Latitudinal gradients and charge sign dependent modulation of galactic cosmic rays

Abstract

Ulysses, launched in October 1990, began its second out-of-ecliptic orbit in September 1997 and its second fast latitude scan in November 2000. At the time of the submission (May 2001) the spacecraft was located close to the heliographic equator at a radial distance of 1.3 AU. In contrast to the first orbit with the Sun declining to low activity, we are now at solar maximum conditions. The latitudinal gradient (∼0.3 %/degree for >100 MeV protons) as well as the charge sign dependent variation of 2.5 GV protons and electrons observed during the previous Ulysses solar minimum orbit can be understood in terms of modulation models taking into account larger perpendicular diffusion to the mean heliospheric magnetic than previously thought. In this paper we present Ulysses COsmic ray and Solar Particle INvestigation Kiel Electron Telescope data at high southern heliographic latitudes and during the first period of the fast latitude scan investigating temporal and spatial modulation around solar maximum and during the solar magnetic field reconfiguration. In contrast to the observation at solar minimum Ulysses observes no or only small latitudinal gradients for galactic cosmic ray protons. While the electron to proton-ratio during the 1994/1995 latitude scan reflected clearly the proton latitudinal distribution, only small variation were found until May 2001 for the current fast latitude scan, hinting that drift is again beginning to play a role in galactic cosmic ray modulation.