Ulysses Cosmic Ray and Solar Particle Investigation/Kiel Electron Telescope observations: Charge sign dependence and spatial gradients during the 1990–2000 A > 0 solar magnetic cycle

[1] Ulysses, launched in October 1990, began its second out- of- ecliptic orbit in September 1997. From 1991 to mid- 2000, solar polar magnetic field observations indicate a magnetic field pointing outward over the northern polar region (A > 0 epoch). While the first fast latitude scan in 1994/ 1995 was performed around solar minimum, the second Ulysses orbit covering high latitudes is being performed around solar maximum. We present data of similar to 2.5 GV electrons and protons from the Kiel Electron Telescope up to southern heliographic latitudes of 80degrees, reached by Ulysses in November 2000. Spatial gradients are determined by comparison with a near- Earth baseline. The radial gradient for an integral proton channel is found to be 2.2 +/- 0.6%/ AU before early 1998 and 3.5%/ AU thereafter. Until mid- 1999 the latitudinal variation is consistent with a zero gradient at low latitudes and an average gradient of about 0.25%/ degree for latitudes above about +/-25degrees. In contrast, the latitudinal gradient decreases markedly after mid- 1999, leading to a spherically symmetric intensity distribution with the approach to solar maximum conditions. The electron to proton (e/ p) ratio is used to study charge sign dependent modulation. According to drift- dominated modulation models, galactic cosmic ray protons and electrons respond differently to the global heliospheric magnetic field. Comparison of the e/ p ratio over the period 1991 to 2000 with model predictions indicates that drift effects are important over a large part of an 11- year period with A > 0. With the approach to solar maximum the e/ p ratio increases, indicative for the fading out of drift effects and the transition to a diffusion- dominated modulation.