Local re-acceleration and a modified thick target model of solar flare electrons

Abstract

The collisional thick target model (CTTM) of solar hard X-ray (HXR) bursts has become an almost 'Standard Model' of flare impulsive phase energy transport and radiation. However, it faces various problems in the light of recent data, particularly the high electron beam density and anisotropy it involves.} {We consider how photon yield per electron can be increased, and hence fast electron beam intensity requirements reduced, by local re-acceleration of fast electrons throughout the HXR source itself, after injection.} {We show parametrically that, if net re-acceleration rates due to e.g. waves or local current sheet electric ($\mathcal{E}$) fields are a significant fraction of collisional loss rates, electron lifetimes, and hence the net radiative HXR output per electron can be substantially increased over the CTTM values. In this local re-acceleration thick target model (LRTTM) fast electron number requirements and anisotropy are thus reduced. One specific possible scenario involving such re-acceleration is discussed, viz, a current sheet cascade (CSC) in a randomly stressed magnetic loop.} {Combined MHD and test particle simulations show that local $\mathcal{E}$ fields in CSCs can efficiently accelerate electrons in the corona and and re-accelerate them after injection into the chromosphere. In this HXR source scenario, rapid synchronisation and variability of impulsive footpoint emissions can still occur since primary electron acceleration is in the high Alfvén speed corona with fast re-acceleration in chromospheric CSCs. It is also consistent with the energy-dependent time-of-flight delays in HXR features.

8 pages, 2 figures

References in corpus (6)

• Impulsive phase flare energy transport by large-scale Alfven waves and the electron acceleration problem
• Chromospheric magnetic field and density structure measurements using hard X-rays in a flaring coronal loop
• Stereoscopic electron spectroscopy of solar hard X-ray flares with a single spacecraft
• Hard X-ray Spectra and Positions of Solar Flares observed by RHESSI: photospheric albedo, directivity and electron spectra
• Dynamics of electron beams in the inhomogeneous solar corona plasma
• Onsets and spectra of impulsive solar energetic electron events observed near the Earth

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• High-resolution Observations of a C9.3 White-light Flare and Its Impact on the Solar Photosphere
• Hybrid simulations of chromospheric HXR flare sources
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