Galactic cosmic ray intensity variations at a high latitude sea level site 1937-1994

We have created an extended cosmic ray data string (1937-1994) by combining data obtained with ion chambers at Cheltenham/Fredericksburg (1937-1972) and Yakutsk (1953-1994). Both represent high-latitude, sea level sites, with an atmospheric cutoff of about 4 GV. Their common median rigidity of response to galactic cosmic ray spectrum is 67 GV. This data string represents the longest continuous cosmic ray intensity variations record yet. Therefore it is useful for the cosmic ray modulation studies over a longer time period. We show that there is no striking quantitative correspondence between the amplitudes of solar activity cycles and cosmic ray modulation over six cycles. However, an inverse correlation is apparent between the two. Moreover, the results on the durations of the two are ambiguous. Flat-topped recovery pattern predicted for even cycles (A > 0), at lower rigidities, is not observed in the ion chamber data. We speculate that three solar activity cycle quasi-periodicity may be present in the extended data string. Its origin is not identified yet. The quasiperiodicity is shown to exist also in the planetary index Ap data. As such, it may represent a characteristic feature of the solar wind source region on the Sun as yet unexplored. The odd cycles, with a negative polarity in the solar northern hemisphere, yield abetter linear correlation between galactic cosmic ray intensity decrease and Ap than even cycles and A > 0 epochs.