Time variations in Galactic cosmic rays as measured from Southeast Asia

Abstract

We summarizetime variations in Galactic cosmic ray (GCR) measurements from the Princess Sirindhorn Neutron Monitor(PSNM) at the summit of Doi Inthanon, Thailand’s highest mountain, since 2007. PSNM is the first neutron monitor (NM) station making long-term measurements in Southeast Asia, with the world’s highest cutoff (threshold) rigidity (momentum per charge) of ≈17 GV. GCR variations with the ∼11-year sunspot cycle or ∼22-year solar magnetic cycle are known as solar modulation. Compared with measurements at lower cutoff, PSNM reveals a distinct pattern of solar modulation related to the interplanetary magnetic field strength. NM viewing directions rotate with Earth, so daily (“diurnal”) variations indicate the GCR anisotropy, and we identified time intervals with unusually strong anisotropy due to a unidirectional GCR gradient. From worldwide NM data, we determine an hourly anisotropy during GCR decreases associated with solar storms, indicating GCR flows consistent with our previous theory that cosmic rays drift into one leg of an interplanetary flux rope and out the other. PSNM was also the first fixed NM to monitor time delays between successive neutrons, providing a proxy for the cosmic ray spectral index using data from a single station, avoiding the systematic uncertainties of cross-station comparisons.