Observation of Complex Time Structures in the Cosmic-Ray Electron and Positron Fluxes with the Alpha Magnetic Spectrometer on the International Space Station

We present high-statistics, precision measurements of the detailed time and energy dependence of the primary cosmic-ray electron flux and positron flux over 79 Bartels rotations from May 2011 to May 2017 in the energy range from 1 to 50 GeV. For the first time, the charge-sign dependent modulation during solar maximum has been investigated in detail by leptons alone. Based on $23.5\times {10}^6$ events, we report the observation of short-term structures on the timescale of months coincident in both the electron flux and the positron flux. These structures are not visible in the $e^{+}/e^{-}$ flux ratio. The precision measurements across the solar polarity reversal show that the ratio exhibits a smooth transition over $830\pm 30$ days from one value to another. The midpoint of the transition shows an energy dependent delay relative to the reversal and changes by $260\pm 30$ days from 1 to 6 GeV.

Figure 1

The time-averaged (a) electron flux, (b) positron flux, and (c) ratio $R_e$. The time-variation range is indicated by the shaded regions, see text. The fit of the model in Refs. [36, 37] to the time-averaged data points is shown by the black curves. The visible time variation of $R_e$ is evidence for charge-sign dependent solar modulation.

Figure 2

Fluxes of primary cosmic-ray positrons (red, left axis) and electrons (blue, right axis) as functions of time, for five of the 49 energy bins. The error bars are the statistical uncertainties. Prominent and distinct time structures visible in both the positron spectrum and the electron spectrum and at different energies are marked by dashed vertical lines.

Figure 3

The ratio $R_e$ of the positron flux to the electron flux as a function of time. The error bars are statistical. The best-fit parametrization according to Eq. (3) is shown by red curves. The polarity of the heliospheric magnetic field is denoted by $A<0$ and $A>0$. The period without well-defined polarity is marked by the shaded area [17].

Figure 4

Results of the fits of the parametrization in Eq. (3) to the ratio $R_e$ as a function of energy (blue circles): (a) $\mathrm{\Delta }t$ and the best-fit constant value of 830 days (red line), (b) $t_{1/2}-t_{\mathrm{rev}}$ with the parametrization according to Eq. (4) (red curve), (c) amplitude $C$ with a dashed line at zero to guide the eye.