Charge-dependent spectral softenings of primary cosmic rays below the knee

In most particle acceleration or propagation theories, the characteristic featuresof the cosmic ray spectra due to acceleration limits or propagation phase changesare charge-dependent 1–4. Alternatively, the interaction scenario would expectmass-dependent spectral features in general. The observational verification of whichrelation takes effect in nature is still lacking because of the difficulty in measuringthe spectra of individual particles up to very high energies. Here we report directmeasurements of the carbon, oxygen and iron spectra from about 20 gigavolts toaround 100 teravolts (60 teravolts for iron) with 9 years of on-orbit data collected bythe Dark Matter Particle Explorer. Distinct spectral softenings have been directlydetected in these spectra for the first time, to our knowledge. Combined with theupdated proton and helium spectra, the spectral softening appears universally ata rigidity of about 15 teravolts. A nuclei-mass-dependent softening is rejected at aconfidence level of >99.999%. Possible interpretations of these results, includinga nearby cosmic ray source5–7 and other models such as the propagation effect8, arediscussed.