Precision Measurement of the Monthly Cosmic Ray Fluxes with the Alpha Magnetic Spectrometer on the International Space Station

by Dr Claudio Corti (University of Hawai'i at Manoa)

Thursday Oct 11, 2018, 3:30 PM → 4:30 PM Pacific/Honolulu

WAT112

The Alpha Magnetic Spectrometer (AMS), on the International Space  Station (ISS) since May 2011, has acquired the largest number of  particles ever measured in space by a single experiment, performing the most precise measurement of galactic cosmic rays (GCR) to-date. The flux of all GCR species at low energy (below 60 GV) is affected by solar activity, which varies in time with a period of about 11  years. Before reaching Earth, particles coming from the galaxy are advected away by the solar wind, they diffuse due to scattering on the irregularities of the magnetic field and drift along the solar magnetic field gradients, curvatures and current sheet. Precise knowledge of the time variation of GCRs is crucial for a better understanding of the background to indirect searches of dark matter and for improving the modeling of the radiation environment in space, enabling safer operations for satellites and human exploration.

The detailed time variation of the proton, helium, electron and positron fluxes was measured by AMS in the first 6 years of data taking. This period covers the ascending phase of solar cycle 24, the solar maximum and the reversal of the Sun’s magnetic field polarity, and the descending phase of solar cycle 24.

For all particles, the high energy spectrum remains stable versus time,  while the low-energy range is strongly modulated by the solar activity. During the solar maximum (end of 2013, beginning of 2014), the flux intensity at the lowest measured energy was reduced by more than 50% with respect to June 2011. After March 2015, the fluxes of all species started  to recover, surpassing the level of the beginning of the mission in 2016. AMS observed a long term decrease of the proton-to-helium ratio, coinciding with the period of flux recovery, and a smooth transition of  the positron-to-electron ratio over more than 800 days from one value to  another.  Accurate measurement of the time variation of the proton-to-helium and positron-to-electron ratios allows for a detailed study of how diffusion affects particles with different charge and the effect of drifts on same-charge, opposite-sign particles.

corti_ams-monthly-fluxes.pdf