# Cosmic radiation cutoff at LOW energies?

The energy spectrum of the cosmic radiation (not CMB) is limited to both sides.

I know about the GZK-cutoff at high energies. Basically the interaction probability for photons of energies above 10^20 eV becomes so high, that all have interacted before they can reach us.

But why is there a limit at lower energies? Earth magnetic field and/or athmosphere, radiation belt? Perhaps someone can explain that to me.

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From where did you here there was a low energy cut-off? Technically, there isn't. Stable particles (alphas, protons, neutrinos etc) can have arbitarily low energies. Of course there comes a point when either; we would tend not to refer to them as cosmic rays (most would be solar wind for example), or when detector technolgy prevents us from seeing them. However, these don't really constitute as a 'cut-off' in the traditional sense of the word. –  qftme Feb 23 '12 at 12:58
In a talk a lower limit at about 1GeV was mentiond as a side note. –  con-f-use Feb 23 '12 at 13:03
Below 1GeV particles aren't generally considered relativistic and therefore, when they bombard our atmosphere from outer-space, they're just not normally refered to as cosmic rays. Furthmore, sub-GeV particles would very quickly thermalise in our atmosphere (Ie exhibit Brownian motion) and therefore retain no information about from where they came. –  qftme Feb 23 '12 at 14:05
This is cross-posted to Astronomy.SE, a practice which is discouraged. In the future please pick one site at a time and only post to the other if you do not get satisfactory answer from your original question in the course of a few days. –  dmckee Feb 23 '12 at 21:04
The GZK process refers to hadrons not photons. Also, it isn't exactly some kind of hard cut-off. The mean free path is on the order of several Mpc (which is comparable to the distance between neighboring galaxies) and even then only 20-50% of the energy is lost at each interaction. So particles with $E>E_{GZK}$ will still traverse quite a distance.