Asymmetries in the modulation of protons and helium nuclei over two solar cycles

Measurements of proton and helium spectra over two cycles of solar modulation show large asymmetries in modulation. These effects are apparent at sunspot minimum conditions in 1965 and 1977, where a much larger intensity is observed in 1977, and also at sunspot maximum conditions where a much lower intensity is observed in 1981 relative to 1970. The details of this asymmetry are discussed and possible causes are considered.