The height distribution of flare hard X-rays in thick and thin target models

Brown, J. C.; McClymont, A. N.

doi:10.1007/bf00152964

Cited by 35 publications

(8 citation statements)

References 27 publications

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“…Following accleration, electrons travel down the flare loop and undergo Coulomb collisions with the ambient plasma, reducing their energy from E 0 to E. Thus, at a given distance, z, along the loop, the spectrum becomes f (E, N(z)), where N(z) = − n(z)dz is the column depth and n(z) is the number density of the ambient plasma (Brown 1972). The energy lost to collisions is given by E 2 = E 2 0 − 2KN, (Brown 1972), where K = 2πe 4 Λ and Λ is the Coulomb logarithm for an ionised plasma, which is used here as observed emission originates from heights at which the solar atmosphere is well ionised (Brown & McClymont 1975;Emslie 1978).…”

Section: Thick Target Modellingmentioning

confidence: 99%

“…During the rise phase of a typical flare, the flux of HXRs reaches a peak and the spectral index hardens (Parks & Winckler 1969;Benz 1977;Fletcher & Hudson 2002). Based on the theoretical derivations of nonthermal X-ray intensity with height in the coronal acceleration scenario (Brown & McClymont 1975), this is expected to result in a descent of the location of peak nonthermal emission in the time coming up to the HXR peak. It was suggested that this downward motion of nonthermal X-ray sources was observed in the C1.1 class early impulsive flare that occurred on 28 November 2002 (SOL2002-11-28T04:37, Sui et al 2006).…”

Section: Introductionmentioning

confidence: 99%

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Solar flare X-ray source motion as a response to electron spectral hardening

O'Flannagain

Gallagher

Brown

et al. 2013

A&A

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Context. Solar flare hard X-rays (HXRs) are thought to be produced by nonthermal coronal electrons stopping in the chromosphere or remaining trapped in the corona. The collisional thick target model (CTTM) predicts that more energetic electrons penetrate to greater column depths along the flare loop. This requires that sources produced by harder power-law injection spectra should appear further down the legs or footpoints of a flareloop. Therefore, the frequently observed hardening of the injected power-law electron spectrum during flare onset should be concurrent with a descending hard X-ray source. Aims. We test this implication of the CTTM by comparing its predicted HXR source locations with those derived from observations of a solar flare which exhibits a nonthermally-dominated spectrum before the peak in HXRs, known as an early impulsive event.Methods. The HXR images and spectra of an early impulsive C-class flare were obtained using the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). Images were reconstructed to produce HXR source height evolutions for three energy bands. Spatially integrated spectral analysis was performed to isolate nonthermal emission and to determine the power-law index of the electron injection spectrum. The observed height-time evolutions were then fitted with CTTM-based simulated heights for each energy, using the electron spectral indices derived from the RHESSI spectra. Results. The flare emission was found to be dominantly nonthermal above ∼7 keV, with emission of thermal and nonthermal X-rays likely to be simultaneously observable below that energy. The density structure required for a good match between model and observed source heights agreed with previous studies of flare loop densities. Conclusions. The CTTM has been used to produce a descent of model HXR source heights that compares well with observations of this event. Based on this interpretation, downward motion of nonthermal sources should occur in any flare where there is spectral hardening in the electron distribution during a flare. However, this is often masked by thermal emission associated with flare plasma preheating. To date, flare models that predict transfer of energy from the corona to the chromosphere by means other than a flux of nonthermal electrons do not predict this observed source descent. Therefore, flares such as this will be key in explaining this elusive energy transfer process.

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Section: Thick Target Modellingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Solar flare X-ray source motion as a response to electron spectral hardening

O'Flannagain

Gallagher

Brown

et al. 2013

A&A

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“…To study possible effects of collisions, we compared the change in the electron spectrum and the resulting footpoint spectrum for collisional energy loss and energy loss due to the electric field. The change in the electron spectrum due to collisions depends on the column depth through which the beam passes and was computed by Leach & Petrosian (1981) and Brown & McClymont (1975). We assume a column depth derived from the density in the loop times half the loop length.…”

Section: Collisions and Other Possible Scenariosmentioning

confidence: 99%

Observational evidence for return currents in solar flare loops

Battaglia

Benz

2008

A&A

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Context. The common flare scenario comprises an acceleration site in the corona and particle transport to the chromosphere. Using satellites available to date it has become possible to distinguish between the two processes of acceleration and transport, and study the particle propagation in flare loops in detail, as well as complete comparisons with theoretical predictions. Aims. We complete a quantitative comparison between flare hard X-ray spectra observed by RHESSI and theoretical predictions. This enables acceleration to be distinguished from transport and the nature of transport effects to be explored. Methods. Data acquired by the RHESSI satellite were analyzed using full sun spectroscopy as well as imaging spectroscopy methods. Coronal source and footpoint spectra of well observed limb events were analyzed and quantitatively compared to theoretical predictions. New concepts are introduced to existing models to resolve discrepancies between observations and predictions. Results. The standard thin-thick target solar flare model cannot explain the observations of all events. In the events presented here, propagation effects in the form of non-collisional energy loss are of importance to explain the observations. We demonstrate that those energy losses can be interpreted in terms of an electric field in the flare loop. One event seems consistent with particle propagation or acceleration in lower than average density in the coronal source. Conclusions. We find observational evidence for an electric field in flare loops caused by return currents.

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“…X-ray source heights have previously been used to test models of accelerated electrons in the solar atmosphere. Brown & McClymont (1975) showed that X-ray source height measurements can potentially discriminate between the thick-and thin-target models. Similarly, and used source heights to derive a chromospheric density measurement directly from observed spectra with RHESSI (Lin et al 2002), with a similar analysis performed by Kontar, Hannah, & MacKinnon (2008) to measure field and density variations with height.…”

Section: Introductionmentioning

confidence: 99%

X-Ray Source Heights in a Solar Flare: Thick-Target Versus Thermal Conduction Front Heating

Reep

Bradshaw

Holman

2016

ApJ

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Observations of solar flares with RHESSI have shown X-ray sources traveling along flaring loops, from the corona down to the chromosphere and back up. The 28 November 2002 C1.1 flare, first observed with RHESSI by Sui, Holman, & Dennis (2006) and quantitatively analyzed by O'Flannagain et al. ( 2013), very clearly shows this behavior. By employing numerical experiments, we use these observations of X-ray source height motions as a constraint to distinguish between heating due to a non-thermal electron beam and in situ energy deposition in the corona. We find that both heating scenarios can reproduce the observed light curves, but our results favor non-thermal heating. In situ heating is inconsistent with the observed X-ray source morphology and always gives a height dispersion with photon energy opposite to what is observed.

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The height distribution of flare hard X-rays in thick and thin target models

Cited by 35 publications

References 27 publications

Solar flare X-ray source motion as a response to electron spectral hardening

Solar flare X-ray source motion as a response to electron spectral hardening

Observational evidence for return currents in solar flare loops

X-Ray Source Heights in a Solar Flare: Thick-Target Versus Thermal Conduction Front Heating

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