Abstract
We revise the published neutron monitor raw data for the increase caused by the solar neutron event of the 24 May 1990. With these data we calculate the attenuation length, λ, of solar neutrons in the Earth's atmosphere assuming either a minimum path as given by the spread of elastically scattered neutrons, or using the minimum mass path estimated by Smart, Shea, and O'Bren (1995) due to an atmospheric refraction effect. In both cases λ reduces to a value around 100 g cm−2, which is more in accordance with data on neutron cross-sections (Shibata, 1994). These two phenomenological calculations suggest that solar neutrons do not propagate in straight lines in the atmosphere. The previous estimate of the attenuation length, λ=208 g cm−2, was calculated assuming straight-ahead transport (Smart, Shea, and O'Bren, 1995). Dorman, Valdes-Galicia, and Dorman (1999) performed a numerical simulation and an analytical approximation to the problem of solar neutron scattering and attenuation in the Earth's atmosphere. These solutions incorporate the refraction effect as a natural consequence of the greater absorption experienced by neutrons scattered to large zenith angles. They are able to reproduce the normalised observed counting rates of neutron monitors for this event.
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Valdés-Galicia, J., Dorman, L. & Rodríguez, M. Atmospheric refraction of solar neutrons during the event of 24 May 1990. Solar Physics 191, 409–417 (2000). https://doi.org/10.1023/A:1005248932125
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DOI: https://doi.org/10.1023/A:1005248932125