This is the second part of the Multi-Messenger Astronomy series. We turn from neutral photons to charged particles: cosmic rays, the second-oldest astronomical messenger.
What cosmic rays are
Cosmic rays are charged particles arriving at Earth from outside the Solar System: about 90% protons, 9% helium nuclei, 1% heavier elements (carbon, oxygen, iron). Plus a tiny fraction of electrons, positrons, and antimatter primaries.
They span an enormous energy range — about 13 orders of magnitude — from the ~$10^7$ eV particles slowed by the solar wind up to the $10^{20}$ eV “Oh-My-God particle” caught by the Fly’s Eye detector in 1991.
The cosmic-ray spectrum is approximately power-law: flux $\propto E^{-2.7}$ at GeV energies, steepening to $E^{-3}$ above the cosmic-ray knee at $3 \times 10^{15}$ eV, then breaking at the ankle near $5 \times 10^{18}$ eV, and possibly cutting off at the GZK scale above $5 \times 10^{19}$ eV.
How they’re detected
Direct detection (below ~1 PeV): satellites and balloon experiments measure the primary cosmic ray directly. AMS-02 on the ISS, the Pamela satellite, balloon experiments.
Air-shower detection (above ~100 GeV): cosmic rays striking Earth’s atmosphere produce extensive air showers — cascades of secondary particles spreading over square-kilometer scale at the ground. Detector arrays at the ground sample the resulting particle distributions. Plus fluorescence telescopes that watch the air shower’s UV emission.
The Pierre Auger Observatory in Argentina (3,000 km² of surface array + 27 fluorescence telescopes) and the Telescope Array in Utah are the leading ultra-high-energy cosmic ray facilities.
Why cosmic rays are limited messengers
The fundamental limit is magnetic deflection. Charged particles travel along magnetic-field lines and are deflected by them. The Milky Way has galactic magnetic fields of ~3 microgauss. Intergalactic magnetic fields are much weaker but not zero.
The deflection angle of a cosmic ray scales as $\theta \propto Z/E$, where Z is the charge and E is the energy. For a proton at $10^{18}$ eV traveling 1 kpc through galactic fields, the deflection is many radians — the proton has no memory of where it came from.
Only at $> 5 \times 10^{19}$ eV — the GZK scale — does the deflection drop to a few degrees, enabling possible source-pointing. Above $10^{20}$ eV, deflection might be small enough to identify individual sources.
But statistics are scarce. The flux at $10^{20}$ eV is roughly one particle per square kilometer per century. Even Auger’s 3,000 km² array has detected only ~30 events above $10^{20}$ eV in its 17 years of operation.
What cosmic rays tell us
Despite the limitations, cosmic rays carry critical information:
Composition: The mass distribution at different energies. Galactic cosmic rays are mostly protons. Above the knee (a few PeV), the composition becomes heavier — consistent with the picture that galactic accelerators produce a maximum rigidity (R = momentum / charge), so heavier elements reach higher energies at the same R.
Spectrum shape: The knee, ankle, and possible cutoff features constrain source models and propagation physics. The transition from galactic to extragalactic origin somewhere between the knee and ankle is a major question.
Diffuse anisotropy: Even with magnetic deflection, statistical correlation analysis can identify excess flux from particular sky regions. Auger has reported anisotropy at ~5σ correlated with the position of nearby active galactic nuclei.
The connection to neutrinos
Cosmic rays and high-energy neutrinos are produced by the same accelerators. The accelerator boosts protons to extreme energies; the protons subsequently produce both:
- Cosmic rays — the protons themselves escape and propagate to Earth.
- Neutrinos — through pion production in proton-proton or proton-photon interactions at the source.
The ratio of neutrino-to-cosmic-ray production depends on the source environment. Dense environments (lots of target gas or photons) produce more neutrinos per accelerated proton; sparse environments produce more direct cosmic-ray escape.
This is why high-energy neutrino astronomy is the missing piece for identifying cosmic-ray sources. Neutrinos travel undeflected; they reveal which sources are actively accelerating protons.
Cosmogenic neutrinos
The interaction of ultra-high-energy cosmic-ray protons with the cosmic microwave background — the GZK process — produces pions that decay to neutrinos. These cosmogenic neutrinos carry information about cosmic-ray sources averaged over the Hubble volume.
Predicted cosmogenic flux peaks around $10^{18}-10^{19}$ eV. The flux is below the sensitivity of IceCube’s current cubic-kilometer scale but within reach of next-generation arrays. The 220 PeV KM3NeT event from February 2023 sits at $2 \times 10^{17}$ eV — at the low-energy edge of cosmogenic predictions, with substantial reconstruction uncertainty.
What’s coming
GRAND — a proposed 200,000-antenna radio array — targets ultra-high-energy neutrinos with sensitivity in the $10^{18}-10^{20}$ eV regime.
IceCube-Gen2 radio — dedicated radio component of the expanded South Pole array — will provide independent sensitivity.
Pierre Auger upgrade (AugerPrime) continues providing the world’s most precise composition measurements above $10^{18}$ eV.
Cosmic Particle Explorer 2025+ — proposed space-based instruments for direct cosmic-ray detection at high energies.
The combined cosmic-ray + neutrino + photon picture in the late 2020s and 2030s should finally identify the dominant sources of ultra-high-energy cosmic rays.
The next part of this series turns to the newest astronomical messenger: gravitational waves.
Frequently asked
Why are cosmic rays limited as astronomical messengers?
They're charged. Galactic and intergalactic magnetic fields bend the path of charged particles during the trip from source to Earth. Below ~10¹⁹ eV, cosmic-ray arrival directions are essentially randomized — you can't trace them back to their sources. Only at the very highest energies (above ~5 × 10¹⁹ eV) do magnetic-deflection angles become small enough for source-pointing to be possible.
What's the highest cosmic-ray energy ever measured?
About 3 × 10²⁰ eV — the 'Oh-My-God particle' detected by Fly's Eye in 1991, plus a few comparable events from Auger and Telescope Array. These energies correspond to macroscopic kinetic energy carried by a single nucleus — about 50 joules, equivalent to a fast-pitched baseball compressed into one particle.
What is the GZK cutoff?
Above about 5 × 10¹⁹ eV, cosmic-ray protons can pion-photoproduce off the cosmic microwave background, losing energy with every interaction. The effect — predicted by Greisen, Zatsepin, and Kuzmin in 1966 — should produce a steep drop in the cosmic-ray spectrum at this energy. The drop is observed and is consistent with the GZK prediction.
How do cosmic rays relate to high-energy neutrinos?
Whenever cosmic-ray protons collide with target material (gas or radiation), they produce pions which decay to neutrinos. The high-energy cosmic neutrino flux that IceCube measures is the signature of cosmic-ray accelerators in distant sources. The 220 PeV KM3NeT event is a candidate cosmogenic neutrino — secondary from cosmic-ray interactions with the CMB.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2026, March 26). Multi-Messenger Astronomy — Part 2: Cosmic rays as charged messengers. Neutrino Times. https://neutrino-times.com/articles/multi-messenger-part-2-cosmic-rays/
Chicago
Neutrino Times Editorial Team. "Multi-Messenger Astronomy — Part 2: Cosmic rays as charged messengers." Neutrino Times, March 26, 2026. https://neutrino-times.com/articles/multi-messenger-part-2-cosmic-rays/.
MLA
Neutrino Times Editorial Team. "Multi-Messenger Astronomy — Part 2: Cosmic rays as charged messengers." Neutrino Times, 26 Mar. 2026, https://neutrino-times.com/articles/multi-messenger-part-2-cosmic-rays/.
BibTeX
@misc{neutrino-times-multi-messenger-part-2-cosmic-rays,
author = {Neutrino Times Editorial Team},
title = {Multi-Messenger Astronomy — Part 2: Cosmic rays as charged messengers},
howpublished = {Neutrino Times},
year = {2026},
month = {mar},
url = {https://neutrino-times.com/articles/multi-messenger-part-2-cosmic-rays/},
note = {Accessed: 2026-03-26}
} RIS
TY - GEN TI - Multi-Messenger Astronomy — Part 2: Cosmic rays as charged messengers AU - Neutrino Times Editorial Team PY - 2026 DA - 2026-03-26 PB - Neutrino Times UR - https://neutrino-times.com/articles/multi-messenger-part-2-cosmic-rays/ ER -