Science Highlights from the First Solar Orbit
In addition to latitudinal effects, the energetic particle and cosmic ray measurements made by Ulysses have revealed a striking global periodic variation. Recurrent increases in the low-energy particle flux with ~26-day period were observed up to high latitudes in the southern hemisphere. On the other hand, at energies >100 MeV, the nucleon intensities show corresponding periodic decreases. Both of these phenomena are thought to be associated with Corotating Interaction Regions (CIR) that form as a consequence of stream interactions between fast and slow solar wind flows.
CIR shocks accelerate the low-energy particles, while the magnetic field compressions associated with the stream interactions impede the access of high-energy particles to the inner heliosphere. In the light of the first comprehensive study of the 3-dimensional evolution of CIRs, the question raised by the Ulysses measurements is how do the effects of CIRs operate at high latitudes, where the CIRs themselves are no longer seen? A variety of models have been developed to explain the observations, but a consensus has yet to be reached.