Properties of Cosmic Helium Isotopes Measured by the Alpha Magnetic Spectrometer

Precision measurements by the Alpha Magnetic Spectrometer (AMS) on the International Space Station of ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ fluxes are presented. The measurements are based on 100 million ${}^4\mathrm{He}$ nuclei in the rigidity range from 2.1 to 21 GV and 18 million ${}^3\mathrm{He}$ from 1.9 to 15 GV collected from May 2011 to November 2017. We observed that the ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ fluxes exhibit nearly identical variations with time. The relative magnitude of the variations decreases with increasing rigidity. The rigidity dependence of the ${}^3\mathrm{He}/{}^4\mathrm{He}$ flux ratio is measured for the first time. Below 4 GV, the ${}^3\mathrm{He}/{}^4\mathrm{He}$ flux ratio was found to have a significant long-term time dependence. Above 4 GV, the ${}^3\mathrm{He}/{}^4\mathrm{He}$ flux ratio was found to be time independent, and its rigidity dependence is well described by a single power law $\propto R^{\mathrm{\Delta }}$ with $\mathrm{\Delta }=-0.294\pm 0.004$. Unexpectedly, this value is in agreement with the B/O and B/C spectral indices at high energies.

Figure 1

The AMS ${}^3\mathrm{He}$ (red points, right scale) and ${}^4\mathrm{He}$ (blue points, left scale) fluxes as functions of time for five rigidity bins (a) from 2.15 to 2.40 GV, (b) from 2.97 to 3.29 GV, (c) from 3.64 to 4.02 GV, (d) from 4.02 to 4.43 GV, and (e) from 14.1 to 15.3 GV. The errors are the quadratic sum of the statistical and time-dependent systematic errors.

Figure 2

The AMS ${}^3\mathrm{He}/{}^4\mathrm{He}$ flux ratio as a function of time for five rigidity bins (a) from 2.15 to 2.40 GV, (b) from 2.97 to 3.29 GV, (c) from 3.64 to 4.02 GV, (d) from 4.02 to 4.43 GV, and (e) from 14.1 to 15.3 GV. The errors are the quadratic sum of the statistical and time-dependent systematic errors. The dashed lines show fit results of Eq. (2) below 4 GV and a constant fit above 4 GV.

Figure 3

The AMS ${}^3\mathrm{He}/{}^4\mathrm{He}$ ratio with total errors as a function of the kinetic energy per nucleon, together with earlier experiments [13]. The dashed line shows the prediction of the Galprop model [29] with the average solar modulation, in the force-field approximation [30], corresponding to the data collection period.

Figure 4

(a) The time-averaged ${}^3\mathrm{He}/{}^4\mathrm{He}$ flux ratio as a function of rigidity $R$ (red points) with statistical and uncorrelated systematic errors added in quadrature. The solid green curve shows a single power law fit $C(R/4\mathrm{GV}{)}^{\mathrm{\Delta }}$ above 4 GV. The shaded blue area indicates the results of single power law fits to $C(R/4\mathrm{GV}{)}^{\delta }$ below 4 GV for each of the 21 time periods. (b) The ${}^3\mathrm{He}/{}^4\mathrm{He}$ flux ratio spectral index (red points) as a function of $R$. As seen, above 4 GV, the spectral index shows no dependence with rigidity (green band). The extrapolation of $\mathrm{\Delta }$ to higher energies is shown (green dotted line). The B/O (blue filled squares) and B/C (blue open squares) flux ratio spectral indices [5, 6] are also shown. As seen, the ${}^3\mathrm{He}/{}^4\mathrm{He}$ spectral index extrapolated to the highest energies is in good agreement with the B/O and B/C spectral indices.