CHARACTERISTICS OF ENHANCED AND LOW-AMPLITUDE COSMIC-RAY DIURNAL-VARIATION

The occurrence of a large number of high- and low-amplitude cosmic-ray diurnal wave trains during the two solar cycles (20 and 21) over the years 1965-1990 has been examined as a function of solar activity. The high-amplitude days with the time of maximum in the similar to 18:00 hr corotation direction do not indicate any significant correlation wi th solar activity. But, the low-amplitude days are inversely correlate d with solar activity and the time of maximum shifts to earlier hours (similar to 15:00 hr direction). The slope of the power-spectrum densi ty roughly characterized by power spectral index'n' in the high-freque ncy range 3.5 x 10(-5) Hz to 8.3 x 10(-4) Hz (time scales of 20 min to 8 hr) is different for the two classes of events. A suggestion is mad e that the enhanced and low-amplitude cosmic-ray diurnal variations ar e produced by different types of interplanetary magnetic field distrib utions.