Publication: Tracking Cosmic-Ray Spectral Variation during 2007-2018 Using Neutron Monitor Time-delay Measurements
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Issued Date
2020-02-10
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15384357
0004637X
0004637X
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2-s2.0-85082435139
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Mahidol University
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SCOPUS
Bibliographic Citation
Astrophysical Journal. Vol.890, No.1 (2020)
Suggested Citation
C. Banglieng, H. Janthaloet, D. Ruffolo, A. Sáiz, W. Mitthumsiri, P. Muangha, P. Evenson, T. Nutaro, R. Pyle, S. Seunarine, J. Madsen, P. S. Mangeard, R. Macatangay Tracking Cosmic-Ray Spectral Variation during 2007-2018 Using Neutron Monitor Time-delay Measurements. Astrophysical Journal. Vol.890, No.1 (2020). doi:10.3847/1538-4357/ab6661 Retrieved from: https://repository.li.mahidol.ac.th/handle/123456789/54539
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Title
Tracking Cosmic-Ray Spectral Variation during 2007-2018 Using Neutron Monitor Time-delay Measurements
Abstract
© 2020. The American Astronomical Society. All rights reserved.. The energy spectrum of Galactic cosmic-ray (GCR) ions at Earth varies with solar activity as these ions cross the heliosphere. Thus, this "solar modulation" of GCRs provides remote sensing of heliospheric conditions throughout the ∼11 yr sunspot cycle and ∼22 yr solar magnetic cycle. A neutron monitor (NM) is a stable ground-based detector that measures cosmic-ray rate variations above a geomagnetic or atmospheric cutoff rigidity with high precision (∼0.1%) over such timescales. Furthermore, we developed electronics and analysis techniques to indicate variations in the cosmic-ray spectral index using neutron time-delay data from a single station. Here we study solar modulation using neutron time-delay histograms from two high-altitude NM stations: (1) the Princess Sirindhorn Neutron Monitor at Doi Inthanon, Thailand, with the world's highest vertical geomagnetic cutoff rigidity, 16.7 GV, from 2007 December to 2018 April; and (2) the South Pole NM, with an atmosphere-limited cutoff of ∼1 GV, from 2013 December to 2018 April. From these histograms, we extract the leader fraction L, i.e., inverse neutron multiplicity, as a proxy of a GCR spectral index above the cutoff. After correction for pressure and precipitable water vapor variations, we find that L roughly correlates with the count rate but also exhibits hysteresis, implying a change in spectral shape after a solar magnetic polarity reversal. Spectral variations due to Forbush decreases, 27 day variations, and a ground-level enhancement are also indicated. These methods enhance the high-precision GCR spectral information from the worldwide NM network and extend it to higher rigidity.