Variations in the Inferred Cosmic-Ray Spectral Index as Measured by Neutron Monitors in Antarctica
1
Issued Date
2024-10-01
Resource Type
ISSN
0004637X
eISSN
15384357
Scopus ID
2-s2.0-85207306551
Journal Title
Astrophysical Journal
Volume
974
Issue
2
Rights Holder(s)
SCOPUS
Bibliographic Citation
Astrophysical Journal Vol.974 No.2 (2024)
Suggested Citation
Muangha P., Ruffolo D., Sáiz A., Banglieng C., Evenson P., Seunarine S., Oh S., Jung J., Duldig M.L., Humble J.E. Variations in the Inferred Cosmic-Ray Spectral Index as Measured by Neutron Monitors in Antarctica. Astrophysical Journal Vol.974 No.2 (2024). doi:10.3847/1538-4357/ad73d6 Retrieved from: https://repository.li.mahidol.ac.th/handle/123456789/101837
Title
Variations in the Inferred Cosmic-Ray Spectral Index as Measured by Neutron Monitors in Antarctica
Corresponding Author(s)
Other Contributor(s)
Abstract
A technique has recently been developed for tracking short-term spectral variations in Galactic cosmic rays (GCRs) using data from a single neutron monitor (NM), by collecting histograms of the time delay between successive neutron counts and extracting the leader fraction L as a proxy of the spectral index. Here we analyze L from four Antarctic NMs from 2015 March to 2023 September. We have calibrated L from the South Pole NM with respect to a daily spectral index determined from published data of GCR proton fluxes during 2015-2019 from the Alpha Magnetic Spectrometer (AMS-02) on board the International Space Station. Our results demonstrate a robust correlation between the leader fraction and the spectral index fit over the rigidity range 2.97-16.6 GV for AMS-02 data, with uncertainty of 0.018 in the daily spectral index as inferred from L. In addition to the 11 yr solar activity cycle, a wavelet analysis confirms a 27 day periodicity in the GCR flux and spectral index corresponding to solar rotation, especially near sunspot minimum, while the flux occasionally exhibits a strong harmonic at 13.5 days. The magnetic field component along a nominal Parker spiral (i.e., the magnetic sector structure) is a strong determinant of such spectral and flux variations, with the solar wind speed exerting an additional, nearly rigidity-independent influence on flux variations. Our investigation affirms the capability of ground-based NM stations to accurately and continuously monitor cosmic-ray spectral variations over the long-term future.