Observation of the Identical Rigidity Dependence of He, C, and O Cosmic Rays at High Rigidities by the Alpha Magnetic Spectrometer on the International Space Station

We report the observation of new properties of primary cosmic rays He, C, and O measured in the rigidity (momentum/charge) range 2 GV to 3 TV with $90\times {10}^6$ helium, $8.4\times {10}^6$ carbon, and $7.0\times {10}^6$ oxygen nuclei collected by the Alpha Magnetic Spectrometer (AMS) during the first five years of operation. Above 60 GV, these three spectra have identical rigidity dependence. They all deviate from a single power law above 200 GV and harden in an identical way.

Figure 1

The AMS (a) helium, (b) carbon, and (c) oxygen fluxes [17] multiplied by ${\tilde{R}}^{2.7}$ with their total errors as functions of rigidity. Earlier measurements of helium, see Fig. 4 in Ref. [28], and carbon [12] fluxes in rigidity are also shown. (d) The dependence of the helium, carbon, and oxygen spectral indices on rigidity. In (d), for clarity, the horizontal positions of the helium and oxygen data points are displaced with respect to carbon. As seen, above 60 GV (indicated by the unshaded region) the spectral indices are identical.

Figure 2

The AMS (a) helium, (b) carbon, and (c) oxygen fluxes as functions of kinetic energy per nucleon $E_K$ multiplied by $E_K^{2.7}$ together with previous measurements [4, 5, 6, 7, 8, 9, 10, 11, 12, 28, 29]. For the AMS measurement $E_K=(\sqrt{Z^2{\tilde{R}}^2+M^2}-M)/A$ where $Z$, $M$, and $A$ are the ${}^4\mathrm{He}$, ${}^{12}\mathrm{C}$, or ${}^{16}\mathrm{O}$ charge, mass, and atomic mass numbers, respectively. Data from other experiments were extracted using Ref. [30].

Figure 3

The (a) $\mathrm{He}/\mathrm{O}$ and (b) $\mathrm{C}/\mathrm{O}$ ratios as functions of rigidity compared with the galprop model predictions [31], dashed red curves. The solid blue lines indicate the fits of constant values to the ratios with the dashed blue lines their total errors ($1\sigma$). As seen, above 60 GV both the $\mathrm{He}/\mathrm{O}$ and $\mathrm{C}/\mathrm{O}$ ratios are well described by constant values of $27.9\pm 0.6$ and $0.91\pm 0.02$ with ${\chi }^2/\mathrm{d}.\mathrm{o}.\mathrm{f}.=16/27$ and ${\chi }^2/\mathrm{d}.\mathrm{o}.\mathrm{f}.=25/27$, respectively.

Figure 4

The rigidity dependence of the helium (left black axis), carbon (left green axis), and oxygen (right red axis) fluxes. For clarity, horizontal positions of the helium and oxygen data points above 400 GV are displaced with respect to the carbon. As seen, above 60 GV the three fluxes have identical rigidity dependence.