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John C. Foster, Massachusetts Institute of Technology Haystack Observatory
Title: "Prompt Acceleration of Radiation Belt Electrons to Ultra-Relativistic Engeries"
Abstract: The energization of radiation belt particles to relativistic energies is a prime objective of NASA’s dual-satellite Van Allen Probes mission. Adiabatic acceleration to highly-relativistic energies can occur gradually over an interval of weeks to months waves in the outer radiation belt (L ~ 3-6) accompanying the slow earthward diffusion of electrons due to scattering by VLF waves. In addition, more rapid local acceleration in the core regions of the outer belt (L ~ 4) has been identified as an important mechanism for repopulating the disturbance-depleted radiation belts in a matter of hours when an appropriate population of seed particles and strong VLF waves are present.
Here we describe the use of the temporal/spatial information afforded by flying dual satellites closely spaced along the same orbit to investigate processes responsible for accelerating electrons to ultra-relativistic energies on a time scale of minutes. We examine the effects of sequential substorm intervals during the 17 March 2013 event. Impulsive substorm injection into the core of the outer belt (L~4) can energize both cold and plasma sheet electrons to energies of 50 - 500 keV. Strong VLF waves are generated by the abundant 50 keV electrons, while the high-energy tail of the electron distribution (> 500 keV) is energized by the same waves. Dual spacecraft observations serve to unravel the spatial/temporal characteristics of this non-linear, non-adiabatic local energization process. For 4.5 MeV electrons at L* ~4 we observe a 5x increase in flux in 10 min following substorm onset, and 100x increase in 1 hour. Phase space density considerations suggest local energization of the seed electron population by ~1 MeV in each of the sequential substorms.
The second example considers at the effects of the 8 October 2013 solar wind shock on the radiation belt electrons. The two Van Allen Probes inside the dayside plasmaphere at L ~ 3 and L ~ 5 provide a detailed description of both the magnetosonic pulse that is launched by the shock and the resulting acceleration of electrons it produces across a broad span of energies. As the pulse propagates through the magnetosphere, drift resonance of ~4 MeV electrons with the initial electric field pulse and subsequent ULF waves selects these electrons for maximum energy gain with the result that a persistent new population of ~4 MeV radiation belt electrons was formed at L ~ 3.5.
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