This study deals with the short-term variations of cosmic ray intensity during the interval 1973-78. Daily means of high latitude neutron and meson monitors from the same station and those of a low latitude neutron monitor have been analysed using the Chree method of superposed epochs. The zero epoch for the Chree analyses corresponds to the day of a substantial increase (AV > 200 km s-~ ) in the solar wind speed to values of >~ 550 km s-1 and which persists at such high values for an interval of at least three days. The investigation reveals the existence of two types of cosmic ray intensity variations with distinctly different spectral characteristics. During the interval 1973-76, relative changes in the neutron and meson monitor rates are nearly equal indicating an almost fiat rigidity spectrum of variation. During 1977-78, however, the spectrum acquires a negative spectral character similar to that observed for Forbush decreases. We suggest that events of the interval 1973-76 are essentially due to high speed streams associated with solar coronal holes and that events of the interval 1977-78 are due to fast streams from solar active regions with flare activity.
Using the experimental data of the high counting rate neutron and meson monitors, the solar tri‐diurnal anisotropy of galactic cosmic radiation has been investigated for the period 1962–79. The enhancement of the average tri‐diurnal amplitude observed by all the detectors during 1973–75 provides sufficient signal to noise ratio to obtain its variational characteristics. The observations are consistent with an interplanetary anisotropy having a power law rigidity spectrum exponent ≈ 1. The tri‐diurnal amplitude varies as cos³ λ, where λ is the effective asymptotic latitude of the detector. The annual average tri‐diurnal amplitude shows a significant positive correlation with the semi‐diurnal amplitude for the period 1968–79. The correlation, however, is poor for the earlier period 1962–67, which may be due to the presence of significant modulation of semi‐diurnal anisotropy with periods of 22‐year sunspot magnetic cycle. An analysis using groups of days with high and low solar wind speed shows greater amplitude of both the tri‐diurnal and semi‐diurnal waves for the group of days with high wind speed. This result coupled with the observation of a factor of 2 increase in both amplitudes during 1973–75, a period when high speed solar wind streams were prevalent, suggests that solar polar coronal holes may influence both the solar tri‐diurnal and semi‐diurnal variation of galactic cosmic ray intensity.
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