Cosmic-Ray Modulation during Solar Cycles 24-25 Transition Observed with CALET on the International Space Station

Abstract

We present the solar modulation of electrons and protons observed by the CALorimetric Electron Telescope onboard the International Space Station for about 7 years since October 2015, during the transition phase from the descending phase of the 24th solar cycle to the ascending phase of the 25th solar cycle. The observed variations of electron and proton count rates at an identical average rigidity of 3.8 GV show a clear charge-sign dependence of the solar modulation of galactic cosmic rays (GCRs), which is consistent with the prediction of a numerical drift model of the GCR transport in the heliosphere. It is also found that the ratio of 3.8 GV proton count rate to the neutron monitor count rate in the ascending phase of solar cycle 25 is clearly different from that in the descending phase of cycle 24. Correlations between the electron (proton) count rate and the heliospheric environmental parameters, such as the current sheet tilt angle, are a useful tool in further developing a numerical model of solar modulation.