Spatially inhomogeneous acceleration of electrons in solar flares

Stackhouse, Duncan J.; Kontar, Eduard P.

doi:10.1051/0004-6361/201730708

Cited by 7 publications

(9 citation statements)

References 54 publications

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“…Here, the particles experience a fluctuating energy increase owing to the higher likelihood of (accelerating) head-on collisions compared to (decelerating) rear-end collisions. This kind of stochastic acceleration is, like direct acceleration, typically predicted to produce a power-law energy distribution for the accelerated particles, both in models based on the Fokker-Planck formalism (Miller et al 1996;Stackhouse & Kontar 2018) and in test particle simulations (Dmitruk et al 2003;Onofri et al 2006).…”

Section: Initial Particle Distributionsmentioning

confidence: 98%

Accelerated particle beams in a 3D simulation of the quiet Sun

Frogner,

Gudiksen,

Bakke

2020

Preprint

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Context. Observational and theoretical evidence suggests that beams of accelerated particles are produced in flaring events of all sizes in the solar atmosphere, from X-class flares to nanoflares. Current models of such particles in flaring loops assume an isolated 1D atmosphere. Aims. A more realistic environment for modelling accelerated particles can be provided by 3D radiative magnetohydrodynamics codes. Here we present a simple model for particle acceleration and propagation in the context of a 3D simulation of the quiet solar atmosphere spanning from convection zone to corona. We then examine the additional transport of energy introduced by the particle beams. Methods. Locations of particle acceleration associated with magnetic reconnection are identified by detecting changes in magnetic topology. At each location, the parameters of the accelerated particle distribution are estimated from local conditions. The particle distributions are then propagated along the magnetic field, and the energy deposition due to Coulomb collisions with the ambient plasma is computed. Results. We find that particle beams originate in extended acceleration regions distributed across the corona. Upon reaching the transition region, they converge and produce strands of intense heating penetrating the chromosphere. Within these strands, beam heating consistently dominates conductive heating below the bottom of the transition region. This indicates that particle beams will qualitatively alter the energy transport even outside of active regions.

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Section: Initial Particle Distributionsmentioning

confidence: 98%

Accelerated particle beams in a 3D simulation of the quiet Sun

Frogner,

Gudiksen,

Bakke

2020

Preprint

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“…In Stackhouse & Kontar (2018), electrons are accelerated over an extended acceleration region using a spatially dependent diffusion coefficient, where the spatial distribution is given as an exponential function. In order to explore different acceleration regions, we adapt the acceleration diffusion coefficient in Stackhouse & Kontar (2018) to allow us to choose various spatial distributions. Thus, in this paper, the acceleration diffusion coefficient D(v, z) is given by:…”

Section: Turbulent Acceleration Scattering and Timescalesmentioning

confidence: 99%

“…The insightful introduction of a spatially dependent turbulent acceleration diffusion coefficient in Stackhouse & Kontar (2018) shows the importance of accounting for a spatial variation in turbulence, producing a softer spectrum than the spatially averaged description of electron acceleration and transport given in the leaky box model (Chen & Petrosian 2013). Following the observational results of Stores et al (2021) and the preliminary work of Stackhouse & Kontar (2018), we perform a detailed parameter study examining how the presence and varying properties of a spatially extended turbulent region in the corona changes the spectral and spatial (imaging) properties of observed flare-accelerated electrons deduced from X-ray spectroscopy and imaging. Section 2 provides an overview of the electron acceleration and transport model, Section 3 discusses the main results, and Section 4 summarizes the study and its application to HXR data.…”

Section: Introductionmentioning

confidence: 99%

Spectral and Imaging Diagnostics of Spatially Extended Turbulent Electron Acceleration and Transport in Solar Flares

Stores

Jeffrey

McLaughlin

2023

ApJ

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Solar flares are efficient particle accelerators with a large fraction of released magnetic energy (10%–50%) converted into energetic particles such as hard X-ray producing electrons. This energy transfer process is not well constrained, with competing theories regarding the acceleration mechanism(s), including MHD turbulence. We perform a detailed parameter study examining how various properties of the acceleration region, including its spatial extent and the spatial distribution of turbulence, affect the observed electron properties, such as those routinely determined from X-ray imaging and spectroscopy. Here, a time-independent Fokker–Planck equation is used to describe the acceleration and transport of flare electrons through a coronal plasma of finite temperature. Motivated by recent nonthermal line broadening observations that suggested extended regions of turbulence in coronal loops, an extended turbulent acceleration region is incorporated into the model. We produce outputs for the density-weighted electron flux, a quantity directly related to observed X-rays, modeled in energy and space from the corona to chromosphere. We find that by combining several spectral and imaging diagnostics (such as spectral index differences or ratios, energy or spatial-dependent flux ratios, and electron depths into the chromosphere) the acceleration properties, including the timescale and velocity dependence, can be constrained alongside the spatial properties. Our diagnostics provide a foundation for constraining the properties of acceleration in an individual flare from X-ray imaging spectroscopy alone, and can be applied to past, current, and future observations including those from RHESSI and Solar Orbiter.

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“…It is interesting to note that the fall in non-thermal velocity (turbulence) is approximately linear along the blue line connecting the loop apex and loop leg at times corresponding to the peak in HXRs and after (and approximately linear along the yellow radial line connecting the different loop tops). In recent modeling studying the effects of an extended spatial distribution of turbulence on electron acceleration e.g., Stackhouse & Kontar (2018), the turbulent acceleration diffusion coefficient is modeled as a decreasing exponential in space (along the coronal loop) since the form of the spatial distribution of turbulence accelerating electrons in the flare is usually unknown. However, although the exact relationship between the diffusion coefficient and the macroscopic velocity v nth is not entirely clear, we can see that the decrease in v nth in space along the loop is much slower than exponential, suggesting that the turbulence may influence the acceleration or transport of electrons over a larger spatial extent than modeled to date.…”

Section: Spatial Changes In V Nthmentioning

confidence: 99%

The Spatial and Temporal Variations of Turbulence in a Solar Flare

Stores,

Jeffrey,

Kontar

2021

Preprint

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Magnetohydrodynamic (MHD) plasma turbulence is believed to play a vital role in the production of energetic electrons during solar flares and the non-thermal broadening of spectral lines is a key sign of this turbulence. Here, we determine how flare turbulence evolves in time and space using spectral profiles of Fe xxiv, Fe xxiii and Fe xvi, observed by Hinode/EIS. Maps of non-thermal velocity are created for times covering the X-ray rise, peak, and decay. For the first time, the creation of kinetic energy density maps reveal where energy is available for energization, suggesting that similar levels of energy may be available to heat and/or accelerate electrons in large regions of the flare. We find that turbulence is distributed throughout the entire flare; often greatest in the coronal loop tops, and decaying at different rates at different locations. For hotter ions (Fe xxiv and Fe xxiii), the non-thermal velocity decreases as the flare evolves and during/after the X-ray peak shows a clear spatial variation decreasing linearly from the loop apex towards the ribbon. For the cooler ion (Fe xvi), the nonthermal velocity remains relativity constant throughout the flare, but steeply increases in one region corresponding to the southern ribbon, peaking just prior to the peak in hard X-rays before declining. The results suggest turbulence has a more complex temporal and spatial structure than previously assumed, while newly introduced turbulent kinetic energy maps show the availability of the energy and identify important spatial inhomogeneities in the macroscopic plasma motions leading to turbulence.

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Spatially inhomogeneous acceleration of electrons in solar flares

Cited by 7 publications

References 54 publications

Accelerated particle beams in a 3D simulation of the quiet Sun

Accelerated particle beams in a 3D simulation of the quiet Sun

Spectral and Imaging Diagnostics of Spatially Extended Turbulent Electron Acceleration and Transport in Solar Flares

The Spatial and Temporal Variations of Turbulence in a Solar Flare

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