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Fletcher, L.; Hudson, H. S.

doi:10.1023/a:1022479610710

Cited by 128 publications

(87 citation statements)

References 14 publications

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“…The flux change rate and energy release rate also show episodic variations except that for these quantities the change in area also contributes to the temporal variations. These results are in agreement with previous studies showing correlations of X-ray light curves with velocity (Fletcher & Hudson 2001), electric field (Qiu et al 2002), and flux change rate (Jing et al 2005). A new result is that the area measured in the blue wing of H also shows an episodic variation and thus plays a role in the time variation of the magnetic energy release.…”

Section: Magnetic Energy Releasesupporting

confidence: 92%

“…Upon integration of the electric field over the ribbon length, we can further determine the electric potential drop or, equivalently, the rate of reconnecting magnetic flux as an important measure for both magnetic reconnection and particle acceleration (Priest & Forbes 2000. These theoretical ideas opened a new area of study in which the flux reconnection rate derived from ribbon motion is compared with the timing of hard X-ray and microwave emissions, and the result often provides qualitative support for magnetic reconnection as the flare driver on the basis of a similarity of time profiles (e.g., Fletcher & Hudson 2001Asai et al 2002Asai et al , 2004aQiu et al 2002;Jing et al 2005;Isobe et al 2002Isobe et al , 2005Noglik et al 2005).…”

Section: Introductionmentioning

confidence: 99%

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Magnetic Energy Release during the 2002 September 9 Solar Flare

Lee

Gary

Choe

2006

ApJ

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We study the magnetic energy release during the 2002 September 9 flare using the high-cadence (40 ms) H filtergram at Big Bear Solar Observatory (BBSO), along with hard X-ray and microwave data from the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI ) and the Owens Valley Solar Array (OVSA), respectively. We take the Poynting vector approach with the standard two-dimensional geometry of the reconnecting current sheet (RCS) but suggest a new technique to infer the area of the RCS, in order to complete the magnetic energy calculation entirely with observed quantities. We found five peaks of impulsive magnetic energy release, concentrated within 10-30 s periods, that are episodic with the peaks of the hard X-ray light curve. The maximum amount of energy released per peak reaches $2:6 ; 10 30 ergs s À1 , and the electron energy deposition rate derived from the RHESSI spectra falls into the range of 10%-80% of the magnetic energy release rate. We briefly discuss this result in comparison with other studies thus far made toward understanding of the magnetic reconnection in solar flares and suggest the pulsating current sheet model as the most plausible interpretation of our result.

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Section: Magnetic Energy Releasesupporting

confidence: 92%

Section: Introductionmentioning

confidence: 99%

Magnetic Energy Release during the 2002 September 9 Solar Flare

Lee

Gary

Choe

2006

ApJ

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“…This therefore allows for the detection of any source motion between the coronal looptop and chromospheric footpoints. 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.…”

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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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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“…The measurements of the X-ray source positions (first moments) pinpoint source locations with 1 or better accuracy and allow us to infer the chromospheric density structure (Aschwanden et al 2002;Liu et al 2006;Kontar et al 2008b). The motions of HXR footpoint locations have been used to infer the reconnection rate in solar flares (Fletcher & Hudson 2002;Krucker et al 2003;Fivian et al 2009). Using X-ray visibilities Schmahl et al 2007) Kontar et al (2008b) have measured not only the positions but the HXR footpoint sizes (second moment) at various energies and heights and found that HXR sources decrease with energy and consequently with height above the photosphere.…”

Section: Introductionmentioning

confidence: 99%

Positions and sizes of X-ray solar flare sources

Kontar¹,

Jeffrey²

2010

A&A

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Aims. The positions and source sizes of X-ray sources taking into account Compton backscattering (albedo) are investigated. Methods. Using a Monte Carlo simulation of X-ray photon transport including photo-electric absorption and Compton scattering, we calculate the apparent source sizes and positions of X-ray sources at the solar disk for various source sizes, spectral indices and directivities of the primary source. Results. We show that the albedo effect can alter the true source positions and substantially increase the measured source sizes. The source positions are shifted by up to ∼0.5 radially towards the disk centre and 5 arcsec source sizes can be two times larger even for an isotropic source (minimum albedo effect) at 1 Mm above the photosphere. The X-ray sources therefore should have minimum observed sizes, and thus their FWHM source size (2.35 times second-moment) will be as large as ∼7 in the 20−50 keV range for a disk-centered point source at a height of 1 Mm (∼1.4 ) above the photosphere. The source size and position change is greater for flatter primary X-ray spectra, a stronger downward anisotropy, for sources closer to the solar disk centre, and between the energies of 30 and 50 keV. Conclusions. Albedo should be taken into account when X-ray footpoint positions, footpoint motions or source sizes from e.g. RHESSI or Yohkoh data are interpreted, and we suggest that footpoint sources should be larger in X-rays than in either optical or EUV ranges.

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Cited by 128 publications

References 14 publications

Magnetic Energy Release during the 2002 September 9 Solar Flare

Magnetic Energy Release during the 2002 September 9 Solar Flare

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

Positions and sizes of X-ray solar flare sources

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