A Solar Polar North-South Asymmetry for Cosmic-Ray Propagation in the Heliosphere: The ULYSSES Pole-to-Pole Rapid Transit

Abstract

The pole-to-pole fast transit of the heliosphere at ~2 AU from the Sun by the international Ulysses spacecraft has made it possible for the first time to investigate in three dimensions the solar wind, interplanetary magnetic field, and---as reported herein---the global propagation of Galactic cosmic rays and anomalous helium. We use measurements of cosmic-ray protons with a mean energy ~2 GeV and helium component fluxes in the energy range 30--70 MeV per nucleon to illustrate new phenomena of charged particle propagation through the heliosphere. The latitude gradients are the same in both hemispheres---namely, ~0.3% deg-1 for cosmic rays, and ~0.7% deg-1 for anomalous helium. It was a surprise to discover that, although the heliospheric modulation of these components is remarkably symmetric globally, the plane of symmetry is offset southward from the Sun's heliographic equator by ~10 deg of latitude and that the fluxes over the north pole exceed those at the south pole by ~6%--15%. Although unexplained at present, the implications of this offset are discussed with respect to interplanetary magnetic field and solar wind observations.