The Earth is immersed in a "sea" of high-energy nuclei known as cosmic rays. In the late 1800s it was realized that there is an ionizing radiation present at the surface of the Earth. A major question at the time was whether or not the radiation originated within the Earth or atmosphere or if it was coming from an extraterrestrial source. The mystery was solved in 1912, when Victor Hess performed a series of high-altitude balloon flights to record the radition as a function of altitude. He demonstrated that the origin of the cosmic radiation was coming from beyond Earth's atmosphere. Hess won the Nobel Prize for this discovery in 1936.
At present we know that cosmic rays are composed of all atomic nuclei, from the simple hydrogen nucleus (a proton) to the iron nucleus and beyond. Transuranic elements have been observed in cosmic rays.
The energy spectrum of cosmic rays has been measured up to 1021 eV (electron-volts) and is shown at left. At the highest energies, cosmic rays have roughly the same energy as a well-struck tennis ball, but packed into a single atomic nucleus or particle.
The energy spectrum of cosmic rays has been relatively well-studied up to 1018 eV. To a first approximation, the spectrum is a rapidly falling power law in energy, dN/dE∝E-α, with an overall index α of about 2.8. However, the spectrum does show significant structure, such as a steepening of the spectral index above 1015 eV from α=2.7 to α=3.0. This change in the spectral index is called the "knee." A second steepening at about 5·1017eV (α=3.3) is followed by a harder spectral index (α=2.7) above 5·1018 eV.
Breaks in the cosmic ray spectrum are thought to be correlated with changes in the composition and sources of the particles. It is believed that below the "knee" most cosmic rays are protons accelerated in supernova remnants inside our Galaxy. In this picture, the decrease in flux at the "knee" can be interpreted as a change in the magnetic confinement of the cosmic rays. As their energies and gyroradii increase, the cosmic rays above a critical energy can escape the magnetic fields in the Galaxy.
Our current picture of cosmic rays below the "knee" — acceleration of particles in supernova remnants — can explain the power law spectrum observed in the cosmic ray flux. In fact the Fermi Gamma-ray Space Telescope observed the tell-tale 'pion bump' from two supernova remnants. However several fundamental questions still remain. For example, the pion bump signature occurs at lower energies (about 100 MeV). Do higher energy TeV gamma rays also come from these sources? Are there other sources out there like those two supernova remnants? What exactly is the acceleration mechanism and does it vary from source to source?
Extensive Air Showers
When a high-energy cosmic ray enters the atmosphere it loses its energy via interactions with air molecules. At high energies these interactions create particles. These new particles go on to create more particles, etc. This multiplication process is known as a particle cascade.
This process continues until the average energy per particle drops below about 80 MeV (million electron-volts). At this point the interactions lead to the absorption of particles and the cascade begins to die. This altitude is known as shower maximum. The particle cascade looks like a pancake of relativistic particles traveling through the atmosphere at the speed of light. Though the number of particles in the pancake may be decreasing, the size of the pancake always grows as the interactions cause the particles to diffuse away from each other.
When the pancake reaches the ground it is roughly 100 meters across and 1-2 meters thick. If the primary cosmic ray was a photon the pancake will contain electrons, positrons, and gamma rays. If the primary cosmic ray was a nucleus the pancake will also contain muons, neutrinos, and hadrons (protons, neutrons, and pions). The number of particles left in the pancake depends upon the energy of the primary cosmic ray, the observation altitude, and fluctuations in the development of the shower. This particle pancake is known as an extensive air shower (or simply an air shower).