Ulysses - COSPIN-KET


Modulation of anomalous and galactic cosmic rays

Involved scientists

B. Heber, M.S. Potgieter, A. Burger, F. McDonald, H. Kunow, P. Ferrando, J.B. Blake, C. Paizis

Description and results

Galactic cosmic rays are energetic particles entering the heliosphere from the interstellar medium. Anomalous cosmic rays are believed to be at the termination shock accelerated pickup ions with maximum energies of ~300 MV. These particles are modulated by the solar wind and the solar magnetic field carried out by the solar wind. The important processes describing particle propagation in the heliosphere are convection, diffusion, adiabatic deceleration, gradient and curvature drifts. Because of drift effects electrons and protons should be modulated differently. At solar minimum during an A>0-solar magnetic epoch like in the 1970's and 1990's protons are expected to drift in into the inner heliosphere over the poles and out along the heliospheric current sheet, while electrons should show the opposite trajectories (see right inlet of the following figure.

A data example
Figure 1: A data example (original size)

These drift effects were expected to lead to large latitudinal gradients, as shown in the left insert. In Belov et al. (1999), Heber et al. (1996a), (1996b), (1998), (2002) and McDonald et al. (1997), (1999) we determined these gradients in the inner heliosphere as well as in the outer heliosphere by using Ulysses, IMP, and Voyager measurements. Here we could show, that the gradients are much lower than anticipated, and only present as long as Ulysses is embedded in the fast solar wind originating from the polar coronal holes. As was suggested by Jokipi and Kota the diffusion coefficient perpendicular to the heliospheric magnetic field in direction to the poles has to be enlarged to account for the smaller latitudinal gradient. However, the rigidity dependence of the diffusion tensor has only be investigated for the parallel component and very little is known about the perpendicular components. Burger et al. (2000) found good evidence that these components have different rigidity dependence than the parallel one. However, qualitatively a new heliospheric magnetic field model, as proposed by Fisk (1996), could also account for lower latitudinal gradients. This model has not been incorporated into the different modulation models, so that we can not undertake quantitative comparison.

But drift effects lead also to a different flux variation with the inclination of the heliospheric current sheet. In Heber et al. (1999a), (1999b), (2001), (2002) we found that this is only observed when the inclination is below ~30 degree close to solar minimum in 1996 and 1997. As Ulysses moves to higher heliographic latitudes in 2000/2001 solar activity had reached its maximum activity and the solar magnetic field reversed in 2001 (Belov et al. 2003, Ferreira et al. 2003, Heber et al. 2002, 2003, and Mckibben et al. 2003). Since the ULYSSES COSPIN/KET instrument is the only continuos operating instrument in space, that measures galactic cosmic ray electrons and protons in the inner heliosphere simultaneously and the solar reversal normally does not occur in both hemispheres at the same time, measurements at high latitudes in one hemisphere has us provide with unique contribution on how the reversal of the Suns magnetic field influences the propagation of galactic cosmic rays in the inner heliosphere, by determining latitudinal gradients as well as the electron to proton ratio.
In 2006 Ulysses will be again at high heliographic latitudes, but then it will encounter again solar minimum condition with the oposite heliospheric magnetic field polarity. Measurements at that time will be cruical to understand the importance of the full 22-year solar cycle on cosmic ray modulation.