Observation of Fine Time Structures in the Cosmic Proton and Helium Fluxes with the Alpha Magnetic Spectrometer on the International Space Station

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Observation of Fine Time Structures in the Cosmic Proton and Helium Fluxes with the Alpha Magnetic Spectrometer on the International Space Station

M. Aguilar et al. (AMS Collaboration)

Phys. Rev. Lett. 121, 051101 – Published 31 July 2018

Abstract

We present the precision measurement from May 2011 to May 2017 (79 Bartels rotations) of the proton fluxes at rigidities from 1 to 60 GV and the helium fluxes from 1.9 to 60 GV based on a total of $1 \times 10^{9}$ events collected with the Alpha Magnetic Spectrometer aboard the International Space Station. This measurement is in solar cycle 24, which has the solar maximum in April 2014. We observed that, below 40 GV, the proton flux and the helium flux show nearly identical fine structures in both time and relative amplitude. The amplitudes of the flux structures decrease with increasing rigidity and vanish above 40 GV. The amplitudes of the structures are reduced during the time period, which started one year after solar maximum, when the proton and helium fluxes steadily increase. Above $\sim 3 GV$ the $p / He$ flux ratio is time independent. We observed that below $\sim 3 GV$ the ratio has a long-term decrease coinciding with the period during which the fluxes start to rise.

Received 21 November 2017
Revised 9 May 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.051101

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Cosmic ray composition & spectra Cosmic ray propagation Cosmic ray sources Cosmic rays & astroparticles

Solar wind

Gravitation, Cosmology & Astrophysics

Authors & Affiliations

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Observation of Complex Time Structures in the Cosmic-Ray Electron and Positron Fluxes with the Alpha Magnetic Spectrometer on the International Space Station

M. Aguilar et al. (AMS Collaboration)

Phys. Rev. Lett. 121, 051102 (2018)

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

M. Aguilar et al. (AMS Collaboration)

Phys. Rev. Lett. 121, 051103 (2018)

Article Text

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Supplemental Material

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References

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Issue

Vol. 121, Iss. 5 — 3 August 2018

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Images

Figure 1
The three-dimensional detailed behavior of the AMS (a) proton and (b) helium fluxes as functions of rigidity from 1 to 10 GV and from 1.9 to 10 GV, respectively, and time. The color code indicates the flux intensity in units of $[m^{2} \cdot sr \cdot s \cdot GV]^{- 1}$ . During the period of observation, both fluxes have a distinct minimum in February 2014 (blue line) and a maximum in February 2017 (red line).
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Figure 2
The AMS proton (blue, left axis) and helium (red, right axis) fluxes as function of time for 8 rigidity bins. The error bars are the quadratic sum of the statistical and time dependent systematic errors. Detailed structures (green shading and dashed lines to guide the eye) are clearly present below 40 GV. The vertical dashed lines denote boundaries between these structures at I) September 27, 2011; II) March 7, 2012; III) July 20, 2012; IV) May 13, 2013; V) February 7, 2014; VI) December 1, 2014; VII) March 19, 2015; VIII) November 17, 2015; IX) June 20, 2016; X) November 28, 2016. The red vertical dashed lines denote structures that have also been observed by AMS in the electron flux and the positron flux [33].
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Figure 3
Comparison of the fine structure time dependence of (a) the AMS proton flux for [1.19–1.40] GeV together with the measurement by EPHIN aboard SOHO for [1.12–1.29] GeV [36], (b) the AMS helium flux for $[1.11 - 1.28] GeV / n$ , (c) the relative variation of the AMS proton flux integrated over $R \geq 6.47 GV$ as a function of time together with the relative variation of the rate reported by the Oulu, Finland neutron monitor [37], and (d) the monthly averaged sunspot number [38] with the period of solar magnetic field polarity ( $A$ ) reversal (vertical dashed lines) from $A < 0$ to $A > 0$ , November 2012 to March 2014, of solar cycle 24 [39]. One year after solar maximum, both the $p$ and He fluxes start to rise and, as seen, there is a negative correlation with the sunspot number. AMS data are converted from rigidity $R$ to kinetic energy per nucleon $E_{K} = (\sqrt{Z^{2} R^{2} + M^{2}} - M) / A$ , where $M$ is the proton or the ${}^{4}{He}$ mass. The AMS error bars are the quadratic sum of the statistical and total systematic errors.
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Figure 4
The AMS $p / He$ flux ratio as function of time for 9 characteristic rigidity bins. The errors are the quadratic sum of the statistical and time dependent systematic errors. The solid lines are the best fit of Eq. (2) for the first 5 rigidity bins from [1.92–2.15] GV to [2.97–3.29] GV. The vertical band (February 28, $2015 \pm 42$ days) is the average of the best fit values of $t_{i}$ for these rigidity bins.
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