Changes in the Low-Energy Particle Cutoff and Primary Spectrum of Cosmic Rays

The low rigidity cutoff for primary particles in the cosmic-ray spectrum reappeared in 1956. From these new results and the earlier measurements in 1948, 1951, and 1954, it is clear that the shift of the low-rigidity cutoff to a very small value is restricted in time to an interval within which solar activity reached a minimum in the 11-year solar cycle. This effect was accompanied by other changes in the primary spectrum; namely (a) the total cosmic-ray intensity and (b) the exponent for the power law spectrum, both passed through maxima near the solar minimum in 1954.The 1956 results further support the view that these changes in the primary spectrum have their origin in a mechanism controlled by solar activity---most likely the diffusion of cosmic-ray particles through inter-planetary disordered magnetic fields transported by plasma clouds of solar origin. If this is so, then only for a brief period near solar minimum is there the possibility of access to the true galactic spectrum for particles below approximately 30 Bv.