Multi-spacecraft observations of the 27-day periodicity in galactic protons from 2018 to 2019 by the HEPD-01 detector on board the CSES-01 satellite and other experiments

Galactic cosmic ray (GCR) intensities exhibit recurrent variations caused by their passage through plasma interaction regions co-rotating with the Sun, with the ∼27-day periodicity being the most prominent one. To study this periodicity, data collected by the High-Energy Particle Detector (HEPD-01) on board the China Seismo-Electromagnetic Satellite (CSES-01) in Low-Earth Orbit have been used to derive daily proton fluxes from August 2018 to August 2019, in the energy range between ∼55 and ∼200 MeV. Daily fluxes from HEPD-01 have been analyzed along with proton fluxes measured during the same period by ERNE and EPHIN, on board the SOHO spacecraft, and by AMS-02, on board the International Space Station. Using a classical time-frequency analysis, we observed a slight energy dependence for the power of the ∼27-day variation as a function of time, with the periodicity maximum occurring earlier for HEPD-01 than for high-energy data from AMS-02. Additionally, the rigidity dependence of the amplitude of the aforementioned GCR variation cannot be described by the same power law at both low and high energies, as a consequence of different physical mechanisms playing roles at different rigidity ranges. HEPD-01 GCR measurements cover the energy range from tens to a few hundreds of MeV, which is not accessible to existing detectors (EPHIN and ERNE covering from a few MeV up to tens or a hundred MeV, respectively, and AMS-02 in the GeV-TeV energy range) providing important information for understanding GCR periodicities.