What Are Cosmic Rays You Say?

Cosmic rays are everywhere, but where do they come from and what exactly are they? Their name suggests they are a form of electromagnetic ray-diation, coming from the cosmos, but the truth is far more intriguing.

Scientists uncovered their first clue that something was amiss way back in 1783 when Charles-Augustin de Coulomb (of electric charge namesake fame) observed an electrically charged sphere instantly and mysteriously lose it's charge. In time he realized that another particle of unknown origin must be colliding with the sphere to make it lose it's charge. What Coulomb could not figure out was what the mystery particle was or where it came from!
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Fast forward 129 years to April 17, 1912, Victor Franz Hess, loaded his instruments onto a high-altitude balloon to test his hypothesis that the mystery particles were coming from outer space. He did this during a total solar eclipse to rule out the Sun as a potential source. This was Victor's eighth such flight where he measured ionization levels as he ascended. He flew the balloon to 17,500 ft and recorded much higher concentrations of ionizing radiation at altitude than when he was on the ground. Proving indeed, that the rays came from the cosmos!

Explaining the origins of those mysterious "cosmic rays" would take another 101 years to figure out. In February 2013, NASA's Fermi Gamma-ray Space Telescope revealed the first clear-cut evidence the expanding debris of exploded stars produced some of the fastest-moving matter in the universe. The findings showed that two supernova remnants (IC443 "Jellyfish Nebula", and W44) accelerated protons to near the speed of light as they were trapped within their shock wave. These protons interacted with nearby interstellar gas clouds, which then emitted gamma rays. Among the products of these star explosions are gamma-ray photons, which (unlike cosmic rays) are not affected by magnetic fields. The gamma rays studied had the same energy signature as subatomic particles called neutral pions. In other words, the matching energy signatures showed that protons could move at fast enough speeds within supernovas to create cosmic rays!

While we know they can be created in supernovas, there may be other sources for cosmic ray creation (active galactic nuclei, gamma ray bursts, quasars, etc). It also isn't exactly clear how supernovas are able to make these cosmic rays go so fast. We are talking 99.99999999999999999999951% the speed of light here! The ultra high energy varieties smash into earth with 100 MILLION times the energy that we can produce at the LHC. However, these "primary" rays usually collide with atoms in the Earth's upper atmosphere and rarely make it through to the ground, "secondary" particles are ejected from this collision and do reach earth's surface. By the time these cosmic rays get to Earth, it's impossible to trace where they came from. That's because their path has been changed as they traveled through multiple magnetic fields (the galaxy's, the solar system's and Earth's itself.) According to NASA, cosmic rays therefore come equally from all directions of the sky. So scientists are trying to trace back cosmic ray origins by looking at what the cosmic rays are made of. Scientists can figure this out by looking at the spectroscopic "signature" each nucleus gives off in radiation, and also by weighing the different isotopes (types) of elements that hit cosmic ray detectors.

The result, NASA adds, shows very common elements in the universe. Roughly 90 percent of cosmic ray nuclei are hydrogen (protons) and 9 percent are helium (alpha particles). Hydrogen and helium are the most abundant elements in the universe and the origin point for stars, galaxies and other large structures. The remaining 1 percent are all elements, and it's from that 1 percent that scientists can best search for rare elements to make comparisons between different types of cosmic rays. Scientists can also date the cosmic rays by looking at radioactive nuclei that decrease over time. Measuring the "half life" of each nuclei gives an estimate of how long the cosmic ray has been out there in space.

In 1949, the Fermi telescope's namesake, physicist Enrico Fermi, suggested the highest-energy cosmic rays were accelerated in the magnetic fields of interstellar gas clouds. In the decades that followed, astronomers showed supernova remnants were the galaxy's best candidate sites for this process. A charged particle trapped in a supernova remnant's magnetic field moves randomly throughout the field and occasionally crosses through the explosion's leading shock wave. Each round trip through the shock ramps up the particle's speed by about 1 percent. After many crossings, the particle obtains enough energy to break free and escape into the galaxy as a newborn cosmic ray.

And there you have it, if anyone mentions Cosmic Rays again you know the history, genesis, and makeup of those amazing high energy particles. It took us just over 220 years to figure those little guys out, but in the end, science gave us the answer!

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Victor Franz Hess - 1915