On the spectral hardening at ~> 300 keV in solar flares

Abstract

It has been noted for a long time that the spectra of observed continuum emissions in many solar flares are consistent with double power laws with a hardening at energies $\sim >$ 300 keV. It is now largely believed that at least in electron-dominated events the hardening in photon spectrum reflects an intrinsic hardening in the source electron spectrum. In this paper, we point out that a power law spectrum of electron with a hardening at high energies can be explained by diffusive shock acceleration of electrons at a termination shock with a finite width. Our suggestion is based on an early analytical work by Drury et al., where the steady state transport equation at a shock with a tanh profile was solved for a $p$-independent diffusion coefficient. Numerical simulations with a $p$-dependent diffusion coefficient show hardenings in the accelerated electron spectrum which are comparable with observations. One necessary condition for our proposed scenario to work is that high energy electrons resonate with the inertial range of the MHD turbulence and low energy electrons resonate with the dissipation range of the MHD turbulence at the acceleration site, and the spectrum of the dissipation range $\sim k^{-2.7}$. A $\sim k^{-2.7}$ dissipation range spectrum is consistent with recent solar wind observations.

20 pages, 2 figures, accepted by ApJ

References in corpus (3)

• Solar wind turbulent spectrum at plasma kinetic scales
• Electron-Electron Bremsstrahlung Emission and the Inference of Electron Flux Spectra in Solar Flares
• Temporal and Spatial Analyses of Spectral Indices of Nonthermal Emissions Derived from Hard X-Rays and Microwaves

Cited by in corpus (16)

• Particle Acceleration by a Solar Flare Termination Shock
• Imaging and spectroscopic observations of magnetic reconnection and chromospheric evaporation in a solar flare
• Non-Equilibrium Processes in the Solar Corona, Transition Region, Flares, and Solar Wind \textit{(Invited Review)}
• The Acceleration and Confinement of Energetic Electrons by a Termination Shock in a Magnetic Trap: An Explanation for Nonthermal Loop-top Sources during Solar Flares
• The Dynamical Behavior of Reconnection-driven Termination Shocks in Solar Flares: Magnetohydrodynamic Simulations
• Determination of Stochastic Acceleration Model Characteristics in Solar Flares
• Numerical Modeling of Energetic Electron Acceleration, Transport, and Emission in Solar Flares: Connecting Loop-top and Footpoint Hard X-Ray Sources
• Radio Spectroscopic Imaging of a Solar Flare Termination Shock: Split-Band Feature as Evidence for Shock Compression
• Effects of shock and turbulence properties on electron acceleration
• An observational revisit of band-split solar type-II radio bursts
• Radio Spectral Imaging of an M8.4 Eruptive Solar Flare: Possible Evidence of a Termination Shock
• Dynamical modulation of solar flare electron acceleration due to plasmoid-shock interactions in the looptop region
• Gyrosynchrotron emission generated by nonthermal electrons with energy spectra of a broken power law
• Double-power-law feature of energetic particles accelerated at coronal shocks
• A model of double coronal hard X-ray sources in solar flares
• Suprathermal electron acceleration by a quasi-perpendicular shock: simulations and observations