TIME AND SPACE VARIATIONS OF THE GALACTIC COSMIC-RAY ELECTRON SPECTRUM IN THE 3-D HELIOSPHERE EXPLORED BY ULYSSES

The Ulysses spacecraft, launched in Oct. 90 after solar maximum of sol ar cycle 22, has explored the heliographic latitudes above 70 degrees S in summer 94, and has then made a rapid latitude scan down to the he liographic equator reached in March 95, in a period close to solar min imum. We present hereafter the Galactic electron spectra, with energie s above 0.3 GeV, from end of 90 to the end of 94, as measured by the C OSPIN/KET instrument onboard the spacecraft. The variations of the ele ctron flux (combining time and space variations) are compared to those of the hadronic counterpart at similar rigidities, also measured by t he KET, in order to investigate possible charge-sign effects of the mo dulation process. For rigidities between 0.9 and 3 GV, the electron/ha dron ratio was found to decrease after launch up to the beginning of 9 2 at 3 GV, the end of 92 at 0.9 GV, i.e. long after the end of the mag netic field polarity reversal. From the end of 92, when Ulysses was at a latitude of only 20 degrees S, up to the end of the data presented here which comprise the south polar pass, the electron/hadron ratio st ayed remarkably constant at all rigidities. The absence of any charge- sign dependence of the modulation recovery as a function of latitude s uggests a weak importance of large scale drifts in the modulation proc ess during the Ulysses observation time. The recovery between end of 9 0 and end of 94, measured by the relative variations of the flux as a function of energy, is maximum around 1 GeV. The electron spectrum sho ws a strong change of shape at low energy between beginning of 91 and end of 94: the 91 flux increases continuously with decreasing energies , while the 94 flux peaks at about 700 MeV. Whether this is due to the high latitude of Ulysses in 94 or to a time dependent effect cannot b e decided with our single spacecraft measurement. In the framework of the simple one dimensional modulation model, this change of spectral s hape is interpreted as a change of the rigidity dependence of the radi al diffusion coefficient in the heliosphere.