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Astron. Astrophys. 316, 528-537 (1996)

Latitude variations of ~7 MeV and >300 MeV cosmic ray electron fluxes in the heliosphere: Ulysses COSPIN/KET results and implications

P. Ferrando1, A. Raviart1, L.J. Haasbroek2, M.S. Potgieter2, W. Dröge3, B. Heber3, H. Kunow3, R. Müller-Mellin3, H. Sierks3, G. Wibberenz3, and C. Paizis4

1 CEA, DSM/DAPNIA/Service d'Astrophysique, CE-Saclay, F-91191 Gif-sur-Yvette Cedex, France
2 Space Research Unit, Potchefstroom University, 2520 Potchefstroom, South-Africa
3 Institut für Kernphysik, Universität Kiel, D-24118 Kiel, Germany
4 Istituto di Fisica Cosmica, Universitá di Milano, I-20133 Milano, Italy

Received 23 February 1996 / Accepted 19 April 1996

Abstract. The 160o latitude scan performed in less than one year between summer 1994 and summer 1995 by Ulysses allowed us to obtain unique data about the latitudinal dependence of the cosmic ray fluxes. We present in this paper results on the electron component derived from the COSPIN/KET instrument onboard Ulysses, both at MeV and GeV energies.

The variations of the 1 to 7 GV electron fluxes do not show any feature which could be attributed to a latitude dependence. This observation deviates from the variations of the proton fluxes of similar rigidities. The electron/proton ratio is shown to depend unambiguously on latitude during the fast latitude scan of Ulysses. This latitudinal dependence can in fact be traced back in our data to at least mid 1993 when Ulysses left the streamer belt region.

The time profiles of high energy electrons and protons are also compared with the predictions of a time dependent modulation model. It is found that our data can be reasonably well accounted for by assuming time-dependent increasing drift effects, asymmetric perpendicular diffusion and a rigidity (P) dependence for the parallel mean free path derived by Bieber et al. (1994), i.e. img3.gif between 0.9 and 2.5 GV.

Regarding the img1.gif 7 MeV electrons, we do not see any flux excess at the poles. This is in contradiction with the claim made by Simnett et al. (1995) of an entry of MeV galactic electrons over the poles. The flux excess they claim to have detected is inconsistent with our electron data by 30 to 60 standard deviations, depending on the spectral shape. We argue that this inconsistency may be attributed to a proton induced background in the Simnett et al. data. The KET data imply a conservative upper limit of 3.2 elec/m2.s.sr.MeV for the galactic electron flux at this energy.

Key words: cosmic rays - Sun: activity - interplanetary medium

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