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Van Allen Belt: Why are electrons generally further out?

  1. Nov 29, 2014 #1
    Why are electrons generally further out than protons in the Van Allen Belt?
  2. jcsd
  3. Nov 30, 2014 #2


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    The Van Allen belts were "the first significant discovery of the space age" in 1958. A couple of years ago NASA launched some probes to study these belts in greater detail.


    Early observationshttp://www.nature.com/nature/journal/v515/n7528/full/nature13956.html#ref1, http://www.nature.com/nature/journal/v515/n7528/full/nature13956.html#ref2 indicated that the Earth’s Van Allen radiation belts could be separated into an inner zone dominated by high-energy protons and an outer zone dominated by high-energy electrons. Subsequent studieshttp://www.nature.com/nature/journal/v515/n7528/full/nature13956.html#ref3, http://www.nature.com/nature/journal/v515/n7528/full/nature13956.html#ref4 showed that electrons of moderate energy (less than about one megaelectronvolt) often populate both zones, with a deep ‘slot’ region largely devoid of particles between them. There is a region of dense cold plasma around the Earth known as the plasmasphere, the outer boundary of which is called the plasmapause. The two-belt radiation structure was explained as arising from strong electron interactions with plasmaspheric hiss just inside the plasmapause boundaryhttp://www.nature.com/nature/journal/v515/n7528/full/nature13956.html#ref5, with the inner edge of the outer radiation zone corresponding to the minimum plasmapause locationhttp://www.nature.com/nature/journal/v515/n7528/full/nature13956.html#ref6. Recent observations have revealed unexpected radiation belt morphologyhttp://www.nature.com/nature/journal/v515/n7528/full/nature13956.html#ref7, http://www.nature.com/nature/journal/v515/n7528/full/nature13956.html#ref8, especially at ultrarelativistic kinetic energieshttp://www.nature.com/nature/journal/v515/n7528/full/nature13956.html#ref9, http://www.nature.com/nature/journal/v515/n7528/full/nature13956.html#ref10 (more than five megaelectronvolts). Here we analyse an extended data set that reveals an exceedingly sharp inner boundary for the ultrarelativistic electrons. Additional, concurrently measured datahttp://www.nature.com/nature/journal/v515/n7528/full/nature13956.html#ref11 reveal that this barrier to inward electron radial transport does not arise because of a physical boundary within the Earth’s intrinsic magnetic field, and that inward radial diffusion is unlikely to be inhibited by scattering by electromagnetic transmitter wave fields. Rather, we suggest that exceptionally slow natural inward radial diffusion combined with weak, but persistent, wave–particle pitch angle scattering deep inside the Earth’s plasmasphere can combine to create an almost impenetrable barrier through which the most energetic Van Allen belt electrons cannot migrate.



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