Periodic signals in Ulysses' energetic particle events upstream and downstream from the Jovian bow shock

Abstract

We present results from a statistical analysis of energetic particle events, observed by the Heliosphere instrument for spectra, composition, and anisotrophy at low energies (HI-SCALE) instrument onboard Ulysses, upstream from the bow shock of Jupiter, during the inbound and outbound trajectory of the spacecraft. A harmonic analysis on the intensity time series of 192 distinct 61- 77 keV upstream ion events, suggests a modulation with a period of ∼40 min or ∼15- 20 min in 48% of the total number of events, at the significance level of P=0.05. In some cases, the periodicity is not significant in the ion intensities, while it is significant in the ion anisotropy and/or spectral data. Our analysis shows that the periodic modulation was more frequently observed on the out-of-ecliptic trajectory (67 events), than on the ecliptic trajectory (26 events). An ∼40/15- 20 min periodicity was also found in energetic ion and electron events in the magnetosheath. The results from the statistical analysis of HI-SCALE measurements are consistent with a magnetospheric source for the majority of the upstream events. The statistical results are also consistent with preferential leakage of energetic ions from the south high-latitude Jovian magnetosphere.