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Observational analysis reveals spectral variations of cosmic rays in relation to sunspot cycles, suggesting a connection to proton spectral index.
The leader fraction, L, is defined as the fraction of neutron monitor counts that are not temporally associated with a later count in the same neutron monitor counter due to the same cosmic ray shower. L was extracted from time-delay histograms and serves as a precise indicator of spectral variations in cosmic rays above the cutoff rigidity. In this work, we analyze long-term variations in L measured by the Princess Sirindhorn Neutron Monitor (PSNM) at Doi Inthanon, Thailand, which has the highest vertical cutoff rigidity for a fixed neutron monitor station (16.7 GV). PSNM has recorded time-delay histograms since 2007, during which time the data acquisition system has continually been upgraded, necessitating normalizations to ensure the consistency of the long-term L dataset. We investigated the spectral variation of Galactic cosmic rays above 17 GV over two sunspot minimum and two sunspot maximum periods, including hysteresis effects. Furthermore, we compare L from PSNM with the daily proton spectral index derived from recently published Alpha Magnetic Spectrometer (AMS-02) data aboard the International Space Station, covering 2011–2019. This comparison provides the relationship between neutron monitor leader fraction variations and direct measurements of the proton spectral index at high rigidities.
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Banglieng et al. (2025) studied this question.
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