Approximation of the reception coefficients of cosmic rays neutron component for latitude measurement

When solving scientific and applied problems, such as latitude monitoring, it is important to correctly exclude primary cosmic ray variations from observation data. Therefore, the purpose of this study was to develop and implement a method for correcting monitoring data, the key point of which was to obtain reception coefficients as a function of latitude. This resulted in an approximation of the rigidity dependence of the zeroth and first harmonic cosmic rays anisotropy coefficients, calculated for a ground-based cosmic ray detectors network. Analysis of the obtained results showed that the approximation was performed with high accuracy, and the results are suitable for use in latitude measurements during marine expeditions.

💡 Research Summary

The paper addresses a practical problem in latitude determination during marine expeditions: the need to remove primary cosmic‑ray (CR) variations from ground‑based neutron monitor observations. Existing methods provide “reception coefficients” that relate variations outside the magnetosphere to those measured at a detector, but these coefficients have only been calculated at discrete locations (Yasue et al., 1982). This limits their use for vessels traversing continuously varying latitudes.

The authors first review the Global Survey Method (GSM), which combines three classic approaches – the coupling‑coefficient method (atmospheric effects), trajectory calculations in the geomagnetic field (magnetospheric effects), and spherical harmonic analysis (anisotropy extraction). In GSM, the zero‑order amplitude (isotropic part) and the first‑order spherical harmonic vector ξ are determined for each hour. The observed variation ν₀₀ at a detector i is expressed as a linear combination of ξ and the anisotropy amplitude a, weighted by reception coefficients C₀₀ (zero harmonic) and C₁₁ (first harmonic). The paper details the coordinate transformations from the geophysical GEO system to the terrestrial GEO′ system, the rotation by the local hour angle ϕ = 2π/24(t + ½), and the inclusion of the particle drift angle φ₁₁, which together define the direct problem (predicting ground‑level variations) and the inverse problem (recovering ξ and a from observations).

The data set consists of the discrete coefficients published by Yasue et al. (1982) for neutron monitors and by Fujimoto et al. (1984) for muon telescopes. These coefficients were computed for several power‑law spectral indices γ (0.5, 1.0, 1.5) and for an upper rigidity of modulation R_U = 100 GV, assuming the geomagnetic field configuration of 1975. The authors note that the geomagnetic field has evolved over the past five decades, with a global decrease of about 0.2 GV in cutoff rigidity and anomalous trends in the South and North Atlantic.

To make the coefficients usable at arbitrary latitudes, the authors fit them with a non‑linear Granitsky‑Dorman function:

C(R_c) = exp