[Rastoin 1996 a]

Time and space variations of the galactic cosmic ray electron spectrum in the 3-D heliosphere explored by Ulysses.

C. Rastoin, P. Ferrando, A. Raviart, R. Ducros, P. O. Petrucci, C. Paizis, H. Kunow, R. Mueller-Mellin, H. Sierks and G. Wibberenz.
Time and space variations of the galactic cosmic ray electron spectrum in the 3-D heliosphere explored by Ulysses.
Astronomy and Astrophysics 307, 981-995. 1996

Contents

The Ulysses spacecraft, launched in October 1990 after solar maximum of solar cycle 22, has explored the heliographic latitudes above 70°S in summer 1994, and has then made a rapid latitude scan down to the heliographic equator reached in March 1995, in a period close to solar minimum. We present hereafter the galactic electron spectra, with energies above 0.3 GeV, from end of 1990 to the end of 1994, as measured by the COSPIN/KET instrument onboard the spacecraft.

The variations of the electron flux (combining time and space variations) are compared to those of the hadronic counterpart at similar rigidities, also measured by the KET, in order to investigate possible charge-sign effects of the modulation process. For rigidities between 0.9 and 3 GV/c, the electron/hadron ratio was found to decrease after launch up to the beginning of 1992 at 3 GV/c, the end of 1992 at 0.9 GV/c, i. e. long after the end of the magnetic field polarity reversal. From the end of 1992, when Ulysses was at a latitude of only 20°S, up to the end of the data presented here which comprise the south polar pass, the electron/hadron ratio stayed remarkably constant at all rigidities.

The absence of any charge-sign dependence of the modulation recovery as a function of latitude suggests a weak importance of large scale drifts in the modulation process during the Ulysses observation time. The recovery between end of 1990 and end of 1994, measured by the relative variations of the flux as a function of energy, is maximum around 1 GeV.

The electron spectrum shows a strong change of shape at low energy between beginning of 1991 and end of 1994: the 1991 flux increases continuously with decreasing energies, while the 1994 flux peaks at about 700 MeV. Whether this is due to the high latitude of Ulysses in 1994 or to a time dependent effect cannot be decided with our single spacecraft measurement. In the framework of the simple one dimensional modulation model, this change of spectral shape is interpreted as a change of the rigidity dependence of the radial diffusion coefficient in the heliosphere.

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