Forbush decrease of June 8, 1969: Causes of the unusually long recovery

Abstract

The cosmic-ray decreases and interplanetary disturbances, that occurred at 1 AU during the period, June 8–July 21, 1969, have been investigated by using the cosmic-ray intensity data recorded with ground-based monitors at Mt. Washington and Deep River, as well as the Interplanetary Magnetic Field (magnitude and direction) and the Solar Wind Plasma bulk speed, density and temperature at 1 AU. We observed a two-step Forbush decrease on June 8–9, 1969, which was due to the structure within the shock and sheath preceding the interplanetary coronal mass ejection. We also observed two cosmic-ray depressions on June 16–20 and July 13–15, 1969, which were attributed to a long-lived corotating high-speed solar wind stream. The outward propagating interplanetary shock waves that occurred at about 0700 UT on June 16, 1969, and at about 1400 UT on July 13, 1969 and which were associated with the long-lived corotating high-speed solar wind stream, most probably swept away the galactic cosmic rays, causing the delay in the Forbush decrease recovery at 1 AU, and, hence, the unusually long recovery of the two-step Forbush decrease at 1 AU with onset on June 8, 1969. The additional depressions by the interplanetary shocks associated with the long-lived corotating high-speed solar wind streams which were superposed on the recovery phase of the Forbush decrease of June 8, 1969, were shown to be larger on the lower energy galactic cosmic-ray particles, and therefore the duration of the recovery phase would be much longer in the lower energy region, an expectation which is consistent with the observations.