The Source of the 2017 Cosmic-ray Half-year Modulation Event

Abstract In 2017, as the solar cycle approached solar minimum, an unusually long and large depression was observed in galactic cosmic-ray (GCR) protons, detected with the Alpha Magnetic Spectrometer, lasting for the second half of that year. The depression, as seen in the Bartel-rotation-averaged proton flux, has the form of a Forbush decrease (FD). Despite this resemblance, however, the cause of the observed depression does not have such a simple explanation as an FD, due to coronal mass ejections (CMEs), which typically last for a few days at 1 au rather than half a year. In this work, we seek the cause of the observed depression and investigate two main possibilities. First, we consider a minicycle—a temporary change in the solar dynamo that changes the behavior of the global solar magnetic field and, by this, the modulation of GCRs. Second, we investigate the behavior of solar activity, both CMEs and corotating/stream interaction regions (CIRs/SIRs), during this period. Our findings show that although there is some evidence for minicycle behavior prior to the depression, the depression is ultimately due to a combination of recurrent CMEs, SIRs, and CIRs. A particular characteristic of the depression is that the largest impacts that help to create and maintain it are due to four CMEs from the same, highly active, magnetic source that persists for several solar rotations. This active magnetic source is unusual, given the closeness of the solar cycle to solar minimum, which also helps to make the depression more evident.