Banglieng C.Muangha P.Ruffolo D.Sáiz A.Mitthumsiri W.Evenson P.Nutaro T.Nuntiyakul W.Mahidol University2026-02-082026-02-082025-12-30Proceedings of Science Vol.501 (2025)https://repository.li.mahidol.ac.th/handle/123456789/114889The 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.MultidisciplinaryConference PaperSCOPUS10.22323/1.501.12732-s2.0-10502906186018248039