[Heber 1997 a]
Solar modulation of galactic cosmic rays: the 3D
heliosphere.
B. Heber, M. S. Potgieter and A. Raviart.
Solar modulation of galactic cosmic rays: the 3D heliosphere.
Adv. Space Res. 19, 795-804. 1996
Contents
The cosmic ray flux observed by the Kiel Electron Telescope on board
the Ulysses space probe varies with solar activity as well as with
heliospheric position. The Ulysses fast latitude scan performed in less
than one year between September 1994 to August 1995 allowed the authors
to obtain unique data about the latitudinal dependence of cosmic ray
fluxes close to solar minimum conditions. In this paper the authors
discuss the flux variation of protons, alpha-particles and electrons in
the few GV rigidity range, and compare their measurements with
predictions from a 2-dimensional drift dominated modulation model. They
focus their interest on the energy dependence of the latitudinal gradient
of the nuclei component, the north-south distribution of > 106 MeV
protons and charge dependence of 2.5 GV particles during the fast
latitude scan. Whereas the latitudinal gradient for alpha-particles
monotonically decreases with increasing energy, protons show a maximum
latitudinal gradient at some hundred MeV. This maximum was not expected
to exist, but can be explained in modified modulation models by
increasing the diffusion perpendicular to the magnetic field in polar
direction. The counting rate of > 106 MeV protons in the northern
and southern hemisphere is not symmetric with respect to the heliographic
equator but with respect to magnetic latitude. The electron/proton-ratio
of 2.5 GV particles shows that electrons and protons have similar
temporal modulation and that the latitudinal dependence of this ratio
is dominated by the latitudinal proton variation. The modified model
can qualitatively reproduce the observed behavior during the fast
latitude scan.
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