We already gave a general look at the various components of the cosmic rays in Earth orbit. Now let’s look closer at them. We start from the galactic cosmic rays (GCR).
GCR originates within our Galaxy and they are accelerated by supernovae explosions. The higher energy component comes from outside the galaxy and its acceleration mechanism is still not clear.
We already showed that cosmic ray energies range from 10 MeV up to 1012 MeV. Obviously, the particle flux varies with the kinetic energy as it is shown in the following figure.
It’s worth noting that for energies less than 1010 eV (that is 104 MeV or 10 GeV) the flux is 1 particle per second per square meter, at 1015 eV (around the knee region) the flux is 1 particle per year per square meter and at higher energies the flux is about 1 particle per year per square kilometer.
At these energies the cosmic rays are essentially made by protons. The heavier the nuclei the lower the flux (apart from the odd/even effect). In the following figure we can compare fluxes of protons, alpha particles (helium nuclei), electrons, Carbon nuclei and Iron nuclei.
To understand the energy magnitude we are talking about and the effects of energetic particles I want to talk about a very interesting phenomenon (without entering details): the Extensive Air Shower. A particle entering the atmosphere meets with an increasing number of nuclei and molecules, mainly Nitrogen and Oxygen. Sooner or later the cosmic ray will collide with one of them. This interaction could originate secondary particles by converting its kinetic energy into mass according to the famous formula
E = mc2
One single particle with kinetic energy of 10 EeV (that is 1019 eV) entering the atmosphere originates up to 100 billions particles at sea level over an area of some square kilometers as shown in this simulation
These ultra high energy cosmic rays are studied by ground based stations as the Auger observatory in Argentina. They are composed by a group of detectors that measure both the atmospheric fluorescence due to particle showers and the particles that reach the ground. Fluorescence telescopes measures ultraviolet light produced by the shower and they are so sensitive that can detect a shower 15 km far. Ground detectors are 1600 water tanks, each filled with 12000 liters, and far 15 km from each other. They detect Cherenkov light that is produced by particles that travel faster than light in water (Cherenkov light principle is similar to sonic boom one).
And all is distributed over an area of more than 10 squares miles…
These ultra energetic particles have an extragalactic origin (because galactic magnetic field is not strong enough to contain them) and in 2007 the scientists of the Auger Collaboration announced that AGN (Active Galactic Nuclei) are the most probable sources of this kind of cosmic rays.
This post is a little off topic for this blog, but I consider these ultra energetic cosmic rays so interesting… In the near future I will talk about less extreme energies that are more related to ALTEA and its research on the International Space Station
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