The flux of Galactic cosmic rays (GCRs) can undergo a Forbush decrease (FD) during the passage of a shock, sheath region, or magnetic flux rope associated with a coronal mass ejection (CME). Cosmic-ray observations during FDs can provide information complementary to in situ observations of the local plasma and magnetic field, because cosmic-ray distributions allow remote sensing of distant conditions. Here we develop techniques to determine the GCR anisotropy before and during an FD using data from the worldwide network of neutron monitors, for a case study of the FD starting on 2013 April 13. We find that at times with strong magnetic fluctuations and strong cosmic-ray scattering, there were spikes of high perpendicular anisotropy and weak parallel anisotropy. In contrast, within the CME flux rope there was a strong parallel anisotropy in the direction predicted from a theory of drift motions into one leg of the magnetic flux rope and out the other, confirming that the anisotropy can remotely sense a large-scale flow of GCRs through a magnetic flux structure.

Ionization in Earth's troposphere is mainly due to Galactic cosmic rays. Occasionally, solar storms produce intense relativistic ion beams that significantly increase such ionization. One of the largest recorded solar radiation storms, on 20 January 2005, resulted in up to 55‐fold increases in the count rates of ground‐based particle detectors in polar regions. We use McMurdo and Inuvik neutron monitor data to estimate accurate time profiles of ion energy spectra above the atmosphere at each location. Using data‐driven atmospheric models, we perform Monte Carlo simulations of particle‐air interactions and calculate atmospheric ionization and potential biological dosage versus altitude and time for each location. We found that if airplane passengers had traversed the south polar region, they could have been exposed to the typical annual cosmic radiation dosage at sea level within 1 h. These techniques can help evaluate possible influences of solar activity on atmospheric properties.

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The flux of Galactic cosmic rays (GCRs) can undergo a Forbush decrease (FD) during the passage of a shock, sheath region, or magnetic flux rope associated with a coronal mass ejection (CME). Cosmic-ray observations during FDs can provide information complementary to in situ observations of the local plasma and magnetic field, because cosmic-ray distributions allow remote sensing of distant conditions. Here we develop techniques to determine the GCR anisotropy before and during an FD using data from neutron monitors worldwide. For example, for the FD starting on 2013 April 13, we find that at times with strong magnetic fluctuations and strong cosmic-ray scattering, there were spikes of high perpendicular anisotropy and weak parallel anisotropy. These results, along with the near constancy of parallel anisotropy across magnetic field reversals, are consistent with diffusive barriers causing the decrease in GCR flux before the arrival of the flux rope. In contrast, within the CME flux rope there was a strong parallel anisotropy in the direction predicted from a theory of drift motions into one leg of the magnetic flux rope and out the other, confirming that the anisotropy can remotely sense a large-scale flow of GCRs through a magnetic flux rope.