Cosmic Rays and their propagation

Cosmic rays (CRs) are the high-energy particles striking the Earth from anywhere beyond Earth’s atmosphere. Their energy spectrum is vast, ranging from MeV to ZeV (Z for Zetta: 10^21).

CRs are made up mostly of protons (~89%), He and heavier nuclei (~10%), and only ~1% of electrons, the particles AESOP-Lite is aimed to detect.


Fluxes of nuclei of the primary cosmic radiation in particles per energy-per-nucleus are plotted vs energy-per-nucleus. The inset shows the H/He ratio at constant rigidity. Ref.[6]

The all-energy cosmic ray spectrum, showing well-know features, such as the “knee” and the “ankle” Ref. [1

As they propagate from their source to us, these particles, because they have an electric charge, will scatter off the Galactic, Heliospheric, and the Earth’s magnetic field. The lower their energy, the more susceptible CRs are to this modulation

What happens to the CRs below 20 GeV as they propagate through the Galaxy and into our heliosphere ?

The solar activity cycle

Every 11 years the Sun moves through from a period of solar minimum to a period of solar maximum, ie the solar cycle.  In the heliosphere, CRs propagate along themagnetic field lines, interacting with perturbations, highly modulating the signal at
E<10 GeV.


As solar activity rises (top panel), the count rate recorded by a neutron monitor in Thule, Greenland decreases (bottom panel)  Ref. [2]

The CRs signal on Earth is strongly dependent on the level of solar activity: that is what we call the the solar modulation of cosmic rays.

Polarity cycle and charge-sign dependence 


Charge sign dependent solar modulation of electron as observed by
AESOP. The dotted blue line shows the positron abundance in a A+ state, while the dotted red line shows the value in a A- state, illustrating a dramatic drop

Every 11 years, at solar maximum activity, the polarity of the Sun’s magnetic field switches: the north becomes south, and vice versa.
We observe a charge-sign dependence in CR propagation through the heliosphere: positive and negative particles don’t travel along the same magnetic field regions.


Simultaneous measurements of CR e+/e-  serve as a crucial test of our present understanding of how large charge-sign dependent modulation in the heliosphere is, as a function of energy and position over a complete solar activity cycle. It is expected that the effects of drifts on CRs should become more evident closer to minimum activity.

Positron Excess



PAMELA,  AMS-02 and Fermi-LAT measurements confirm an excess in the high-energy positron fraction, above what is expected from positrons produced in cosmic-ray interactions (Ref. [4]) The grey band indicates the expected range in the positron fraction, Ref. [5])

Positrons constitute ~5% to ~30% of the total electron spectrum above 100 MeV. most of which are generally believed to be of galactic secondary origin.

However, in wildly discussed results, PAMELA, Fermi-LAT, AMS-02 reported a
positron fraction higher than the one predicted for a purely secondary model at energies above 1 GeV. This excess points towards the existence of primary sources of positrons, such as pulsars or possible dark matter annihilation product.


AESOP-Lite will extend the measurements made by PAMELA and AMS-02 in the low-energy regime below 300 MeV.



Ref. [1] Cosmic Rays: Recent Progress and some Current Questions – Hillas, Anthony M. astro-ph/0607109.

Ref. [2]  Bartol Research Institute Neutron Monitor Program

Ref. [3] “Positron Abundance in Galactic Cosmic Rays,” J. Clem and P. Evenson, Astrophysical Journal, 568, 216, 2002.

Ref. [4]  M. Aguilar et al. (AMS Collaboration), First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5-350 GeV, Phys. Rev. Lett 110, 141102 – Published 3 April 2013.

Ref. [5] R. J. Protheroe, “On the Nature of the Cosmic Ray Positron Spectrum,” Astrophys. J. 254, 391 (1982).

Ref. [6C. Patrignani et al. (Particle Data Group), Chin. Phys. C, 40, 100001 (2016) and 2017 update.

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