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Acceleration from field-aligned potential drops
Haerendel, Gerhard
AA(Max-Planck-Institut für extraterrestrische Physik, Garching bei München, Germany)
Astrophysical Journal Supplement Series (ISSN 0067-0049), vol. 90, no. 2, p. 765-774 (ApJS Homepage)
Publication Date:
Nuclear and High-Energy Physics
NASA/STI Keywords:
Beta Particles, Field Aligned Currents, Magnetohydrodynamic Stability, Particle Acceleration, Stellar Magnetic Fields, Current Density, Energy Conversion, Plasma Generators, Shear Stress, Wave-Particle Interactions
Bibliographic Code:


Unstable field-aligned currents are seen as the origin of field-aligned potential drops. They convert energy stored in magnetic shear stresses into kinetic energy. A good fraction of this energy is carried by runaway electrons and ions out of the acceleration region. The paper emphasizes the analogy with mechanical fractures. Simple expressions for the energy conversion rate and the parallel potential drop are derived, the two being linked by the critical current density needed for instability. The origin of the currents (generator) lies mostly in a region remote from that of energy conversion (fracture zone). The transmission of shear stresses and energy from the generator plasma, where the primary forces are applied to the fracture zone is also considered. A closed set of relations allows quantitative evaluation of the energetic particle production efficiency . The decoupling of the plasma on either side of the fracture zone which allows fast stress relief is described in detail, as well as a stationary model of the Alfven wave interaction between fracture zone and generator plasma. A simple concept of the nature of the anomalous resistivity generated by the unstable current leads to an expression for the magnetic diffusivity inside the fracture zone and an estimate of the latter's extent parallel to the magnetic field, whereas its width and length transverse to B follow from the macroscopic relations. Finally and as an example, the theory is applied to the problem of fast electron (and ion) acceleration well above 1 MeV seen to occur in many solar flares. It is obvious that this process belongs to the most powerful production processes of high-energy particles in stellar magnetic fields.

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