Precision Measurement of Cosmic-Ray Nitrogen and its Primary and Secondary Components with the Alpha Magnetic Spectrometer on the International Space Station

A precision measurement of the nitrogen flux with rigidity (momentum per unit charge) from 2.2 GV to 3.3 TV based on $2.2\times {10}^6$ events is presented. The detailed rigidity dependence of the nitrogen flux spectral index is presented for the first time. The spectral index rapidly hardens at high rigidities and becomes identical to the spectral indices of primary He, C, and O cosmic rays above $\sim 700\mathrm{GV}$. We observed that the nitrogen flux ${\mathrm{\Phi }}_{\mathrm{N}}$ can be presented as the sum of its primary component ${\mathrm{\Phi }}_{\mathrm{N}}^P$ and secondary component ${\mathrm{\Phi }}_{\mathrm{N}}^S$, ${\mathrm{\Phi }}_{\mathrm{N}}={\mathrm{\Phi }}_{\mathrm{N}}^P+{\mathrm{\Phi }}_{\mathrm{N}}^S$, and we found ${\mathrm{\Phi }}_{\mathrm{N}}$ is well described by the weighted sum of the oxygen flux ${\mathrm{\Phi }}_{\mathrm{O}}$ (primary cosmic rays) and the boron flux ${\mathrm{\Phi }}_{\mathrm{B}}$ (secondary cosmic rays), with ${\mathrm{\Phi }}_{\mathrm{N}}^P=(0.090\pm 0.002)\times {\mathrm{\Phi }}_{\mathrm{O}}$ and ${\mathrm{\Phi }}_{\mathrm{N}}^S=(0.62\pm 0.02)\times {\mathrm{\Phi }}_{\mathrm{B}}$ over the entire rigidity range. This corresponds to a change of the contribution of the secondary cosmic ray component in the nitrogen flux from 70% at a few GV to $<30\%$ above 1 TV.

Figure 1

(a) The AMS nitrogen flux ${\mathrm{\Phi }}_{\mathrm{N}}$ [16] multiplied by ${\tilde{R}}^{2.7}$ with total errors as a function of rigidity. (b) The dependence of the nitrogen spectral index on rigidity together with the rigidity dependence of the spectral indices of primary He, C, and O cosmic rays and secondary Li, Be, and B cosmic rays. For clarity, the horizontal positions of the Li and B data points and He and O data points are displaced with respect to the Be and C data points, respectively. The shaded regions are to guide the eye. As seen, the nitrogen spectral index is situated between the primary and secondary cosmic ray spectral indices, hardens rapidly with rigidity above $\sim 100\mathrm{GV}$ and becomes identical to the spectral indices of the primary cosmic rays above $\sim 700\mathrm{GV}$.

Figure 2

The AMS nitrogen flux ${\mathrm{\Phi }}_{\mathrm{N}}$ as a function of kinetic energy per nucleon $E_K$ multiplied by $E_K^{2.7}$ together with earlier measurements [4, 5, 6, 7] and with the predictions of cosmic ray propagation model galprop [10] (dashed red curve).

Figure 3

(a) The AMS nitrogen flux ${\mathrm{\Phi }}_{\mathrm{N}}$ fit to the weighted sum of the oxygen flux ${\mathrm{\Phi }}_{\mathrm{O}}$ and the boron flux ${\mathrm{\Phi }}_{\mathrm{B}}$ over the entire rigidity range. (b) The AMS $({\mathrm{\Phi }}_{\mathrm{N}}/{\mathrm{\Phi }}_{\mathrm{O}})$ ratio as a function of rigidity. (c) The AMS $({\mathrm{\Phi }}_{\mathrm{N}}/{\mathrm{\Phi }}_{\mathrm{B}})$ ratio as a function of rigidity. The contributions of the primary and secondary components are indicated by the shading (yellow and green, respectively). As seen from (b) and (c), the contribution of the secondary component in the nitrogen flux decreases, and the contribution of the primary component correspondingly increases, with rigidity.

Figure 4

Comparison of the AMS measurements of the primary cosmic ray fluxes [8] and the secondary cosmic rays fluxes [9] with the nitrogen flux [16] multiplied by ${\tilde{R}}^{2.7}$ with their total errors as functions of rigidity above 30 GV. For display purposes only, the C, O, Li, Be, B, and N fluxes were rescaled as indicated. For clarity, the horizontal positions of the He, O, Li, and B data points above 400 GV are displaced. As seen, the three secondary fluxes have identical rigidity dependence above 30 GV as do the three primary fluxes above 60 GV, but they are different from each other. The rigidity dependence of the nitrogen flux is distinctly different from the dependence of both the primary fluxes and the dependence of the secondary fluxes.