Impulsive bursts of energetic particles in the high‐latitude duskside magnetosphere of Jupiter

Abstract

Recent studies using the entire set of proton and electron measurements of the cosmic and solar particle investigations (COSPIN) on Ulysses revealed that the impulsive, sometimes quasi‐periodic bursts discovered during the Ulysses' pass through Jupiter's duskside magnetosphere consisted not only of relativistic electrons and radio wave emissions but also protons and helium nuclei of energies from ∼0.7 to ∼10 MeV/nucleon. In this paper we present a detailed analysis of observations of the proton and helium bursts and of their correlation with relativistic electron bursts. Using an automated computer algorithm, we found 37 nucleonic bursts, all of which occurred in the duskside magnetosphere. Typically, the nucleonic bursts lasted only ∼20 s to ∼2 min and had a very rapid onset and shutoff, showing a spike‐like profile. The flux during the bursts often increased by a factor >10 above the background and had a very strong unidirectional anisotropy directed along the magnetic fields outward from the southern polar region of the planet. The half width of the pitch angle 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 velocity dispersion expected for particles arriving from a source at a significant distance. From this velocity dispersion relation and the O4CS magnetic field model we found that the source of the bursts was located a few Jovian radii above the surface over the southern high‐latitude polar region. Of the 37 nucleonic bursts, about half were associated with an electron burst originating at the same source and at the same time. Energy spectra for protons in the bursts often showed a peak at ∼1 MeV, and above this energy there was spectral hardening relative 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 observed in the auroral region of Earth's magnetosphere. If the same acceleration process operates on both Earth and Jupiter, these observations imply that Jovian aurorae are probably associated with the high‐latitude boundaries of the polar cap rather than with the Io torus.