The induced electric field produced by magnetohydrodynamic processes can accelerate test particles very efficiently under certain circumstances. Numerical experiments indicate that particles are accelerated to very high energy by the magnetohydrodynamic fields accompanying reconnection that is triggered by finite amplitude turbulent fluctuations. This turbulent neutral point mechanism includes both coherent and stochastic components of acceleration. Turbulence appears to influence the acceleration in two ways: It enhances the reconnection electric field while producing a stochastic electric field that gives rise to momentum diffusion; it also produces magnetic “bubbles” and other irregularities that can temporarily trap test particles in the strong reconnection electric field for times comparable to the magnetofluid characteristic time. We propose a scaling of the acceleration process to parameter regimes of interest which indicates that this type of acceleration may be an important factor in space physics and astrophysics.
A possible source of free energy available for accelerating charged particles is conversion of magnetic energy to particle energy in reconnecting magnetic fields. Recent simulations using test particles suggests that reconnection may efficiently accelerate particles to the maximum energies that are observed in several astrophysical contexts. A simple analytic formula is used in conjunction with the simulation results to predict the maximum energy achievable in a particular plasma environment with the result that in solar flares reconnection is capable of accelerating particles to several GeV. In magnetospheric substorms the predicted maximum can reach several hundred keV, and near magnetic sector crossings in the solar wind the maximum energy can approach 100 keV.
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