Properties of Neon, Magnesium, and Silicon Primary Cosmic Rays Results from the Alpha Magnetic Spectrometer

We report the observation of new properties of primary cosmic rays, neon (Ne), magnesium (Mg), and silicon (Si), measured in the rigidity range 2.15 GV to 3.0 TV with $1.8\times {10}^6\mathrm{Ne}$, $2.2\times {10}^6\mathrm{Mg}$, and $1.6\times {10}^6\mathrm{Si}$ nuclei collected by the Alpha Magnetic Spectrometer experiment on the International Space Station. The Ne and Mg spectra have identical rigidity dependence above 3.65 GV. The three spectra have identical rigidity dependence above 86.5 GV, deviate from a single power law above 200 GV, and harden in an identical way. Unexpectedly, above 86.5 GV the rigidity dependence of primary cosmic rays Ne, Mg, and Si spectra is different from the rigidity dependence of primary cosmic rays He, C, and O. This shows that the Ne, Mg, and Si and He, C, and O are two different classes of primary cosmic rays.

Figure 1

(a) Ne and Mg fluxes multiplied by ${\tilde{R}}^{2.7}$ and $\mathrm{Ne}/\mathrm{Mg}$ flux ratio, and (b) Si and Mg fluxes multiplied by ${\tilde{R}}^{2.7}$ and $\mathrm{Si}/\mathrm{Mg}$ flux ratio with their total errors as functions of rigidity. For display purposes only, the Ne and Si fluxes were rescaled as indicated. For clarity, Ne and Si data points above 400 GV are displaced horizontally. The solid curves show the fit results with Eq. (2). As seen, the Ne and Mg fluxes have identical rigidity dependence above 3.65 GV and the three fluxes have identical rigidity dependence above 86.5 GV, as indicated by the location of the arrows.

Figure 2

The AMS (a) neon, (b) magnesium, and (c) silicon fluxes as functions of kinetic energy per nucleon $E_K$ multiplied by $E_K^{2.7}$ together with earlier measurements. For the AMS measurement $E_K=(\sqrt{Z^2{\tilde{R}}^2+M^2}-M)/A$, where $Z$, $M$, and $A$ are ${}_{10}{}^{20}\mathrm{Ne}$, ${}_{12}{}^{24}\mathrm{Mg}$, and ${}_{14}{}^{28}\mathrm{Si}$ nuclei charge, mass, and atomic mass numbers, respectively.

Figure 3

The dependence of the Ne, Mg, and Si spectral indices on rigidity. For clarity, the Ne and Si data points are displaced horizontally. As seen, the Ne and Mg spectral indices are identical in this rigidity range and the three flux spectral indices harden identically with rigidity above $\sim 200\mathrm{GV}$.

Figure 4

The AMS $\mathrm{Ne}/\mathrm{O}$, $\mathrm{Mg}/\mathrm{O}$, and $\mathrm{Si}/\mathrm{O}$ flux ratio spectral indices obtained with fits of Eq. (4) as a function of rigidity. For clarity, $\mathrm{Ne}/\mathrm{O}$ and $\mathrm{Si}/\mathrm{O}$ spectral indices data points are displaced horizontally. The vertical dashed line shows the interval boundary of 86.5 GV. As seen, above 86.5 GV all spectral indices are identical with average value $⟨\delta ⟩=-0.045\pm 0.008$.

Figure 5

The rigidity dependence of the Ne, Mg, and Si fluxes compared to rigidity dependence of the He, C, and O fluxes from Ref. [30] above 86.5 GV. For display purposes only, the He, C, O, Ne, and Si fluxes were rescaled as indicated. For clarity, He, O, Ne, and Si data points above 400 GV are displaced horizontally. The green shaded area shows the fit result of He, C, and O fluxes from Ref. [30] with Eq. (5) together with fit errors [16]. The magenta shaded area shows the fit result of Ne, Mg, and Si fluxes from Ref. [16] with Eq. (5) when varying ${\gamma }_{\mathrm{NeMgSi}}={\gamma }_{\mathrm{HeCO}}+⟨\delta ⟩$, by $\pm 0.008$, from the value of $⟨\delta ⟩=-0.045\pm 0.008$.