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How significant are runaway electrons returning to the acceleration region in solar flares?

Published onAug 18, 2020
How significant are runaway electrons returning to the acceleration region in solar flares?

It is well established that when an electric field is applied to a plasma, a fraction of the latter will run away. However, models describing the accelerated electron beam/return-current system have generally failed to take these runaway electrons into account, which means that the magnitude of the return-current electric field is assumed to be much less than the Dreicer field. We investigate the conditions for which runaway electrons cannot be neglected and their effect on the beam/return current system, as well as the associated bremsstrahlung spectrum. We develop a model in which an accelerated electron beam drives a steady-state co-spatial return current electric field, which locally balances the direct beam current and freely accelerates a fraction of background (return-current) electrons. The model is self-consistent, i.e. the electric field induced by the co-evolution of the direct beam and runaway current is taken into account. We then apply our model to RHESSI observations of two flares with different spectral characteristics. One flare whose X-ray spectra during the impulsive phase are well represented by a single power-law, and the other requiring strong broken power-law spectra (difference between the upper and lower energy spectral indices is greater than 0.6).


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