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Ann. Geophys., 21, 1359-1366, 2003
www.ann-geophys.net/21/1359/2003/
© European Geosciences Union 2003
Charge-sign dependent modulation in the heliosphere over a 22-year cycle
S. E. S. Ferreira1,3, M. S. Potgieter1, B. Heber2, and H. Fichtner3
1Unit for Space Physics, School of Physics, Potchefstroom University for CHE, 2520 Potchefstroom, South Africa
2Fachbereich Physik, Universität Osnabrück, 49069 Osnabrück, Germany
3Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, Germany
Abstract. A time-dependent model
based on a numerical solution of Parker’s transport equation is used to model
the modulation of cosmic ray protons, electrons and helium for full 11-year and
22-year modulation cycles using a compound approach. This approach incorporates
the concept of propagating diffusion barriers based on global increases in the
heliospheric magnetic field as they propagate from the Sun throughout the
heliosphere, combined with gradient, curvature and current sheet drifts and the
other basic modulation mechanisms. The model results are compared to the
observed 11-year and 22-year cycles for 1.2 GV electrons and 1.2 GV Helium at
Earth for the period 1975–1998. The model solutions are also compared to the
observed charge-sign dependent modulation along Ulysses’ trajectory for the
period 1990–1998. This compound approach to long-term modulation, especially
charge-sign dependent modulation, is found to be remarkably successful. It is
shown that the model can easily account for the latitude dependence for cosmic
ray protons and the lack thereof for cosmic ray electrons by assuming large
perpendicular diffusion in the polar direction. This approach contributes to an
improved understanding of how diffusion and drifts vary from solar minimum to
maximum modulation, and what the time-dependence of the heliospheric diffusion
coefficients may be.
Key words. Interplanetary physics
(energetic particles; cosmic rays; general or miscellaneous)
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