Physics
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmsh41b..03j&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #SH41B-03
Physics
7514 Energetic Particles (2114), 7807 Charged Particle Motion And Acceleration
Scientific paper
The existence of suprathermal charged particles (energies between thermal particles and cosmic-rays) is well- established. Fisk etal.(2000) suggested that the suprathermal particles observed in the solar wind have a velocity spectrum f(v) = const × v-5 which is "ubiquitous". This is clearly a very significant discovery and deserves close attention. I will review the implications of this observation for the physics of charged particles in the heliospheric plasma. These suprathermal particles must be accelerated locally in the plasma since the cooling due to the expansion of the solar-wind plasma is faster than any reasonable transport from the Sun. Hence local electric fields must do the accelerating. The solar-wind plasma, to a very good approximation, behaves as an ideal hydromagnetic fluid on the relevant scales. Hence, the only electric fields capable of accelerating suprathermal charged particles arise from local flow velocity variations due to turbulent fluctuations, compressions, shock waves or reconnection events. These mechanisms are widely thought to accelerate particles to suprathermal energies in a variety of astrophysical sites. The constraints imposed by the observed ∝ v-5 spectrum will be discussed. Shocks robustly produce power laws, but the -5 index is only one of a continuous range of possible and plausible spectra. Nothing in shock physics has been found which preferentially selects the observed value. Turbulent, or stochastic acceleration does not robustly produce power laws, much less ones with a -5 index. Hence the parameters of the charged-particle interaction with the compressions or turbulence must be very carefully tuned, possibly by some feedback mechanism. The physics behind this fine-tuning remains controversial. Arguments based on general constraints such as thermodynamics have not yet been widely accepted. There as yet is no consensus mechanism which accounts for the observed ≍ v-5 spectrum.
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