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

Abstract

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 ∼2.5 GV electrons and protons from the Kiel Electron Telescope up to southern heliographic latitudes of 80°, 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 ±25°. 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.