Cosmic rays at the top of the atmosphere

Abstract

The atmosphere of the Earth provides more than ten interaction lengths for protons going straight down. If the observations were made at sea level such a proton would retain on the average less than 0.001 of its energy. The energy loss fluctuates from event to event and the energy spectrum of cosmic ray protons would be difficult to reconstruct. Heavier cosmic ray nuclei have significantly shorter interaction lengths and lose energy much faster. For these simple reasons the observations of the cosmic rays are much easier outside the atmosphere.