Variations in the Inferred Cosmic-Ray Spectral Index as Measured by Neutron Monitors in Antarctica

TL;DR

This study demonstrates that neutron monitor data can reliably track short-term variations in cosmic-ray spectral indices, correlating well with space-based measurements and revealing solar rotation effects over multiple years.

Contribution

It introduces a calibration method linking neutron monitor leader fraction to cosmic-ray spectral indices, validating ground-based spectral monitoring against AMS-02 data.

Findings

Strong correlation between leader fraction and spectral index (uncertainty 0.018)

Detection of 27-day solar rotation periodicity in cosmic-ray data

Magnetic sector structure influences spectral and flux variations

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 during 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) aboard 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 0.018 in the daily spectral index as inferred from $L$. In addition to the 11-year 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 exhibited a strong harmonic at 13.5 days, and that 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 in the long-term future.