What Is Hitting the Atmosphere From Space? Particles With Improbable Energy
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Charged particles arrive from space continuously, mostly protons, and a few carry energies far beyond anything a laboratory can produce. Where the most energetic ones come from is still an open question.
What arrives and what happens next
Cosmic rays are high-energy charged particles arriving from space, overwhelmingly protons with a smaller fraction of heavier nuclei and a few per cent electrons, and the name is a historical accident since they are particles rather than radiation. When one strikes a nucleus in the upper atmosphere it produces a cascade, with the collision creating new particles that themselves collide and create more, so a single primary particle generates a shower of millions spreading over a wide area by the time it reaches the ground. Most of what reaches the surface is muons, which are heavy relatives of the electron, and they pass through everything including people at a rate of roughly one per square centimetre per minute.
Where they come from
Sources differ across an enormous range of energies:
- •The sun, which supplies the lowest-energy particles and increases output during flares
- •Supernova remnants within the galaxy, which accelerate particles at expanding shock fronts
- •Other galactic sources for the higher energies, including neutron stars and stellar winds
- •Sources outside the galaxy for the very highest energies, since the galaxy cannot confine them
- •Active galactic nuclei and gamma ray bursts as leading extragalactic candidates
- •The origin of the most energetic particles remains genuinely unresolved
The impossible particles
A handful of detected events carry energies that are difficult to comprehend. The most energetic recorded, detected in 1991 and nicknamed the oh-my-god particle, carried roughly the kinetic energy of a fast-thrown cricket ball concentrated in a single atomic nucleus, which is some tens of millions of times what the largest accelerator produces. Such particles are extremely rare, arriving at a rate of about one per square kilometre per century, which is why detection requires arrays covering thousands of square kilometres. They also should not reach us from far away, since above a certain energy a particle loses energy by interacting with the microwave background radiation filling space, which limits how far one can travel and therefore places their sources relatively nearby in cosmological terms. Identifying those sources has not succeeded.
How they are detected
Measuring particles that arrive at a rate of one per square kilometre per century requires methods that differ completely from ordinary detectors. Surface arrays cover enormous areas with detectors spaced hundreds of metres apart, sampling the shower where it reaches the ground and reconstructing the original particle's energy and direction from the arrival times and densities across the array, with the largest covering three thousand square kilometres in Argentina. Fluorescence telescopes watch the atmosphere on dark nights for the faint ultraviolet glow the shower produces as it excites nitrogen, which traces the shower's development through the air. Balloon and satellite instruments catch lower-energy particles directly above the atmosphere. And the earliest detections used cloud chambers and photographic emulsions carried up mountains.
What they affect
The practical consequences are more numerous than expected. The cascade produces carbon-14 in the atmosphere, which is the basis of radiocarbon dating and which means the technique depends on cosmic ray flux having been reasonably steady. Particles striking electronics flip bits in memory, which is a real and quantified failure mode addressed with error correction in servers and with hardened components in aircraft and spacecraft, and it becomes more significant as circuit features shrink. Radiation dose to aircrew and to astronauts comes substantially from this source and is monitored. The particles also produce isotopes in exposed rock that are used to date surfaces. And they were the source of the first discoveries in particle physics, including the positron and the muon, before accelerators existed.
The takeaway
Mostly protons arriving from space collide in the upper atmosphere and produce cascades of millions of particles, with muons reaching the ground continuously. Supernova remnants supply galactic energies and extragalactic sources the highest, which remain unidentified. The cascade produces the carbon-14 that radiocarbon dating depends on and flips bits in electronics.