Rigidity Dependence of the Long-term Galactic Cosmic Ray Variation by AMS-02

Using measurements from the Alpha Magnetic Spectrometer (AMS-02) aboard the International Space Station, we have examined the long-term variations in galactic cosmic ray (GCR) proton fluxes in 2011–2018. The AMS-02 data allow for the study of time profiles and the rigidity dependence of the long-term variations observed directly in space in a wide rigidity range from 1 to 100 GV. We have investigated the rigidity dependence of the amplitude of the long-term GCR variations described by the power-law fitting over the solar cycle. For a physical interpretation, we have considered the relationship between long-term GCR variations and heliospheric magnetic field turbulence using power spectral density frequency exponents. The apparent solar cycle variability can be seen in the time profile of the spectral index γ of the power-law rigidity spectrum of long-term GCR variations for 2006–2018 during solar cycle 24. The spectral index γ shows a tendency to be higher for the solar maximum phase compared to the solar minimum phase. Furthermore, we find evidence of the energy-dependent rigidity spectrum of the long-term GCR variations observed by AMS-02. We reveal the soft rigidity spectrum of the GCR isotropic intensity variations for the solar maximum and the hard rigidity spectrum for the solar minimum owing to the essential temporal rearrangements of the structure in the heliospheric magnetic field turbulence from the maxima to minima epoch of solar activity. The long-term GCR variations by AMS-02 show the softening of the spectra toward higher energies.