Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsh23c..07z&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #SH23C-07
Physics
[2104] Interplanetary Physics / Cosmic Rays, [2114] Interplanetary Physics / Energetic Particles, [2152] Interplanetary Physics / Pickup Ions, [2159] Interplanetary Physics / Plasma Waves And Turbulence
Scientific paper
Suprathermal charged particles observed in many places inside the heliosphere often show a p-5 distribution function. Fisk and Gloeckler (2008, ApJ, v686, p1466) argued it is a universal phenomenon. They suggested it is produced by particle acceleration in compressible plasma waves. It this paper, we present a detailed calculation of particle acceleration by a large-amplitude continuously compressed/expanding plasma wave train. Although specific wave forms of plasma speed and diffusion coefficient are prescribed for the simplicity of calculation, the particle acceleration exhibits some common properties. The compressible wave will broaden the momentum distribution of particles with more possibility to accelerate them to higher energies. The broadening of momentum distribution is significant only when the diffusion scale κ/V is comparable to or larger than the size of each compression zone and plasma compression ratio is large enough. The accelerated particles downstream of the wave typically have a power-law spectrum at momentum far away from the injection momentum of source particles. The slope of the power-law has a minimum, which is only a function of the amplitude of plasma compression ratio. At a compression ratio of 4, the minimum slope is -4.95 for one type of plasma wave form, but the slope is close to that value even if some other forms of plasma wave are used. The minimum slope is achieved under large range of parameter space as long as the particle diffusion length is greater than the size of individual compression zone but less that the length of the entire wave train, indicating that the same power-law slope should cover a wide range of particle energy. If there is a mechanism that limits the compression ratio of plasma wave to a maximum of 4, then this acceleration mechanism can naturally explain the p-5 distribution commonly observed in space.
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