Charge-sign dependent drift effects in the time-lag of cosmic-ray variation relative to solar activity observed with CALET

Abstract

The 11-yr variation of galactic cosmic-ray flux lags behind the variation of the sunspot number. An average ∼1-yr time-lag is expected from the outward propagating solar wind with the frozen-in photospheric magnetic field varying in the solar cycle, and from the inward diffusive transport of cosmic-ray particles. The long-term neutron monitor data, however, show that the time-lag is significantly longer (shorter) in the odd (even) solar cycle. In this paper, we analyze the time-lag in proton and electron fluxes observed by the Calorimetric Electron Telescope (CALET). It is found that the time-lag is similar in proton and electron fluxes during an A > 0 polarity epoch of the solar dipole magnetic field. In an even solar cycle 24 including a polarity reversal from A < 0 to A > 0, on the other hand, it is found that the time-lag of proton (electron) flux variation is significantly shorter (longer) than the average ∼1-yr lag by analyzing the combined data with CALET and AMS-02. This is the first observation of the charge-sign dependent time-lag. We demonstrate that these observations can be qualitatively interpreted in terms of different 11-yr time profiles of proton and electron fluxes in A > 0 and A < 0 epochs expected from the drift effect.