Observations of galactic cosmic ray modulation during Earth-directed coronal mass ejections

We describe three cosmic-ray decreases observed by the energetic particle telescope ERNE on board SOHO spacecraft during the period January - May, 1997. The decreases in cosmic-ray proton intensities in the energy range above 50 MeV were observed after the passage of interplanetary shocks on February 9, April 10, and May 15. All the shocks were produced by Earth-directed coronal mass ejections. The amplitudes of the decreases compared to the counting rates just before the shock arrival were 8.5 %, 14 %, and 7 %, respectively. The observed recovery times were 1, 4.2, and 5 days, respectively. After the February and May events the pre-shock levels were reached, but in the April event the recovery was only to 3 % below this level. In each case, there were also clear indications of the passage of the ejecta. The energy dependences of the cosmic-ray helium flux decreases at 12-100 MeV/n were similar to each other in the April and May events, while in the February event the decrease was less pronounced.