Latitudinal and radial gradients of galactic cosmic ray protons in the inner heliosphere – PAMELA and Ulysses observations
22 Sep 2011
1INFN, Structure of Rome "Tor Vergata" and Physics Department of University of Rome "Tor Vergata", Via della Ricerca Scientifica 1, I-00133 Rome, Italy
2INFN, Structure of Trieste and Physics Department of University of Trieste, I-34147 Trieste, Italy
3Inst. für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität Kiel, Leibnizstr. 11, 24118 Kiel, Germany
Abstract. Ulysses, launched on 6 October 1990, was placed in an elliptical, high inclined (80.2°) orbit around the Sun, and was switched off in June 2009. It has been the only spacecraft exploring high-latitude regions of the inner heliosphere. The Kiel Electron Telescope (KET) aboard Ulysses measures electrons from 3 MeV to a few GeV and protons and helium in the energy range from 6 MeV/nucleon to above 2 GeV/nucleon. The PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) space borne experiment was launched on 15 June 2006 and is continuously collecting data since then. The apparatus measures electrons, positrons, protons, anti-protons and heavier nuclei from about 100 MeV to several hundreds of GeV. Thus the combination of Ulysses and PAMELA measurements is ideally suited to determine the spatial gradients during the extended minimum of solar cycle 23. For protons in the rigidity interval 1.6–1.8 GV we find a radial gradient of 2.7%/AU and a latitudinal gradient of −0.024%/degree. Although the latitudinal gradient is as expected negative, its value is much smaller than predicted by current particle propagation models. This result is of relevance for the study of propagation parameters in the inner heliosphere.