Stochastic Fermi Acceleration of sub-Relativistic Electrons and Its Role in Impulsive Solar Flares

Abstract

We reexamine stochastic Fermi acceleration (STFA) in the low energy (Newtonian) regime in the context of solar flares. The particle energization rate depends a dispersive term and a coherent gain term. The energy dependence of pitch angle scattering is important for determining the electron energy spectrum. For scattering by whistler wave turbulence, STFA produces a quasi-thermal spectrum. A second well-constrained scattering mechanism is needed for STFA to match the observed 10-100keV non-thermal spectrum. We suggest that STFA most plausibly acts as phase one of a two phase particle acceleration engine in impulsive flares: STFA can match the thermal spectrum below 10kev, and possibly the power law spectrum between 10 and 100keV, given the proper pitch angle scattering. However, a second phase, such as shock acceleration at loop tops, is likely required to match the spectrum above the observed knee at 100keV. Understanding this knee, if it survives further observations, is tricky.

34 pages, 5 figures, accepted for publication in MNRAS

References in corpus (5)

• MHD Turbulence Revisited
• Compressible MHD Turbulence in Interstellar Plasmas
• Particle acceleration by slow modes in strong compressible MHD turbulence, with application to solar flares
• Non-thermal Acceleration from Reconnection Shocks
• Distinguishing Solar Flare Types by Differences in Reconnection Regions

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• Persistent mysteries of jet engines, formation, propagation, and particle acceleration: have they been addressed experimentally?
• Spatial intermittency of particle distribution in relativistic plasma turbulence
• The shock reprocessing model of electron acceleration in impulsive solar flares
• Stochastic Acceleration of Electrons by Fast Magnetosonic Waves in Solar Flares: the Effects of Anisotropy in Velocity andWavenumber Space
• Acceleration of cosmic rays in presence of magnetohydrodynamic fluctuations at small scales
• On the role of stochastic Fermi acceleration in setting the dissipation scale of turbulence in the interstellar medium
• Velocity space compression from Fermi acceleration with Lorentz scattering