Origin of the Proton-to-Helium Ratio Anomaly in Cosmic Rays

Understand

Recent data on Galactic cosmic rays (CRs) revealed that the helium energy spectrum is harder than the proton spectrum.

• The AMS experiment has now reported that the proton-to-helium ratio as function of rigidity $R$ (momentum-to-charge ratio) falls off steadily as p/He $\sim R^\Delta$, with $\Delta\approx$-0.08 between $R\sim$40 GV and $R\sim$2 TV.
• Besides, the single spectra of proton and helium are found to progressively harden at $R\gtrsim$100 GV.
• The p/He anomaly is generally ascribed to particle-dependent acceleration mechanisms occurring in Galactic CR sources.