Figure 1 shows the proton and helium energy spectra measured by PAMELA in the energy range 1 GeV – 1 TeV and 1 GeV – 450 GeV, respectively. The discrepancies with other experiments at energies lower than 30 GeV are due to the different periods of solar activity in which data have been collected.
A spectral hardening appears in the two slopes at the highest energies explored, challenging the present paradigm of acceleration of cosmic rays by a single supernovae remnant. These sudden changes appears more evident at 230 – 240 GV, for proton and helium respectively, as shown in Figure 2, where the data are plotted as a function of rigidity. They could be interpreted as an indication of different populations of cosmic ray sources, as novae stars and explosions in superbubbles.
Figure 3 shows the ratio of the proton to helium fluxes versus rigidity. A clear difference between the proton and helium slopes is evident, showing that the helium slope is harder than the proton one. The ratio exhibits a continuous and smooth decrease and it is well described by a power law down to 5 GV with a spectral index of 0.1.