On 27-day recurrences in the cosmic rays and the solar corotation

The 27-day recurrences of neutron monitor data, at 3.1 GV cut-off for the years 1953-1979, have been analysed by the inspection of the harmonic amplitude and phase computed via a new algorithm. The analysis allows us to distinguish for these recurrences two behaviours: the first one appears to have small amplitudes and periodicity of about 28.5 days around the solar minima; the second one appears to have greater amplitudes and periodicity around 27.5 days. We have investigated the possibility that the former behaviour is due to the control of the cosmic ray modulation by medium and high heliolatitude solar activity phenomena such as coronal holes, while the latter is related to the solar activity responsable for phenomena of greater amplitude. (Forbush Decreases)