Cosmic-Ray Spectral Variations during 2007-2025 with Neutron Monitor Time-Delay Measurements at High Cutoff Rigidity

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 \textasciitilde 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.