New Insights into Cosmic-Ray Variations from Antarctic Neutron Monitors

Recent research has focused on the variations in the inferred cosmic-ray spectral index, utilizing data from neutron monitors located in Antarctica. This study, titled "Variations in the Inferred Cosmic-Ray Spectral Index as Measured by Neutron Monitors in Antarctica," was conducted by a team of researchers led by Pradiphat Muangha and includes contributions from 38 other authors. The analysis covers data collected from four Antarctic neutron monitors between March 2015 and September 2023.

The researchers developed a technique that tracks short-term spectral variations in Galactic cosmic rays (GCRs) by analyzing the time delay between successive neutron counts. They extracted a parameter known as the leader fraction (L), which serves as a proxy for the spectral index. The study calibrated L from the South Pole neutron monitor against a daily spectral index derived from published data of GCR proton fluxes from the Alpha Magnetic Spectrometer (AMS-02) aboard the International Space Station.

Key findings indicate a robust correlation between the leader fraction and the spectral index fit over a rigidity range of 2.97 to 16.6 GV. The results also highlight the influence of solar activity cycles on cosmic ray flux, revealing a 27-day periodicity corresponding to solar rotation, particularly noted during sunspot minima. Additionally, the study found that the magnetic field structure along a nominal Parker spiral significantly impacts spectral and flux variations, with solar wind speed also playing a role.

This research underscores the capability of ground-based neutron monitor stations to continuously and accurately monitor cosmic ray spectral variations, which could have implications for understanding cosmic ray behavior and its effects on Earth. The full paper can be accessed at arXiv:2408.13999.