Cosmic Ray Modulation Observed by the Princess Sirindhorn Neutron Monitor at High Rigidity Cutoff

Neutron monitors (NMs) are the premier instruments for precisely tracking time variations in the Galactic cosmic ray (GCR) flux at the GV-range. For more than 60 years, the worldwide NM network has provided continuous measurements of the solar induced variations of the GCR flux impinging Earth and the data cover about six 11-year solar cycles. The recent rise of space exploration, with PAMELA and AMS-02 spacecraft, brings new energy sensitive measurements of GCR fluxes. Moreover since late 2007, the range of sensitivity of the worldwide NM network has been increased wih the installation of the Princess Sirindhorn Neutron Monitor (PSNM), at the summit of Doi Inthanon, Thailand's highest mountain (2565m altitude). PSNM records the GCR flux with the world's highest vertical rigidity cutoff for a fixed station, 16.8 GV. PSNM data now cover the period from the last solar minimum to the recent solar maximum and give us the opportunity to study the effect of the solar modulation at such high rigidity for the first time. We present here the observations of PSNM since 2007. The observed solar modulation is much weaker than predicted by the force field model with Φ inferred from NM data at low cutoff. We compare measurements with those from NMs located at low rigidity cutoff and with spacecraft data, and discuss the solar modulation at high rigidity. Partially supported by the Thailand Research Fund (RTA5980003) and US National Science Foundation awards PLR-1341562, PLR-1245939, and their predecessors.