Long-term variation in cosmic ray intensity and Forbush decreases at 1AU

Abstract

The long-term variations in cosmic ray intensity have been studied using the pressure corrected cosmic ray intensity data for the period 1963-87 (which includes solar activity cycles 20 and 21) recorded by Deep-River neutron monitor (cut off rigidity = 1.02 GV) on the basis of yearly average in two different ways. In one case, the yearly average is calculated taking into account all the conspicuous Forbush decreases (FDs), while in the other, it is calculated excluding all such decreases from every year's data. Thus, the contribution of FDs to the long-term variation has been calculated for both the solar cycles and for their ascending and descending phases separately. It is observed that cumulative effect of FDs is not the only cause for producing the long-term variation in cosmic ray intensity, and its contribution is different for the solar cycles 20 and 21, having different solar activity and magnetic field polarity.