Study of the Diurnal Variation of Cosmic Rays during Different Phases of Solar Activity

The pressure-corrected hourly counting rate data of neutron monitor stations, situated in different latitudes, have been employed to study the variation of cosmic ray diurnal anisotropy for a long period of 44 years (1965-2008). Some of these neutron monitors are situated in low latitudes with high cutoff rigidity. Annual averages of the diurnal amplitudes and phases have been obtained for each station. It is found that the amplitude of the diurnal anisotropy varies with a period of one (11 year) solar activity cycle, whereas the diurnal phase varies with a period of 22 years (one solar magnetic cycle). The annual average diurnal amplitudes and phases have also been calculated by grouping the days on the basis of ascending and descending periods of each solar cycle (cycles 20, 21, 22 and 23). The results indicate that the systematic and significant differences are observed in the diurnal variation between the descending periods of the odd and even solar cycles. The overall vector averages of the descending period of the even solar cycles (20 and 22) show some what smaller diurnal amplitude as compared to the vector averages of the descending period of the odd solar cycles (21 and 23). In contrast, we find a large diurnal phase shift to earlier hours during the descending period of even solar cycles (20 and 22), as compared to almost no shift in the diurnal phase during the descending period of odd solar cycles. Further, we find that the overall vector average diurnal amplitude of the ascending period of odd and even solar cycles, remain invariant from one ascending periods to the other, or even in between the even and odd solar cycles. However, we do find a significant diurnal phase shift to earlier hours during the ascending periods of odd solar cycles (21 and 23), in comparison to the diurnal phase in the ascending periods of even solar cycles (20 and 22).