Precision Measurement of the Helium Flux in Primary Cosmic Rays of Rigidities 1.9 GV to 3 TV with the Alpha Magnetic Spectrometer on the International Space Station
2015
Knowledge of the precise rigidity dependence of the helium flux is important in understanding the
origin, acceleration, and propagation of cosmic rays. A precise measurement of the helium flux in primary
cosmic rays with rigidity (momentum/charge) from 1.9 GV to 3 TV based on 50 million events is presented
and compared to the proton flux. The detailed variation with rigidity of the helium flux spectral index is
presented for the first time. The spectral index progressively hardens at rigidities larger than 100 GV. The
rigidity dependence of the helium flux spectral index is similar to that of the proton spectral index though
the magnitudes are different. Remarkably, the spectral index of the proton to helium flux ratio increases
with rigidity up to 45 GV and then becomes constant; the flux ratio above 45 GV is well described by a
single power law.
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