Plasma Physics of the Local Cosmos (2004) / Chapter Skim
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6 Energetic Particle Acceleration
6 Energetic Particle Acceleration
Pages 65-76
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From page 65... ...
stochastic acceleration. Both coherent and stochastic electric field acceleration can also occur as a part of the shock acceleration process.
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From page 66... ...
At quasi-perpendicular shocks, in the absence of particle collisions with turbulence or waves, the compressed magnetic field downstream of a shock causes particles to drift along the shock face and to be accelerated in the upstream motional electric field. This coherent mechanism is referred to as shock drift acceleration 2 Heliospheric Shock Acceleration Sites The solar wind flow speed is highly supersonic, and therefore shock waves will form ahead of any obstacle to the flow, or regions where high-speed plasma collides with low-speed plasma.
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From page 67... ...
Recent observational and theoretical studies have shown that, in addition to acceleration at the shocks bounding the compression region, particles are also accelerated by a Fermi-type process within the compression region itself.4 CIR particles can achieve energies up to 10 to 20 MeV Planetary and Cometary Bow Shocks Planets with magnetic fields, such as Ear h, Igniter Saturn, Uranus, and Neptune, have large standing shocks on the sunward side where the solar wind impacts the planet's magnetic field. Upstream of these shocks, energetic particle bursts are routinely observed s in the case of Earth, the solar wind convects past the shock on a time scale of -1 hour.
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From page 68... ...
For convenience this conceptually simple process of acceleration by parallel electric fields is referred to here as direct electric field acceleration. Strong particle acceleration also occurs when coherent electric fields are applied in directions perpendicular to relatively strong magnetic fields, a process referred to here as indirect e/ectnc field acceleration.
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From page 69... ...
If the electric field increases in a period of time that is commensurate with the time period associated with the bounce motions of the particles, or even faster and commensurate with the gyromotions of particles around the magnetic field, then the acceleration parallel or perpendicular to the background magnetic field can be much larger than that achieved by adiabatic betatron acceleration or Fermi accelera
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From page 70... ...
Particles that originate in the distant magnetic tail of a magnetosphere and the solar wind are often moved abruptly toward Earth as a result of bursty reconnection. Inductive electric fields produced by the abruptly changing magnetic fields in these flows accelerate electrons and ions prior to their entry into the radiation belts.
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From page 71... ...
Coherent electric field acceleration cannot explain all aspects of radiation belt enhancement. Satellite measurements of particle velocity distributions suggest that additional heating due to waves with frequencies comparable to the electron gyrofrequency is taking place.
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From page 72... ...
containing a significant fraction, -10 to so percent, of this energy How the Sun releases this energy, presumably stored in the magnetic fields of the corona, and how it rapidly accelerates electrons and ions with such high efficiency, and to such high energies, are currently not known. Hard x-ray spectra obtained with high spectral resolution show a break at ~20 to 100 keV, suggesting that the accelerated electrons have a sharp feature in that energy range.9 Similar features in electron spectra observed in Earth's auroral zone are the result of acceleration by a quasi-stationary (DC)
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From page 73... ...
The Auroral Magnetosphere Electric fields parallel to the background magnetic field have long been thought to play a role in producing Earth's aurora. A field-aligned potential drop was first proposed in the mid-1970s by David Evans to explain the monoenergetic electron beams observed in association with auroral arcs 'A Evidence for the existence of such structures has been provided by double probe measurements, chemical release experiments, and particle data.
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From page 74... ...
wavesi · How probable are extreme radiation belt flux enhancements such as that on March 24, 1991, which produced new MeV electron and trapped solar proton belts on a drift time scale of minutesi · Why did outer zone electron fluxes essentially disappear for 2 months following the May 11, 1999, period in which solar wind density dropped to less than 0.1 cm~ii · How does Jupiter generate its incomparably powerful radiation belt in the absence of solar wind buffeting effectsi · What is the distribution of parallel electric field along B within the auroral acceleration region and how is it maintainedi STOCHASTIC PARTICLE ACCELERATION In his original model for the acceleration of cosmic rays in the interstellar medium, Fermi suggested that the random movement of magnetic scattering centers or clouds could further energize fast-moving particles since they would experience more head-on (energy-gaining) than overtaking (energy-losing)
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From page 75... ...
What is the effect of different turbulence characteristics on stochastic particle accelerationi · in what plasma regimes is stochastic particle acceleration effective compared to alternative acceleration processesi SUMMARY In situ access to energetic particle acceleration mechanisms in the solar system provides a unique opportunity to make direct measurements of processes that can be scaled up to astrophysical counterparts. The rapid acceleration of energetic particles in solar flares and substorms is still not well understood.
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From page 76... ...
While much progress has been made in understanding microphysical processes of parallel electric field acceleration, there is still much uncharted territory. So, for example, it was learned only recently how structured and time-variable the auroral return current region is, and particle acceleration theory has not yet caught up with the recent observations from the FAST satellite.
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