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Astron. Astrophys. 316, 547-554 (1996)

Observations of galactic cosmic rays and the anomalous helium during Ulysses passage from the south to the north solar pole*

R.B. McKibben1, J.J. Connell1, C. Lopate1, J.A. Simpson1,2, and M. Zhang1

1 Enrico Fermi Institute, University of Chicago, Chicago, IL, USA
2 Dept. of Physics, University of Chicago, Chicago, IL, USA

Received 15 July 1996 / Accepted 12 September 1996

Abstract. We compare observations of the modulated intensities and spectra of galactic cosmic ray nuclei and anomalous helium above 30 MeV/n measured by the COsmic and Solar Particle INvestigations (COSPIN) High Energy Telescope (HET) on Ulysses with similar observations from the IMP-8 earth-orbiting satellite between September 1994 and August 1995. During this interval Ulysses made a rapid scan of solar latitude from 80.22oS latitude to 80.22oN latitude at radial distances between 2.2 and 1.3 AU in a period of slowly decreasing solar modulation. The observations confirm our previous conclusions from measurements in the southern hemisphere that solar modulation is remarkably spherically symmetric in the inner heliosphere. Flux increased towards the poles by less than a factor of two for all species measured with no significant changes in the spectra of galactic cosmic ray nuclei and anomalous components. Fluxes over the north pole were in general slightly higher ( img2.gif 10-20%) than fluxes over the south pole. As reported by Simpson et al. (1996), the modulation was symmetrical about a surface at latitude 10oS. At the boundaries of the equatorial zone defined by low speed solar wind the solar wind speed increased by a factor of two within 5 degrees of latitude with no effect on the modulated cosmic ray and anomalous component spectra. This suggests that typical cosmic ray trajectories are not tightly confined in heliographic latitude and freely cross the boundary between the polar and equatorial zones during the modulation process. At the highest latitudes ( img4.gifo from the symmetry surface at img5.gifoS) latitudinal gradients in the cosmic ray intensity were significantly smaller than at lower latitudes.

Key words: interplanetary medium - solar wind - cosmic rays

* The Ulysses cosmic ray data on which this analysis is based are available on the World-Wide Web at file://odysseus.uchicago.edu/WWW/Simpson/Ulysses.html.

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