Cosmic rays are high energetic ionizing particles originating from the sun, galactic and extragalactic objects. If these primary particles collide with oxygen or nitrogen atoms in the upper layers of the earths atmosphere they cause a cascade of secondary particles.

These airshowers of secondary particles have a well-defined profile. Starting at the top of the atmosphere, the cascade grows because of secondary particle collisions. As the cascade propagates further downwards, the flux decrease because the secondary particles lose energy and get absorbed. By consequence this altitude profile should have a maximum, the so-called Pfotzer maximum. This is already proven by testflights of HACORD (see picture).

The altitude profile depends on the incident angle of the cosmic rays. At lower altitudes a higher flux should be measured in the zenith direction than in the horizontal direction, while at high altitudes an isotropic distribution is expected.

Beside the angular dependency the cosmic ray flux also depends on the latitude at which the measurment is done. At higher latitudes it is expected to observe an overall higher flux in all directions due to the absence of earth magnetic shielding.

A final relevant feature of cosmic rays is that it tracks the 11 year solar activity cycle. During a high activity period, less cosmic rays will be measured with respect to a low solar activity period.