At energies below 5 GeV, the PAMELA measurement of the positron fraction deviates significantly from previous measurements. In this regime, the fluxes of cosmic-ray particles are modulated due to interactions with the solar wind when they arrive at the outskirts of the solar system. The solar wind originates from the corona of the Sun. A magnetic field, rooted in the Sun, is frozen into the solar wind plasma, and the Sun's rotation leads to the creation of the large-scale structure in form of an Archimedes spiral. Cosmic-ray particles are scattered on the magnetic fields.
Solar modulation effects can only be studied by measuring at low energies, roughly between 0.1GeV and 10GeV. Due to the geomagnetic field, cosmic-ray particles in this energy range cannot reach locations near the equator, for example the International Space Station, but can only be observed in the vicinity of the Earth's poles.
|Geomagnetic cutoff for the ISS orbit and at starting locations near the Earth's poles ||Effective live time fraction as a function of energy for an experiment launched from the South pole, compared to the ISS orbit |