Spatial evolution of 26-day recurrent galactic cosmic ray decreases: Correlated Ulysses cospin/KET and SOHO COSTEP observations

In December 1995 the Ulysses spacecraft was at a radial distance of 3 AU from the Sun and 60° northern heliographic latitude. To that time the Solar and Heliospheric Observatory (SOHO) started its mission. On board of both spacecraft particle sensors are measuring electrons, protons and helium nuclei in the MeV to GeV energy range. In early 1996 the counting rates of several hundred MeV galactic cosmic rays at Ulysses and at SOHO (Earth orbit) were modulated by recurrent cosmic ray decreases (RCRDs). The RCRDs at SOHO were found to be associated with a corotating interaction region (CIRs). A Lomb (spectral) analysis was performed on the galactic cosmic ray flux from February 1996 to June 1996. Surprisingly, the most probable frequency is ∼28 days and not 26 or 27 days, corresponding to one solar rotation. The amplitude of the RCRDs is ∼2.3% on both spacecraft. The variation in the solar wind speed shows the same periodicities and is anticorrelated to the variation in the cosmic ray flux (see Richardson et al. 1996). In contrast to the RCRDs the amplitude found in the solar wind speed is four times larger at WIND (120 km/s) than at Ulysses (32 km/s). The solar wind proton density and magnetic field strength yielded no significant periodicities, neither at Ulysses nor at WIND. Comparing the RCRDs with coronal hole structures observed in the FE XIV line, we found that a single coronal hole close to the heliographic equator can account for the RCRDs observed "simultaneously" at Ulysses and SOHO. The coronal hole boundaries changed towards lower Carrington longitudes and vanished slowly. The changes of the boundaries during the investigated period could explain a 28 day periodicity.