Recurrent modulation of galactic cosmic rays: A comparative study between IMP, SOHO, STEREO, and Ulysses


It is well known that the galactic cosmic ray (GCR) flux is modulated by corotating interaction regions (CIR) in the vicinity of Earth. When Ulysses first explored high latitude regions in 1996, it was found that the flux of GCRs was still modulated on the time scale of one solar rotation, although neither the solar wind nor the interplanetary magnetic field at these latitudes showed the characteristics of CIRs. This finding led to the modification of our understanding of either the heliospheric magnetic field (HMF, Fisk field) or the transport of particles perpendicular to the HMF. Now, 12 years later, Ulysses explored these high latitude regions again. From September 2007 to September 2008, the GCR flux at Earth showed a clear 27 day solar-rotation modulation. In this contribution, we show that the intensities of GCRs and Jovian electrons at the location of Earth are well modulated with the expected time periods of 27 and 26 days, respectively.

Proceedings of the 31st International Cosmic Rays Conference, ICRC 2009'