Pronounced diurnal variation in cosmic-ray intensity

Enhanced diurnal variations during February 1–6, 1967; April 5–14, 1967; and February 22–28, 1968, have been examined by use of data from several super neutron monitors. It is found that during these days, cosmic-ray intensity in a direction 40°–70° west of the sun-earth line was lower with respect to other directions, in particular the polar and antigarden-hose directions. This anisotropy, which resulted from a reduced intensity in the garden-hose direction in the ecliptic plane, suggests that the entire diurnal variation observed in cosmic-ray intensity cannot be attributed to a simple mechanism involving the corotation of cosmic-ray gas, with the interplanetary field. Corotation of cosmic-ray gas implies an enhancement of intensity from a direction roughly 90° east of the sun-earth line (1800 hr direction). Highly variable north-south asymmetries were also present during these periods. These anisotropies occurred without accompanying geomagnetic disturbances or Forbush decreases indicating that they were not due to solar activity on the visible side of the sun.