The deviation from the power law, observed below 10 GeV, is a consequence of the influence of the solar wind called solar modulation [Gle68]. Flux intensity in this energy range is anti-correlated to the solar activity and follows the sun-spot 11-year cycle. The correlation between the solar activity and the modulation of the cosmic rays flux has been studied by monitoring the flux of atmospheric neutrons. In fact, a flux of low energy neutrons (E ~ 1.E8-1.E9 eV) is produced in the interaction of primary CRs with the atmosphere and it is mostly due to low energy primaries (1-20 GeV), due to the rapid fall of the primary flux intensity with energy. One assumes that far from the solar system there exists an unmodified flux called Local Interstellar Spectrum, which is modified within the solar system by the interaction with the solar wind. This interaction is well described by the Fokker-Planck diffusion equation. Describing the solar wind by a set of magnetic irregularities, and considering these irregularities as perfect elastic scattering centres, one obtains the Fokker-Planck diffusion equation. For energies above 100 MeV this equation can be solved using the "Force Field Approximation" [Cab04]. According to this approximation, at a given distance from the Sun, for example at 1 a.u., the population of CRs at energy E_interstellar is shifted at the energy E_0 as in an energy loss mechanism due to a potential V: E_0 = E_interstellar + Z . V_solarwind(t) The solar wind potential at a given distance from the Sun depends on only one parameter, the time: V = V(t). So it doesn't matter what the interstellar flux is: given a flux on the Earth at a time t, one can find the flux at another time just from the relative variation of the solar wind potential Phi. This variation can be derived from the neutron monitor counts [Bad96]. In the case of the fit used by FLUKA, an offline code [Bad96] makes use of an algorithm which takes into account a specific Phi value, or the counting rate of the CLIMAX neutron monitor [CLIMAX] to provide the prediction for the flux at a specific date or for a given value of the potential which expresses the effect of the interplanetary modulation of the local interstellar spectrum. Even if the model is not a description of the processes and of the manner in which they occur, it reasonably predicts the GCR modulation at Earth.
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