*1.3.1 Galactic cosmic rays (GCR)*

Collectively, the particles that make up cosmic radiation are called Galactic Cosmic Rays (GCR). They are baryons (mainly hydrogen protons (83%) and alpha particles, as well as helium (14%) and heavy (1%) nuclei [14]) and electrons that travel in space and are present everywhere. A substantial part of GCR seems to originate from supernova remnants [15, 16] and GCR are believed to be accelerated from outside the Solar System by neutron stars, black holes and supernovae shocks [17]. The mechanism guiding particle acceleration was first proposed by Fermi who explained the energy transfer from magnetized clouds to individual particles [18]. The Fermi I mechanism, also called the diffusive shock acceleration, applied to a strong shock such as from a supernova explosion predicts a power law particle spectrum which has been observed [18].

The magnitude of the GCR spectrum as observed on Earth, and in the rest of the Solar System is correlated with the solar cycle. When the Sun is most active, the enhanced solar magnetic field causes GCR particles to lose some of their energy, and the lower energy particles are affected the most. As such, the fluence of particles of a few GeV/u drops by up to 20% [14]. GCR models account for this relationship with help of the solar modulation parameter [19, 20]. Such models reconstruct the flux of particles mainly from observations, and the most widely used model is the Badhwar-O'Neill (BON) [21]—BON2014 model. Recently, an improved version has been released, BON2020 which reduces model errors largely owning to revised methods of using the solar modulation potential and calibrating free parameters in the local interstellar spectrum for all GCR ions [22].
