27-Day Recurrent Effects in the Diurnal Anisotropy of Galactic Cosmic Radiation & Active Solar Regions

Abstract

During a period of 30 solar rotations from August 1962 to November 1964, the 27-day recurrent effects in the observed diurnal anisotropy of the galactic cosmic radiation have been examined. It is shown that, on the average, the amplitude of the diurnal anisotropy decreases in association with 27-day recurrent decreases of the cosmic ray mean intensity and M-region geomagnetic storms. The observed recurrent effects are found to be well correlated with the longitudinal distributions of long-lasting Ca plages and of the coronal active regions having enhanced λ 5303 emission. It is suggested that the observations can be understood in terms of co-rotating shocks in the interplanetary space which are produced as a result of the interaction between fast and slow solar wind streams. The reduction in the amplitude of the diurnal anisotropy is suggested to be due to an increase in the ratio of the coefficients of cosmic ray diffusion perpendicular and parallel to interplanetary magnetic field direction. It is suggested that various recurrence periods observed in cosmic ray modulation or geomagnetic activity can be directly related to the number of active regions on the sun.