Imagine the Universe!

Cosmic Ray Spectrum and "the Knee"

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'knee' of cosmic ray spectrum

This is a spectrum composed of measurements from different instruments to cover the energy range from 1011 to 1020eV. It shows the flux (numbers of cosmic rays passing through a surface) at different energies. The graph is logarithmic; a straight line indicates that the number of cosmic rays is proportional to the energy to some power , called the spectral index . In the case of cosmic rays, there are fewer particles at higher energies (the numbers go DOWN as the energy goes UP) so the spectral index is negative.

The "knee" appears as a sharp turn in the slope of the line of data points. In actuality, the turn is very slight, almost imperceptible when viewed in normal units. However, the numbers of cosmic rays observed are so great that the measurements are known with great accuracy, or the error bars are small. The original graph is usually scaled so that the knee is more obvious. This is only possible or believable because the data are so robust.

The graph changes slope again at 10 20 eV, a fact which has been confirmed with more recent measurements with newer instruments that are able to capture cosmic rays with the highest energies.

This figure is figure 2.2 of Sokolsky, P. 1989. Introduction to Ultrahigh Energy Cosmic Ray Physics. Redwood City: Addision-Wesley Publishing Company.


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Imagine the Universe! is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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This page last updated: Monday, 27-Sep-2004 11:26:09 EDT