Tuesday, October 9, 2012

Unlocking the Secrets of Galactic Cosmic Ray Abundances

 A primary cosmic ray particle (proton) collides with a molecule of our atmophere, with secondary muons, pions and neutrinos, positrons (e-) etc. produced.(Source: Wikipedia)

Cosmic rays hold an abiding interest for many despite the fact their apprehension and observation by particle physicists is now mainly confined to particle accelerator laboratories - such as FermiLab. This has left most serious cosmic ray studies (especially of galactic cosmic rays) to remain the province of astrophysics.

In the near-Earth and solar system environment, meanwhile, cosmic rays have been invoked time and again by those in other disciplines - not just solar physics, but climatologists. For example the C14:C12 isotope ratio, such as detected in tree rings, and first noted by the late solar physicist John Eddy ("The New Solar Physics") has led to definitive conclusions regarding anthropogenic greenhouse warming. As Eddy and others have noted, this isotope ratio can also be used as a proxy for solar activity, for example, going back thousands of years to times when there weren't telescopes or where solar spot records are sparse.

In general, C14 is produced in the upper atmosphere via the impact –interaction with high energy cosmic rays, say from galactic sources. Solar activity in turn modulates the intensity of these cosmic rays via the action of the heliosphere which deflects a fraction of the intense cosmic ray flux and other harmful interstellar radiation. At times the Sun is more active, so also will the heliosphere be stronger, shielding the Earth from more intense cosmic rays the effect of which is to reduce the C14 produced in the Earth’s upper atmosphere.

 Conversely, when the Sun is less active – as it was from 2000- 2009 then the heliospheric shield was weaker and more intense cosmic rays penetrated to our upper atmosphere yielding more C14 produced.   It follows from this that if a record could be obtained of the ratio of say C14 to C12 then one would have a proxy indicator of cosmic ray activity as it influences our atmosphere and climate, for any time (With the C14 to C12 ratio extracted from tree rings or other plant tissue). Thus cosmic ray intensity will then be seen to be modulated by the C14:C12 ratio, and the lower this ratio the lower the putative intensity. Fortuitously, a 2000-year record of C14:C12 deviations has been compiled by P.E. Damon ('The Solar Output and Its Variation', and appears in Eddy's monograph.   As Eddy observes concerning this output and the data (ibid.):

“The gradual fall from left to right (increasing C14/C12 ratio) is…probably not a solar effect but the result of the known, slow decrease in the strength of the Earth’s magnetic moment exposing the Earth to ever-increased cosmic ray fluxes and increased radiocarbon production. The sharp upward spike at the modern end of the curve, representing a marked drop in relative radiocarbon, is generally attributed to anthropogenic causes—the mark of increased population and the Industrial Age."  

While this is important and heady stuff, cosmic ray abundances are also of interest, especially from galactic sources. Entering here are the electronic instruments on assorted high altitude balloons as well as spacecraft which have made precise measurements of elemental cosmic ray composition over energy ranges from tens of mega-electron volts to a few tera-electron volts per nucleon.

Most critically, such measurements have disclosed abundances of elements such as lithium (Li), beryllium (Be) and boron (B) which are rare in the Sun and the solid bodies of the solar system (as well as the atmospheres of other stars) but much more manifest in glactic cosmic rays. But a qualification here: these abundances of rare elements come about from secondary cosmic rays formed after nuclear fragmentation of the original (galactic) cosmic rays with nuclei of interstellar gas.

Incredibly, the radioactive nuclei spun off in secondary cosmic rays that astrophysicists now observe appear to have been accelerated nearly 15 million years ago. (cf. Yanasak, N.E. et al, Astrophysical Journal, 2001, Vol. 563, p. 768.) More incredibly, the absence of the isotope Ni 59 (T ½ = 70,000 yrs.) shows these heavy nuclei had to have spend some 100,000 years as interstellar material (likely after nucleosynthesis in a supernova) before the blast wave from another supernova accelerated them to cosmic ray energies where they got stripped of their atomic electrons. (cf. Weidenbeck et al, 1999, Astrophysical Journal, Vo. 523, p. L16-64).

The upshot of these and other papers is that the detailed isotopic composition of the original cosmic rays provides evidence of the rays' origin in regions of the galaxy with large collections of massive stars (e.g. 4 solar masses or more).  Measurements of the cosmic rays in the tera-electron volt range also demonstrate subtle differences in the spectra of different elements, and deviations from simple power laws (see e.g., Ahn et al, 2010, Astrophysical Journal, Vol. 714, p. L89).

Lastly, and most exciting to me - are the measurements of the positron-electron ratio which also indicate exotic (non-prosaic) sources for the cosmic ray positrons we detect. Some have even theorized these emerge from the decay of particles comprising 'dark matter', which up to now has never been directly detected. However, we are aware it has a more or less definitive gravitational "signature" with the gravitational effects about 5 time as abundant as those for normal matter in the universe.

Meanwhile, astrophysicists involved in cosmic ray research await the day (and the instruments!) which deliver cosmic rays from extra-galactic sources. For this we require energies accessed over 10-19  eV. The problem is that the cosmic ray energy spectrum falls steeply  over 4 x 10-19  eV on account of interaction with photons from the cosmic microwave background.

All we can say now is 'stay tuned'!

For further insight, see: http://www.physicstoday.org/resource/1/phtoad/v65/i2/p30_s1?bypassSSO=1

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