Solar modulation of galactic cosmic rays: the 3D heliosphere

Abstract

The cosmic ray flux observed with the Kiel Electron Telescope onboard the Ulysses space probe varies with solar activity as well as with heliospheric position. The Ulysses' fast latitude scan performed in less than one year between September 1994 to August 1995 allowed us to obtain unique data about the latitudinal dependence of cosmic ray fluxes close to solar minimum conditions. In this paper we discuss the flux variation of protons, alpha-particles and electrons in the few GV rigidity range, and compare our measurements with predictions from 2-dimensional drift dominated modulation models. We focus our interest on the energy dependence of the latitudinal gradient of the nuclei component, the North-South distribution of > 106 MeV protons and charge dependence of 2.5 GV particles during the fast latitude scan. Whereas the latitudinal gradient for alpha-particles monotonically decreases with increasing energy, protons show a maximum latitudinal gradient at several hundred MeV. This maximum was not expected to exist, but can be explained in modified modulation models by increasing the diffusion perpendicular to the magnetic field in polar direction. The counting rate of > 106 MeV protons in the northern and southern hemisphere is not symmetric with respect to the heliographic equator but with respect to ``magnetic'' latitude. The e/p-ratio of 2.5 GV particles shows that electrons and protons have similar temporal modulation and that the latitudinal dependence of this ratio is dominated by the latitudinal proton variation. The modified model can qualitatively reproduce the observed behaviour during the fast latitude scan.