A Solar Polar North-South Asymmetry for Cosmic-Ray Propagation in the Heliosphere: The [ITAL]Ulysses[/ITAL] Pole-to-Pole Rapid Transit

Open Access
Publisher Website
Google Scholar
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° 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.