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Abstract Observations of temporary Forbush decreases (FDs) in the Galactic cosmic-ray (GCR) flux due to the passage of solar storms are useful for space-weather studies and alerts. Here, we introduce techniques that use global networks of ground-based neutron monitors and muon detectors to measure variations of GCR rigidity spectra in space during FDs by (1) fitting count rate decreases for power-law rigidity spectra in space with anisotropy up to second order and (2) using the “leader fraction” derived from a single neutron monitor. We demonstrate that both provide consistent results for hourly spectral index variations for five major FDs, and they agree with daily space-based data when available from the Alpha Magnetic Spectrometer. We have also made the neutron monitor leader fraction publicly available in real time. This work verifies that ground-based observations can be used to precisely monitor GCR spectral variation over a wide range of rigidities during space-weather events, with results in real time or from short-term postanalysis. 

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 fractionLas a proxy of the spectral index. Here we analyzeLfrom four Antarctic NMs from 2015 March to 2023 September. We have calibratedLfrom 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 fromL. 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.