Observations of energetic particles with EPAC on Ulysses in polar latitudes of the heliosphere.

Abstract

Measurements with the EPAC energetic-charged-particle instrument aboard Ulysses show between -15° and -65° ions and, to some extent, also electrons apparently accelerated by shocks associated with a corotating interaction region (CIR) operating at low latitudes. Particles could have reached Ulysses along magnetic-field lines which connect to the shocks in the more distant heliosphere. Such connections evidently do not exist above -65°. Between the recurrent streams the authors find the underlying composition to be similar to that of the anomalous component of cosmic rays (ACR). One channel, sensitive also to high-energy protons, shows that, superimposed to the large-scale heliospheric modulation of galactic ions, a 26-day variation of the flux is observed. Such modulation is also observed for the ACR, in phase with the galactic particle modulation, but anticorrelated to the CIR-related low-energy particles. An estimate of the latitudinal and radial gradients of the galactic cosmic rays at 1 GV gives +0.4%/degree and -11%/AU, respectively.