Reappearance of recurrent low-energy particle events at Ulysses/HI-SCALE in the northern heliosphere

Abstract

In the high-latitude northern hemisphere of the heliosphere, the Heliosphere Instrument for Spectra Composition and Anisotropy at Low Energies (HI-SCALE) energetic particle detectors on Ulysses have measured quasi-recurrent ~26-day increases of 40-65 keV electrons during 11 solar rotations from October 1995 through July 1996. They do not appear on all rotations, but when they do, they are associated with increases in 0.5-1.0 MeV protons (preceding the electron increases by several days) and sometimes with decreases in galactic cosmic rays. The northern recurrences form two series shifted half a solar rotation with respect to each other, unlike the very regular and more intense series of 21 recurrences observed by the same instrument throughout the middle-to-high-latitude southern hemisphere from mid-1993 to the beginning of 1995. Correlated energetic particle measurements from IMP 8 at Earth and Voyagers 1 and 2 at 42-62 AU establish that recurrent events during this period were intrinsically stronger in the southern heliosphere than in the north. The variability of the northern recurrences is attributed, using a generalization of the model of Fisk [1996], to temporal changes during 1966 in the near-Sun polar magnetic field configuration. These changes would affect the connection of Ulysses via magnetic field lines to the corotating interaction regions at lower latitudes >10AU beyond the spacecraft, where the low-energy particles are accelerated and the galactic cosmic rays are modulated. The observed evolution of the northern polar coronal structure, as revealed in FeXIV (5303Å) synoptic maps and confirmed by FeXII (195Å) images from the Extreme Ultra-Violet Imaging Telescope on the SOHO spacecraft, is just that which is required, according to the model, to explain the evolution of the low-energy particle recurrences as observed by Ulysses/HI-SCALE in the northern heliosphere.