Tuesday, August 9, 2011

Earth's Antimatter Radiation Belt

A great paper came out on the arXiv last week: O. Adriani et al., The discovery of geomagnetically trapped cosmic ray antiprotons, arXiv:1107.4882v1. Here is the abstract:

The existence of a significant flux of antiprotons confined to Earth's magnetosphere has been considered in several theoretical works. These antiparticles are produced in nuclear interactions of energetic cosmic rays with the terrestrial atmosphere and accumulate in the geomagnetic field at altitudes of several hundred kilometers. A contribution from the decay of albedo antineutrons has been hypothesized in analogy to proton production by neutron decay, which constitutes the main source of trapped protons at energies above some tens of MeV. This Letter reports the discovery of an antiproton radiation belt around the Earth. The trapped antiproton energy spectrum in the South Atlantic Anomaly (SAA) region has been measured by the PAMELA experiment for the kinetic energy range 60--750 MeV. A measurement of the atmospheric sub-cutoff antiproton spectrum outside the radiation belts is also reported. PAMELA data show that the magnetospheric antiproton flux in the SAA exceeds the cosmic-ray antiproton flux by three orders of magnitude at the present solar minimum, and exceeds the sub-cutoff antiproton flux outside radiation belts by four orders of magnitude, constituting the most abundant source of antiprotons near the Earth.
The red data points are the measured antiproton flux in the South Atlantic Anomaly region.
 So, most people have heard of the Van Allen radiation belts.  These are composed protons and electrons trapped by the Earth's magnetic field at a certain distance above the Earth's surface.  The proton component is thought to be due to the decay of neutrons produced by galactic cosmic ray interactions with the Earth's atmosphere.  The cartoon description is as follows: A galactic cosmic ray (fully ionized, highly energetic heavy ion) and an atmospheric molecule experience a nuclear interaction.  A neutron is produced which decays into a proton (and an electron and an antineutrino).  In addition, there will be a contribution from direct proton production from the nuclear interaction - that is ion + neutral nucleus --> protons + lots of other stuff.  These are the basic process which produce the protons that are trapped in the Van Allen belts. 

The production of the antiprotons is not from decay, but only from the direct interaction.  With enough energy (which cosmic rays have - there are particles in the cosmic ray spectrum with energies orders of magnitude above that CERN uses), antiprotons will be produced in the nuclear interactions.  Some of these will escape the atmosphere and should become trapped in the Earth's geomagnetic field.  Now the PAMELA experiment, as reported in this article, have confirmed this experimentally!

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