CERN Accelerating science

AMS looks at Cosmic Galactic Rays

by Panos Charito

The Alpha Magnetic Spectrometer (AMS) is a state-of-the-art particle physics experiment operating on the International Space Station (ISS) since May 2011. In the first 6 years of missions, AMS has detected over 100 billion cosmic ray (CR) particles. Recently, it has released new results on proton, antiproton, lepton, and nuclei energy spectra at unexplored energies and with an unmatched level of accuracy.


Recent results inaugurate a new era in Cosmic ray (CR) physics driven by high precision. With the latest AMS-02 nuclei data [1] (including the boron-to-carbon ratio, proton flux, helium flux and antiproton-to-proton ratio), the team was able to constrain the primary source and propagation parameters of cosmic rays in the Milky Way by considering three schemes with different data sets (with and without antiproton/proton data) and assuming different propagation models.


Initial discovery of CRs dates back to a century ago (1912). The inexplicable rate of discharge of electroscopes, could be explained by hypothesizing a mysterious source of ionizing radiation in the Galaxy. In 1912, Viktor Hess, an Austrian scientist, showed that this radiation was of extraterrestrial origin, by carrying electrometers on balloon flights and finding that the rate of discharge was increasing with height. For this discovery he was awarded the Nobel Prize in 1936. A similar conclusion had already been reached the year before by the Italian scientist Domenico Pacini, who had carried electrometers on a submarine and performing measurements at different sea depths.


Their identification of CR as particles rather than radiation dates to about 20 years later and in 20 more years also the first suggestion that they were associated with Supernovae Remnants (SNRs) was in place. Highly energetic particles consist of essentially every element ranging from hydrogen, accounting for approximately 89% of the GCR spectrum. The idea that Supernova Remnant (SNR) shocks are the primary sites of CR acceleration in the Galaxy is what is generally referred to as the ”Supernova remnant paradigm for the origin of Cosmic Rays”. This paradigm has been under scrutiny now for about 50 years, but only in the last few years some clear evidence in its favour has been found.


If SNRs are the main sources of CRs, effective amplification of the interstellar magnetic field must take place. The last generation of X-ray telescopes, with their superb spatial resolution, have not only confirmed the presence in SNRs of electrons with TeV energies, but have also finally allowed to highlight the presence of amplified magnetic fields in these sources, likely associated with efficient acceleration of hadrons. Also suggestive of efficient acceleration of protons is the measurement of anomalous widths of Balmer lines in some Hα bright remnants while recently, γ-ray astronomy have led, for the first time, to direct observational evidence of the presence of mildly relativistic protons in few SNRs interacting with molecular clouds.


This fundamental progress deriving from observations of SNRs has been paralleled by discoveries coming from direct observation of CRs. The latter have touched both the hadronic and leptonic component of CRs. As far as nuclei are concerned, the paradigm of a featureless spectrum at energies below the knee in the CR spectrum has been disclaimed by balloon and satellite observations showing a spectral hardening of all species at around 200 GeV/nucleon and a different spectrum for the two most abundant species, protons and He nuclei, with the latter being systematically flatter. These features might be revealing us important clues on the process behind particle acceleration and propagation in the Galaxy.


The long duration of the AMS mission, planned to last for the whole ISS lifetime, will cover a complete solar cycle from the ascending phase of cycle 24, through its maximum, and the descending phase into the next solar minimum. This makes AMS an excellent multichannel CR monitor of solar activity. Precision measurements of the CR time evolution, in connection with the changing solar activity, may give us strong insight on the so-called solar modulation effect.


Along with its connection with solar and CR physics, understanding CR modulation addresses a prerequisite for modeling space weather, which is an increasing concern for space missions and air travelers. The study of these effects has been limited for long time by the scarcity of long-term Cosmic Ray data on different species, and by the poor knowledge of the local interstellar spectra.  AMS provides a continuous stream of time-resolved and multichannel Cosmic Ray data that set new objectives, namely: (i) to advance solar modulation observations of Cosmic Ray particles and antiparticles, and (ii) to develop improved and measurement-validated models of Cosmic Ray transport in the heliosphere.


Moreover, the new data allow to understand the puzzling anomalies detected in the energy spectra of CR proton and helium nuclei while also maintaining the universality of the dominant diffusive-shock acceleration mechanism.  In this model, the p/He anomaly is explained by a flux transition between two source components that have different injection spectra and composition.


Another important topic is the recent observation of an eight-month time lag in solar modulation of Cosmic Rays. This effect reveals important properties on the dynamics of the formation and changing conditions of the heliospheric plasma. Crucial tests can be performed by AMS via monthly-resolved measurements of these ratios, or even better, by measurements of individual particle fluxes for protons, antiprotons, electrons and positrons under both polarity conditions and across the magnetic reversal. This demonstrates that time-dependent measurements on CR antimatter can provide precious information on the physics of the heliosphere. Finally, understanding the charge-sign dependence of Cosmic Ray modulation is also essential to search for dark matter signatures in Cosmic Ray fluxes.


AMS opens a dedicated, high-precision, multichannel investigation of solar modulation effects in Galactic Cosmic Rays. To develop reliable and data-driven models of CR modulation, however, the availability of time-resolved measurements over the period of interest is crucial and in this respect, monthly-resolved data from AMS will be very precious.





[1] A collection of these data can be found here: ASI/SSDC CR database:

[2] Y. Zhang et alii, ApJ Lett. 844 (2017) L3
[3] N. Tomassetti, Adv. Space Res. 60 (2017) 815-825 [13]
[4] A. C. Cummings et alii, ApJ 831 (2016) 18 [14]
[5] N. Tomassetti et alii, ApJ Lett. 849 (2017) L32

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