Abstract
The effect of Coulomb losses on the stochastic acceleration of high energy electrons in solar flares is investigated. Including Coulomb losses makes the acceleration less efficient at low energies, the resulting particle spectra show a steepening below an energy Ec, typically 1 – 3 MeV, where the loss rate equals the acceleration rate. It is shown that electrons can efficiently be accelerated to high energies even if they are injected at energies E < Ec. The solutions successfully explain the observed steepening of interplanetary electron spectra below ≈ 3 MeV following impulsive solar flares taking place at low coronal heights. It is concluded that simultaneous second-order Fermi acceleration and Coulomb losses operating in closed flare loops are a likely mechanism for second-step acceleration in impulsive solar flares.
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