The altitude profile of the cosmic ray atmospheric cutoff
July 07, 2021
Neutron monitors are the main ground-based instruments for continuous measurements the cosmic-ray intensity operating over more than five decades. Those instruments are energy-integrating detectors with count rates governed by the atmospheric and geomagnetic cutoffs. The geomagnetic cutoff dominates (up to 17 GV in rigidity) over most of the globe. However, it is negligible in the polar regions, and there, the atmospheric cutoff is important. The atmospheric cutoff depends on the elevation of the instrument above sea level (on the atmospheric depth), and it is estimated as ~1 GV for cosmic-ray protons at sea level. However, the cutoff is not precisely known at higher altitudes. This is specifically important for studies based on high-altitude polar neutron monitors, which count rate is solely defined by the atmospheric cutoff.
We present a newly estimated altitude profile of the atmospheric cutoff for cosmic-ray protons, which can be used in analysis of both galactic cosmic rays and solar energetic particles. We computed the profile using two methods. The first one is based on Monte Carlo simulation of the cosmic-ray induced cascade in the atmosphere with the PLANETOCOSMICS code. The second one uses recently computed and verified neutron monitor yield function by Mishev et al., 2020, which considers the efficiency of the instrument. Both methods agree reasonably well, though the yield-function based one provides a more conservative result, as expected. There are two definitions of solar-particle sub-GLE (sub-Ground-Level-Enhancement) events by Raukunen et al. (2018) and Poluianov et al. (2018) based on different principles. Considering the derived in this study atmospheric cutoff at altitudes about 3000 m a.s.l., we conclude: there is no contradiction between the definitions.
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