ESA Science & Technology30-Jun-2005 16:37:02

Science Highlights from the First Solar Orbit

Cosmic Rays

A fundamental question that has been answered by Ulysses relates to the presence of large-scale directional fluctuations in the polar heliospheric magnetic field. Prior to the launch of Ulysses, it was suggested that the classical picture of smooth, near-radial polar magnetic field lines would be modified by the effects of random walk of the field foot points resulting from super-granulation motion in the photosphere. This stochastic motion would lead to large-scale transverse fluctuations in the polar fields, which in turn would impede the access of cosmic ray particles to the inner heliosphere over the poles.

The data from Ulysses' polar passes have confirmed both the existence of the field fluctuations, and the absence of large latitudinal gradients in the fluxes of cosmic ray particles. Of particular interest is the latitude distribution of the so-called anomalous cosmic ray component (ACR), generally thought to be singly-charged interstellar ions that have been accelerated to MeV energies at the solar wind termination shock. Ulysses results show a modest positive latitudinal gradient for ACR oxygen, nitrogen and neon, qualitively in agreement with models in which the acceleration takes place largely at the poles of the heliosphere, the ACR subsequently diffusing and drifting down to the equatorial regions.

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