Cosmic Rays are often started from one singular particle, but by the time they hit the atmosphere, they cause air showers large and small. This is because there

Cosmic Rays are often started from one singular particle, but by the time they hit the atmosphere, they cause air showers large and small. This is because there's not a whole lot to interact with in space, but there's tons of matter once you start hitting the atmosphere. At ultra high energies, cosmic rays cause extensive air showers leading to radiation in the form of gamma rays, muons, and electrons/positrons through pair production. The electrons are actually called the "electronic shower" There is a distance where the average energy of the photons in the shower is less than 1MeV, causing pair production to cease. This distance into the atmosphere is called Xmax, but it's dependent upon the amount of mass in the atmosphere, not the magnetic field. For lower energy cosmic rays, Xmax is pretty small. For higher energies, Xmax can easily hit us.