Heliospheric Modulation of Galactic Cosmic Rays Observed at the L1 Lagrange Point in Solar Cycle 23

We analyze a unique 14-year record of Galactic Cosmic Ray measurements made by the SOHO Coronal Diagnostic Spectrometer NIS detectors, recording integrated GCR numbers with energies above 1.0 GeV between July 1996 and June 2010. The GCR numbers decrease by 50 percent from the 1997 minimum to the 2000 maximum of the solar cycle, then increase back to the 1997 level in 2007 and continue to rise, reaching in December 2009 a level 22% higher than in 1997. This 22% increase is in contrast with the behaviour of Ulysses/KET GCR protons extrapolated to 1 AU in the ecliptic plane, showing the same level in 2008-2010 as in 1997 (Heber et al. 2009). The increase of SOHO/CDS GCR numbers, continuing until 2009, is correlated with the decrease of the tilt angle of the heliospheric current sheet from 30 degrees in mid-2008 to 5 degrees in late 2009. The GCR level then drops sharply from January 2010, again consistent with a rapid increase of the tilt angle to over 35 degrees. We discuss the effect of the structure of the heliospheric current sheet on cosmic ray transport. The CDS/NIS on SOHO currently remains the only space instrument monitoring daily the GCR protons with energies above 1.0 GeV. Unfortunately, CDS operations are to be terminated in 2011.