THE LIMITING PRIMARY RIGIDITY OF COSMIC-RAY DIURNAL ANISOTROPY

Our calculations of the upper cut-off primary rigidity (R(c)), applicable to the solar diurnal anisotropy, have been extended to cover the declining phase of the solar activity cycle 21. The calculated values of R(c) now encompass a complete solar magnetic cycle (1965-1987). We found that the values of R(c) were very large for the 1982-1985 period, almost twice as large as those observed previously for the periods of maximum solar activity. The largest value occurred in 1983. Also, the value obtained at the solar activity minimum year of 1986 was about twice as large as that obtained in 1965. Our results are compared with those reported by others.