Ulysses - COSPIN-KET


Modulation of anomalous and galactic cosmic rays by Corotating Interaction Regions

Involved scientists

B. Heber, M.S. Potgieter, J.B. Blake, H. Kunow, G. Wibberenz, C. Paizis, P. Ferrando

The Sun in the light of Fe XII (Iron)
Figure 1: The Sun in the light of Fe XII

Description and results

Corotating Interaction Regions (CIRs) are long lasting large-scale plasma structures generated in low and middle latitude regions of the heliosphere by the interaction of a stable fast solar wind stream with the surrounding slow solar wind. The areas emitting the fast solar wind can be associated with dark areas in e.g. images of the Sun in the light of Fe XII (Figure 1). During low solar activity conditions the polar region of each hemisphere is covered by "coronal holes". As shown below, these coronal holes can extend to the heliographic equator or even beyond, leading to typical CIR structures in the solar wind speed and magnetic field strength (Figure 2). Associated with these plasma structures are recurrent MeV-ion events and recurrent decreases of the galactic and anomalous cosmic ray intensities.

Data Sheet
Figure 1: Data Sheet. (original size)

The three-dimensional extent and their role in structuring the quiet heliosphere became clear after Ulysses high latitude observations had became available. While in situ no significant recurrent variation in Plasma nor magnetic field parameter were observed, recurrent decreases of galactic cosmic rays and keV electron events were observed up to ≈70° heliographic latitude. Of special interest is the rigidity dependence of the amplitude of the recurrent decreases. In Heber et al. (1999) we summarized the observations and in Paizis et al. (1999) we determine the rigidity dependence of these decreases for galactic cosmic ray protons and found an amazing similarity to the rigidity dependence of the latitudinal gradient. Especially interesting is the fact, that the amplitude is depending also on the particle species and not only on the regidity. For example anomalous cosmic ray helium has approximately five times larger amplitude than galactic cosmic ray protons of the same rigidity. One of the most important question connected to the solar modulation of cosmic rays is:
Do corotating interaction regions play a role in determining solar modulation of galactic and anomalous cosmic rays during periods of low to moderate solar activity?