The efficiency of electron acceleration during the impulsive phase of a solar flare

Abstract

Solar flares are known to be prolific electron accelerators, yet identifying the mechanism(s) for such efficient electron acceleration in solar flare (and similar astrophysical settings) presents a major challenge. This is due in part to a lack of observational constraints related to conditions in the primary acceleration region itself. Accelerated electrons with energies above $\sim$20~keV are revealed by hard X-ray (HXR) bremsstrahlung emission, while accelerated electrons with even higher energies manifest themselves through radio gyrosynchrotron emission. Here we show, for a well-observed flare on 2017~September~10, that a combination of \emph{RHESSI} hard X-ray and and SDO/AIA EUV observations provides a robust estimate of the fraction of the ambient electron population that is accelerated at a given time, with an upper limit of $\sim 10^{-2}$ on the number density of nonthermal ($\ge 20$~keV) electrons, expressed as a fraction of the number density of ambient protons in the same volume. This upper limit is about two orders of magnitude lower than previously inferred from microwave observations of the same event. Our results strongly indicate that the fraction of accelerated electrons in the coronal region at any given time is relatively small, but also that the overall duration of the HXR emission requires a steady resupply of electrons to the acceleration site. Simultaneous measurements of the instantaneous accelerated electron number density and the associated specific electron acceleration rate provide key constraints for a quantitative study of the mechanisms leading to electron acceleration in magnetic reconnection events.

arXiv comment5 figures, 10 pages

References in corpus (36)

• Differential Emission Measures from the Regularized Inversion of Hinode and SDO data
• Implications of X-ray Observations for Electron Acceleration and Propagation in Solar Flares
• Microwave and Hard X-Ray Observations of the 2017 September 10 Solar Limb Flare
• Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
• Measurement of magnetic field and relativistic electrons along a solar flare current sheet
• Global Energetics of Solar Flares: V. Energy Closure in Flares and Coronal Mass Ejections
• Double Coronal Hard and Soft X-ray Source Observed by RHESSI: Evidence for Magnetic Reconnection and Particle Acceleration in Solar Flares
• Electron Acceleration during Macroscale Magnetic Reconnection
• The Relationship Between Solar Radio and Hard X-ray Emission
• Relations between concurrent hard X-ray sources in solar flares
• Kappa Distribution Model for Hard X-Ray Coronal Sources of Solar Flares
• Turbulent kinetic energy in the energy balance of a solar flare
• Chromospheric magnetic field and density structure measurements using hard X-rays in a flaring coronal loop
• Statistical Properties of Super-hot Solar Flares
• Implications for electron acceleration and transport from non-thermal electron rates at looptop and footpoint sources in solar flares
• Kappa distribution and hard X-ray emission of solar flares
• A Statistical Survey of Hard X-ray Spectral Characteristics of Solar Flares with Two Footpoints
• Collisional relaxation of electrons in a warm plasma and accelerated nonthermal electron spectra in solar flares
• Properties of the Acceleration Regions in Several Loop-structured Solar Flares
• On the variation of solar flare coronal x-ray source sizes with energy
• Solar Physics with the Square Kilometre Array
• Energetic Electron Distribution of the Coronal Acceleration Region: First results from Joint Microwave and Hard X-ray Imaging Spectroscopy
• Onset of turbulent fast magnetic reconnection observed in the solar atmosphere
• A Classification Scheme For Turbulent Acceleration Processes In Solar Flares
• Multi-thermal representation of the kappa-distribution of solar flare electrons and application to simultaneous X-ray and EUV observations
• Understanding breaks in solar flares x-ray spectra: Evaluation of a co-spatial return-current model
• Diffusive Transport of Energetic Electrons in the Solar Corona: X-ray and Radio Diagnostics
• Determination of the total accelerated electron rate and power using solar flare hard X-ray spectra
• Electron Distribution Functions in Solar Flares from combined X-ray and EUV Observations
• The Spatial and Temporal Variations of Turbulence in a Solar Flare
• Global Energetics of Solar Flares: VIII. The Low-Energy Cutoff
• The Specific Acceleration Rate in Loop-structured Solar Flares -- Implications for Electron Acceleration Models
• Forward modelling of particle acceleration and transport in an individual solar flare
• Electron distribution and energy release in magnetic reconnection outflow regions during the pre-impulsive phase of a solar flare
• Instrumental oscillations in RHESSI count rates during solar flares
• Spectral and Imaging Diagnostics of Spatially-Extended Turbulent Electron Acceleration and Transport in Solar Flares

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• Energetic Nonthermal Electrons within the Above-the-looptop Regions in Solar Flares: Acceleration, Feedback, and Quasi-Periodic Pulsations
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