Abstract
Cosmic rays (CRs) are charged particles with energies above 1 MeV, accelerated in sources outside of the Earth’s magnetosphere. The variability of their fluxes, caused by solar modulation and solar eruptive events, is an important field of astroparticle physics, and represents the main focus of this study.
The main instruments to study the cosmic-ray variability are neutron monitors (NMs), located at different locations around the globe, and space-borne experiments. A NM is an integral detector so that its response is integrally related to CR fluxes via NM response function. Space-borne experiments have particle detectors that allow to directly register different CR particles.
In this work both data from NMs and space-borne experiments are combined to study the variability of CRs. In particular, the reconstruction of the solar modulation potential ϕ applying the simplified force-field (FF) model of the solar modulation of cosmic rays was performed using in-situ cosmic-ray fluxes measured by both the PAMELA and AMS-02 experiments together with NM data for the period from 2006 to 2017. Validation of the FF model for periods of different solar activity levels was further performed, and it was found that such an approximation performs better during solar minima, but disagrees with the observations of up to ≈ 10% during solar maximum. This makes the FF model approach not well suited for detailed studies of the solar modulation processes. At the same time, this precision is adequate to quantify the condition of the heliospheric modulation and to study its long-term variability.
To study solar energetic particle (SEP) fluxes using NM data, a new method of “effective rigidity” was proposed, allowing to reconstruct high-energy SEP integral fluences recorded during ground level enhancement (GLE) events. A significant advantage of this novel method is that it is a non parametric one and thus the spectral shape of the SEP fluence can be deduced directly from the reconstructed data. Reconstructions of the SEP fluences for two recent GLEs, #69 and #71, using this newly developed method, yield a very good agreement with the laborious method of the full fluence reconstruction using NM data, and with PAMELA measurements (for GLE #71), but disagree with earlier simplified estimations based on NM data.
The NM data analysis was performed using the NM yield function by Mishev et al. [2013], which was validated using the AMS-02 data for protons and helium for a time period between 2011 to 2017 and showed the best performance among other modern yield functions.
Improved knowledge of the CR variability is crucially important for e.g. CR-induced atmospheric effects, including the production of cosmogenic isotopes
