Data, from the worldwide network of neutron monitors, recorded at Deep River, Hermanus, Rome, Tokyo, and Huancayo, over two solar cycles (Nos. 20 and 21) are analyzed to study the long-term variations of the solar diurnal variations as they relate to solar-wind speed. The median primary rigidities of response (Rm) for these detectors cover the range 16 GV ≤ Rm ≤ 33 GV. We discuss the solar diurnal variations (amplitude and phase) of cosmic rays as a function of solar activity. The behavior of solar diurnal phases is completely different for the two epochs of high-wind speed. Data of solar-wind speed from 1966–1986 are classified according to the state of the daily mean values. Variation in the amplitudes of the diurnal variations, as functions of the median primary rigidity of cosmic rays, for the two selected periods (1973–1975 and 1979–1981) of high and low solar-wind speeds were determined at the selected stations. The rigidity dependence of the averaged solar diurnal variations of cosmic rays related to the high solar-wind speed was studied. The most sensitive rigidity of modulation is around 20 and 30 GV during the 1973–1975 and 1979–1981 periods, respectively. Our results also show that there is a significant correlation in the solar diurnal amplitudes between the two divisions of high and low solar-wind speed days.
The onset of long-term cosmic ray modulation in cycle 23 coupled with a transient increase of anomalous cosmic rays in the distant heliosphere
