The effect of wave-particle interactions on low energy cutoffs in solar flare electron spectra

Abstract

Solar flare hard X-ray spectra from RHESSI are normally interpreted in terms of purely collisional electron beam propagation, ignoring spatial evolution and collective effects. In this paper we present self-consistent numerical simulations of the spatial and temporal evolution of an electron beam subject to collisional transport and beam-driven Langmuir wave turbulence. These wave-particle interactions represent the background plasma's response to the electron beam propagating from the corona to chromosphere and occur on a far faster timescale than coulomb collisions. From these simulations we derive the mean electron flux spectrum, comparable to such spectra recovered from high resolution hard X-rays observations of solar flares with RHESSI. We find that a negative spectral index (i.e. a spectrum that increases with energy), or local minima when including the expected thermal spectral component at low energies, occurs in the standard thick-target model, when coulomb collisions are only considered. The inclusion of wave-particle interactions does not produce a local minimum, maintaining a positive spectral index. These simulations are a step towards a more complete treatment of electron transport in solar flares and suggest that a flat spectrum (spectral index of 0 to 1) down to thermal energies maybe a better approximation instead of a sharp cut-off in the injected electron spectrum.

6 pages, 5 figures, accepted by ApJL

References in corpus (7)

• RHESSI Microflare Statistics II. X-ray Imaging, Spectroscopy & Energy Distributions
• Chromospheric magnetic field and density structure measurements using hard X-rays in a flaring coronal loop
• Dynamics of electron beams in the inhomogeneous solar corona plasma
• Onsets and spectra of impulsive solar energetic electron events observed near the Earth
• Low-energy cutoffs in electron spectra of solar flares: statistical survey
• Numerical consideration of quasilinear electron cloud dynamics in plasma
• Physics of collisionless reconnection in a stressed X-point collapse

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