Corotating Ion Events Associated with Cosmic Ray Modulation

Interplanetary ions in a few MeV/n energies were observed by the GEOTAIL spacecraft at 1 AU during the period from December 1993 to April 1994. The measurements were performed with the HEP-particle telescopes onboard the GEOTAIL. We have examinedthe correlation between the interplanetary ion events and cosmic ray modulation. It is found that the variation of counting rates of neutron monitor is well correlated with the flux variation of low energy ions. The corotating interaction regions formed in the interplanetary space significantly affect the intensities not only of low energy galactic cosmic rays (GCRs) but also of high energy GCRs. It implies that corotating streams cause the decrease of GCR flux because they act as barriers that impede the flows of GCRs toward the Sun.