Comparative Study of Diurnal and Semidiurnal Anisotropies in CR Intensity for the Period 1964-95

The Deep River neutron monitor data has been harmonically analysed for the period 1964-95 covering three solar cycles 21, 22 and 23 for looking the long term trend of the diurnal and semi-diurnal anisotropies of cosmic ray intensity on geomagnetically 60 quiet days. The amplitude of both the harmonics remains statistically constant during 1964-70. The amplitudes of the first harmonic is found to be low during 1965, 1976-77, 1987 and 1995 which are the periods of minimum solar activity. The amplitude of diurnal anisotropy acquired exceptionally large value in 1985 whereas semi-diurnal anisotropy acquired large values in 1974-75 and in 1984, which are the epochs of high speed solar wind stream (HSSWS). The phase of the diurnal anisotropy has shifted to earlier hours in 1973 and 1995 whereas the phase of semi-diurnal anisotropy has significantly shifted to earlier hours during 1967, 1977, 1981-82, 1990-91 and 1995, the periods close to minimum solar activity. The phase shows a shift to later hours during 1964, 1979 and 1986.