Periodicity of 151 days in outer heliospheric anomalous cosmic ray fluxes

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Abstract
Statistically significant variations have been observed in the differential flux of ∼27‐MeV anomalous cosmic ray (ACR) oxygen, helium, and protons at the Voyager 1 spacecraft during 1998 and 1999 (at a helioradius of ∼73 AU). The quasiperiodic variations are in phase, with oxygen and helium having periods near 151 days, while protons exhibit a period of ∼146 days. The Voyager 1 ACRs vary by ∼30% with respect to the trend, and similar galactic cosmic ray variations, if they exist, must be less than ∼5%, probably much less. No similar, significant periodicities have been detected for these same ACR species at Voyager 2 (at 57 AU) during this period. We report on these and other periodicities in the Voyager Low Energy Charged Particle experiment measurements and address the possible connection between this ∼151‐day ACR periodicity and the previously discovered ∼154‐day periodicities in solar flares, the interplanetary magnetic field, and other phenomena.