Cosmic Ray Mystery

Cosmic rays are energetic particles that are observed to arrive at earth from space, and are thought to consist of electrons, protons, nuclei, and photons (with ratio these four "ingredients" varying with energy). The energies of these particle span more than ten orders of magnitude (109 - 1020 eV), the energy spectrum of these particles following a polynomial discription of Energy2.7 (ie. at double the energy, only a quarter as many particles are observed)- a spectral shape consistent with that produced by Fermi first order acceleration mechanism.

In fact the spectral shape follows this relationship so well, that the spectrum for the most part is quite dull!
The knee feature occurs at around 1015eV The ankle feature occurs at around 1018eV

However, features at the higher energy (right end) of the spectrum do start to appear, with firstly the "knee", followed by the "ankle" parts of the spectrum. These parts of the spectrum, that depart from the Energy2.7 relation, are thought to be both due to a change in the the origin of the cosmic rays and effects introduced into the spectrum during particle propogation. These effects occur due to an interaction of the cosmic rays (protons- as far as we know, these dominate at the energies under discussion) with the all pervasive CMB (relic photons left from the coallition of electrons with protons produced from the big bang).

The first (in order of increasing energy) of the features in the spectrum is a feature called the "knee" (since it relates to a steepening of the spectrum), occurring at around 1015eV and is explained as being due to the beginning (in terms of increasing energy) of the end of cosmic rays of supernovae origin.

The second of the featues in the spectrum is a feature called the "ankle" (since it relates to the spectrum becoming less steep), occurring at around 1018eV, is due to the beginning (again, in terms of increasing energy) of extragalactic sources contributing dominantly to the cosmic ray population.

So far it may well seem that cosmic rays aren't the mystery initially professed at the top of the page, in fact a fair amount seems to be understood about them. However, the plot thickens at the higher energies....

The actual particles that being observed as cosmic rays are thought to be protons and atomic nuclei (with ratios of each chemical species roughly reflecting the Galaxies own chemical abundances- ie. protons heavily dominant, followed by Helium nuclei, Carbon, Nitrogen, Oxygen and a fair amount of Iron nuclei).

Such (hadronic) particles are not free to travel unimpeded through space, since space is not entirely empty. Space, seethes with residual photons from the Universe's beginning.


Andrew Martin Taylor