Charge-Sign Dependent Cosmic-Ray Modulation Observed with the Calorimetric Electron Telescope on the International Space Station

We present the observation of a charge-sign dependent solar modulation of galactic cosmic rays (GCRs) with the Calorimetric Electron Telescope onboard the International Space Station over 6 yr, corresponding to the positive polarity of the solar magnetic field. The observed variation of proton count rate is consistent with the neutron monitor count rate, validating our methods for determining the proton count rate. It is observed by the Calorimetric Electron Telescope that both GCR electron and proton count rates at the same average rigidity vary in anticorrelation with the tilt angle of the heliospheric current sheet, while the amplitude of the variation is significantly larger in the electron count rate than in the proton count rate. We show that this observed charge-sign dependence is reproduced by a numerical “drift model” of the GCR transport in the heliosphere. This is a clear signature of the drift effect on the long-term solar modulation observed with a single detector.

Figure 1

Schematic cross-sectional view of CALET with an electron LE event candidate with energy of $\sim 3.9\mathrm{GeV}$.

Figure 2

(a) Time profiles of the sunspot number (solid line) and the HCS tilt angles (blue solid circles) as a function of the central time of each Carrington rotation. (b) Time profiles of the normalized count rates of electrons $C_{e^{-}}$ (blue open circles) and protons $C_p$ (red open circles) for each Carrington rotation (left vertical axis), where statistical uncertainties are shown. The black curve shows the count rate of a neutron monitor at the Oulu station on the right vertical axis, while the blue and red curves show the electron and proton count rates reproduced by the numerical model, respectively. Each shaded area around the reproduced curve indicates the error deduced from the error of the HCS tilt angle and the regression coefficient between the tilt angle and the reproduced curve (see text).

Figure 3

Correlation plot between the normalized count rate and the HCS tilt angle. Blue (red) open circles show the correlation of the observed electron (proton) count rate, respectively, while blue and red solid curves display correlations reproduced by the numerical model (see text).