Spatial variation of >40 MeV/n nuclei fluxes observed during the Ulysses rapid latitude scan

Abstract

In this paper we present fluxes of galactic cosmic ray nuclei with energies above 40 MeV/n observed by the Kiel Electron Telescope on board the Ulysses spacecraft during the fast scan from the South Pole (September 1994) to the North Pole (July 1995). This part of the Ulysses orbit gives us the unique opportunity to investigate spatial modulation of cosmic rays under solar minimum conditions. We show that during this time period temporal variations are well ordered with particle rigidity using 1 AU data from the University of Chicago particle instrument on board IMP8. The latitudinal variation is partic- ularly strong for 38-125 MeV/n helium nuclei and decreases with increasing energy. In contrast, the spatial variation for pro- tons is most dominant in the energy range of a few hundred MeV. Spatial effects are very small for some ten to one hundred MeV protons. The proton results are compared with predictions of a steady-state modulation model which takes into account modifications of the large scale heliospheric magnetic field and an increased level of cosmic ray scattering over the poles. We find that the model is in excellent agreement with the latitudinal variations of >2 GeV protons, whereas it does not reproduce the behaviour at low rigidities. We conclude that either the as- sumed rigidity dependence for the diffusion coefficients at low energies has to be modified or that time-dependent modulation effects dominate at low energies.