar to 1 MeV, and above this energy there was spectral hardening rel ative to preburst and postburst background spectra. These observations suggest that particle acceleration occurs in the high-latitude polar inner magnetosphere of Jupiter and may be similar to those phenomena o bserved in the auroral region of Earth's magnetosphere. If the same ac celeration process operates on both Earth and Jupiter, these observati ons imply that Jovian aurorae are probably associated with the high-la titude boundaries of the polar cap rather than with the Io torus.