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The Field Guide to Particle Physics
https://pasayten.org/the-field-guide-to-particle-physics
©2021 The Pasayten Institute cc by-sa-4.0
The definitive resource for all data in particle physics is the Particle Data Group: https://pdg.lbl.gov.The Pasayten Institute is on a mission to build and share physics knowledge, without barriers! Get in touch.
The Particle Data Group's write up on cosmic rays. See Figure 29.8 for a representation of the "ankle" feature in the spectrum.
https://pdg.lbl.gov/2019/reviews/rpp2019-rev-cosmic-rays.pdfAnother representation of the power laws can be found in Professor Peter Gorham's Coursework on Ultra High Energy Cosmic Rays: http://www2.hawaii.edu/~gorham/UHECR.html
Natalie Wolchover has written two great articles in Quanta on Cosmic Rays, both which talk about what might accelerate these particles.
The Particle That Broke a Cosmic Speed Limit and Cosmic Map of Ultrahigh-Energy Particles Points to Long-Hidden Treasures
Colussi & Hoffmann
In situ photolysis of deep ice core contaminants by Çerenkov radiation of cosmic origin
Gephysical Research Letters: https://doi.org/10.1029/2002GL016112Guzmán, Colussi & Hoffmann
Photolysis of pyruvic acid in ice: Possible relevance to CO and CO2 ice core record anomalies
Atmospheres: https://doi.org/10.1029/2006JD007886A quick primer on Cherenkov Radiation: https://www.radioactivity.eu.com/site/pages/Cherenkov_Effect.htm
Theme music "Sneaking Up on You" by the New Fools, licensed by Epidemic Sound.
Cosmic Rays
Part 4 - Paleoclimatology and Muons
Our atmosphere is one giant filter for cosmic rays. The sparse molecules near the top of our atmosphere begin the process of catching the energy of those energetic particles from space and transferring it into heat or muons. These cosmogenic muons that typically make it all the way down to the surface.Near the surface, the atmosphere is a lot thicker, but it’s still just a collection of ballistic molecules bashing into each other at 1000 miles per hour. Some of those molecules hit us, and some hit the ground. We perceive these molecular impacts as air pressure.
By contrast, cosmogenic muons are moving through this mess at over 600 million miles per hour.
To those muons, the surface of the Earth is barely noticeable. They fly through a lot of things: hundreds of meters of rock, oceans, plants and animals before colliding or decaying. By contrast, those particles of atmospheric gas typically reflect off the surface of the Earth. Rocks just aren’t that permeable to most gas. As we explained in the ALPHA particle miniseries, helium gas generated from radioactive decay deep within the earth collects underground, trapped by rocks.
One thing gas can permeate is surface water.
Quite a bit of our atmospheric gases get dissolved into the ocean. Oxygen in the air allows the fish to breathe too, once dissolved into the water so it can be picked up by their gills. Increased carbon dioxide levels also imply more CO2 gets put under water.
When the water on Earth’s surface freezes, as it might do near the polar ice caps, it traps some of that dissolved gas with it.
This has been happening for millions of years, and until somewhat recently at least, that ice has been compounding. New ice forms above, pushing old ice down.
This has resulted in a LOT of ice.In Antarctica there are areas where the ice is over four kilometers deep! That’s miles of ice! Greenland also carries massive glaciers, two to three kilometers deep, built up in same fashion.
The gases trapped in that glacial ice is a frozen relic of an older atmosphere. The deeper the ice, the older the dissolved gases. As the mixture of molecules in our atmosphere changes over time, it sets down a record in the glacial ice. The deepest ice, millions of years old, can tell us what the atmosphere was like millions of years ago.Extracting that ice is quite the scientific adventure!
This all easy to say in theory - but the practice of Science requires a lot of gory, technical detail. Different measurements from different samples of ice at different depths from different parts of the world need to be calibrated. Ice can form at different rates in different places under different conditions.
But, at least averaged over a given year or decade or so, the atmosphere should be well mixed. Huge weather patterns around the world mix the air, ensuring should be about the same.
And so the Scientific logic goes like this:
Assuming older ice is usually below the younger ice and the atmosphere is well mixed, then given any two ice sheets on earth, there should be a way compare them. The concentrations of different gases dissolved at different times should sequentially be the same. Like multi-colored stripes on a pole. The stripes may be different sizes, but they should be in the same order.
If we can find the same sequences in gas concentrations across different ice sheets then we can start to put together a history of the Earth’s atmosphere.
Near the turn of the 21st century, geophysicists were working on exactly this problem. They were trying to calibrate the gas concentrations trapped in ancient ice samples by comparing ice from Antarctica with Greenland. And things just weren’t adding up. The sequences didn’t align. The gas concentrations were just too different. There was some kind of missing variable in the data.
As it turned out, that variable involved cosmogenic muons.
The Speed of Sound and Light
To understand how muons resolved this Paleoclimatology puzzle, we need to go back to the source. The source of cosmic rays.
In episode two of this series we talked about Fermi Acceleration - the process by which electrically charged particles like protons get accelerated to outrageous velocities by SHOCKWAVES in astrophysical plasmas.
And shockwaves occur in glacial ice too.
To understand shockwaves, let’s think about sound waves.
Sound usually travels in the atmosphere like a wave. A wave of air pressure. Those atmospheric particles slam against each other in an organized and oscillating way, spreading out away from source.
The speed of those waves depends on the amount and types of molecules present, as well as the overall temperature of the atmospheric gas. The sound waves we experience travel at around 343 meters per second, which is about 767 miles per hour.
Here’s the thing, humans routinely fly supersonic jets that travel faster than that.
Supersonic jets - like fighter jets - travel faster than the speed of sound. They travel faster than noise they make. You can’t hear them coming until they’re already past you. And when you do finally hear them, it’s a tremendous noise.
It’s a s...
