Modulation recovery of high energy electrons: ULYSSES observations

Abstract

The ULYSSES spacecraft, launched in Oct 1990 near solar maximum, continues its investigation journey toward the southern polar pass during the decaying phase of solar cycle 22. One of the primary aim of its mission, namely the study of the cosmic-ray solar modulation in the three-dimensional heliosphere, has really started when exploring high lattitudes. In particular, time variations of the cosmic-ray electron intensity and spectrum constitute a crucial test for any modulation theory. We present hereafter the observations of the COSPIN/KET Experiment and we focus on the electron observations above 300 MeV, from launch to mid June 1994, when Ulysses came close to 70 degS. The recovery rates of the Galactic electron flux are studied with reference to that of protons and alphas simultaneously measured by KET above 380 MV. The modulation recovery shows step-like features and the rigidity dependence of the recovery rate is reported with no evidence of charge-dependent effects. During the whole observation time, the relative modulation amplitude of the Galactic electrons peaks between 0.5 and 2.0 GeV.