Current suggestions are that the cosmic rays with energies less than about 10 10 eV are primarily solar cosmic rays produced in solar flares and coronal mass ejections, while those with energies between 10 10 eV and the knee at 10 15 eV are galactic cosmic rays produced in the shocks of supernova remnants. The origin of these changes in the steepness of the spectrum is still the subject of intense study, but it is assumed that they distinguish between populations of cosmic rays originating via different mechanisms.
However, if we look at the spectrum more closely we can see a knee at ~ 10 15 eV and an ankle at ~ 10 18 eV. Roughly speaking, for every 10% increase in energy beyond 10 9 eV, the number of cosmic rays per unit area falls by a factor of 1,000. By plotting this range of energies against the number of cosmic rays detected at each energy we generate a cosmic ray spectrum which clearly shows that the number of cosmic rays drop off dramatically as we go to higher energies. The range of energies encompassed by cosmic rays is truly enormous, starting at about 10 7 eV and reaching 10 20 eV for the most energetic cosmic ray ever detected. The spectrum exhibits a ‘knee’ and an ‘ankle’, both of which deviate from the standard exponential decline (blue line). The cosmic ray spectrum clearly shows that the number of cosmic rays (the cosmic ray flux) detected drops off dramatically as we go to higher energies.
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