The high latitude pass of ULYSSES: energetic particle observations with EPAC.

Abstract

We report on observations of energetic particles (EP) during the ascent of ULYSSES to the south pole, on the fast pass from south to north and in the first few months after the north polar pass. After it left the ecliptic plane the spacecraft moved periodically into a corotating (26 days period) fast solar wind stream and observed the accelerated EP associated with the forward and reverse shocks. Such particles were observed up to 68° in heliographic latitude. Only when ULYSSES returned to lower latitudes did the accelerated beams reappear. The fast pass from south to north polar regions showed on a compressed timescale similar features to those already observed on the ascending part. Apparently the recurrence period of the EP did not change throughout that pass. No evidence for differential rotation could be found in our data. The north polar pass again did not show energetic particles to be present at such high latitudes. In regions where the intensities were low, we identified from the C/O ratio and oxygen spectrum an underlying population of anomalous cosmic rays (ACR), which show a bidirectional field aligned anisotropy. The appearance of the EPs was anticorrelated to the modulated ACR and GCR fluxes. We conclude that ACR and GCR were modulated by the corotating interaction regions (CIR); shocks associated with them accelerated the observed EP streams. During the fast polar pass the latitudinal gradient of GCR protons was determined to be 0.3 to 0.4%/degree.