New Method of Assessment of the Integral Fluence of Solar Energetic (> 1 GV Rigidity) Particles from Neutron Monitor Data

Abstract

A new method to reconstruct the high-rigidity part (≥ 1 GV) of the spectral fluence of solar energetic particles (SEP) for GLE events, based on the world-wide neutron monitor (NM) network data, is presented. The method is based on the effective rigidity Reff and scaling factor Keff. In contrast to many other methods based on derivation of the best-fit parameters of a prescribed spectral shape, it provides a true non-parametric (viz. free of a priori assumptions on the exact spectrum) estimate of fluence. We reconstructed the SEP fluences for two recent GLE events, #69 (20 Jan. 2005) and #71 (17 May 2012), using four NM yield functions: (CD00 – Clem and Dorman in Space Sci. Rev.93, 335, 2000), (CM12 – Caballero-Lopez and Moraal in J. Geophys. Res.117, A12103, 2012), (Mi13 – Mishev, Usoskin, and Kovaltsov in J. Geophys. Res.118, 2783, 2013), and (Ma16 – Mangeard et al. in J. Geophys. Res.121, 7435, 2016b). The results were compared with full reconstructions and direct measurements by the PAMELA instrument. While reconstructions based on Mi13 and CM12 yield functions are consistent with the measurements, those based on CD00 and Ma16 ones underestimate the fluence by a factor of 2 – 3. It is also shown that the often used power-law approximation of the high-energy tail of SEP spectrum does not properly describe the GLE spectrum in the NM-energy range. Therefore, the earlier estimates of GLE integral fluences need to be revised.