Cosmic-ray modulation over the past 10 years observed with CALET on the International Space Station

Abstract

The CALorimetric Electron Telescope (CALET) on the International Space Station has been measuring cosmic rays and gamma rays since October 2015. In addition to the high-energy trigger, CALET operates a low-energy electron (LEE) trigger at high geomagnetic latitudes, enabling measurements of 1–10 GeV electrons. Using this LEE trigger mode, CALET has observed the solar modulation of low-energy electrons and protons over nearly ten years, including the solar minimum and the solar maximum of the 25th solar cycle. The electron and positron flux exhibited a continuous decrease beginning in the ascending phase of the 25th solar cycle, reaching its lowest values in 2024. By comparing electron and proton count rates at the same average rigidity of 3.8 GV, we observed a clear charge-sign dependence of solar modulation. We also identified a clear hysteresis structure over the ascending and descending phases of the solar cycle. This hysteresis persisted across the solar maximum, covering the period of the solar magnetic polarity reversal. Furthermore, CALET detected Forbush decreases of electrons and protons in temporal agreement with ground-based neutron monitor observations. The larger decrease observed by CALET compared with ground-based neutron monitors suggests the rigidity dependence of solar modulation. These results demonstrate CALET’s capability to resolve charge-sign–dependent effects and transient phenomena in the CR propagation in the heliosphere, providing valuable input for models of solar modulation.