The influence of albedo on the size of hard X-ray flare sources

Battaglia, Marina; Kontar, Eduard P.; Hannah, I. G.

doi:10.1051/0004-6361/201014798

Cited by 12 publications

(11 citation statements)

References 43 publications

(52 reference statements)

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“…Most of our footpoint sizes and areas are upper limits where the measurements of the minor axes are upper limits. However, these upper limits are not likely to have been significantly increased by the albedo since Battaglia et al (2011) have shown that albedo increases the FWHM of a footpoint by less than 30%. The presence of extended albedo sources can in principle be inferred in the presence of compact footpoint sources from the relative intensities of the modulation amplitudes given by the finer and coarser subcollimators .…”

Section: Nonthermal Source Areasmentioning

confidence: 97%

Thermal and nonthermal hard X-ray source sizes in solar flares obtained from RHESSI observations

Warmuth

Mann

2013

A&A

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Context. In solar flares, a large amount of thermal and nonthermal energy is released impulsively in the form of heated plasma and accelerated particles. These processes can be studied via hard X-ray (HXR) diagnostics. In addition to spectroscopic observations, a thorough understanding of the thermal and nonthermal particle populations requires the knowledge of the HXR source sizes. Aims. We derive the geometric source parameters of both thermal coronal sources and the nonthermal HXR footpoints in solar flares. We compare and evaluate four different methods for obtaining source sizes, and then derive the most reliable source sizes, as well as the systematic uncertainties. Methods. We obtained time series of HXR images for 24 flares from GOES class C3.4 to X17.2 using the RHESSI instrument. The four imaging techniques employed are CLEAN, Pixon, visibility forward fit, and MEM_NJIT. From this data set, we derived the geometric parameters of the thermal HXR sources and the nonthermal footpoints. Using the different imaging techniques allowed us to quantify systematic measurement uncertainties. Results. We find that the different methods give consistent results on HXR source sizes. The correlations are very good for the thermal sources, and somewhat lower for the footpoints. The MEM_NJIT algorithm gives systematically smaller sizes than the other methods, possibly a result of over-resolution. Thermal source volumes are in the range of 2 × 10 25 −1.2 × 10 28 cm 3 (with a median relative uncertainty of 30%), and nonthermal footpoint areas in the range of 2 × 10 16 −6 × 10 17 cm 2 (median relative uncertainty: 40%). The thermal volumes of our sample are in the same range as those derived for microflares, which would imply that source size is not an important parameter for flare energetics. Conclusions. Using different imaging algorithms for determining HXR source sizes offers the advantage that uncertainties can be better quantified thus making the derived parameters more reliable. Combined with geometric parameters that were derived as time series in a larger number of flares, this will allow the study of the scaling relations and the temporal evolution of thermal and nonthermal HXR sources.

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Section: Nonthermal Source Areasmentioning

confidence: 97%

Thermal and nonthermal hard X-ray source sizes in solar flares obtained from RHESSI observations

Warmuth

Mann

2013

A&A

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“…The affect of albedo may also have contributed to the estimated footpoint area. Battaglia et al (2011) estimated that albedo could result in increased source areas by up to 30% for energies between 10 and 100 keV. Given these considerations, the measured footpoint area from HXR imaging should be considered as an overestimate.…”

Section: X-ray Imagingmentioning

confidence: 99%

Radiative hydrodynamic modelling and observations of the X-class solar flare on 2011 March 9

Kennedy

Milligan

Allred

et al. 2015

A&A

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Aims. We investigated the response of the solar atmosphere to non-thermal electron beam heating using the radiative transfer and hydrodynamics modelling code RADYN. The temporal evolution of the parameters that describe the non-thermal electron energy distribution were derived from hard X-ray observations of a particular flare, and we compared the modelled and observed parameters. Methods. The evolution of the non-thermal electron beam parameters during the X1.5 solar flare on 2011 March 9 were obtained from analysis of RHESSI X-ray spectra. The RADYN flare model was allowed to evolve for 110 s, after which the electron beam heating was ended, and was then allowed to continue evolving for a further 300 s. The modelled flare parameters were compared to the observed parameters determined from extreme-ultraviolet spectroscopy. Results. The model produced a hotter and denser flare loop than that observed and also cooled more rapidly, suggesting that additional energy input in the decay phase of the flare is required. In the explosive evaporation phase a region of high-density cool material propagated upward through the corona. This material underwent a rapid increase in temperature as it was unable to radiate away all of the energy deposited across it by the non-thermal electron beam and via thermal conduction. A narrow and high-density (n e ≤ 10 15 cm −3 ) region at the base of the flare transition region was the source of optical line emission in the model atmosphere. The collision-stopping depth of electrons was calculated throughout the evolution of the flare, and it was found that the compression of the lower atmosphere may permit electrons to penetrate farther into a flaring atmosphere compared to a quiet Sun atmosphere.

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“…Since all C class flares were imaged at 25−50 keV, their footpoints could appear larger than they are as compared to the stronger flares. However, the albedo component has a much lower flux density than the footpoints and is therefore unlikely to influence the FWHM measurements significantly (see also Battaglia et al 2011).…”

Section: Scaling Of Geometric Parameters With Flare Importancementioning

confidence: 99%

Thermal and nonthermal hard X-ray source sizes in solar flares obtained from RHESSI observations

Warmuth

Mann

2013

A&A

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Context. A thorough understanding of solar flares requires determining the physical parameters of both the hot thermal plasma and the accelerated nonthermal electrons. This can be provided by hard X-ray (HXR) observations. In addition to HXR spectroscopy, imaging is needed to measure the geometric HXR source sizes. These parameters may vary as a function of flare size, importance, and time. Aims. We determine the scaling relations of the geometric source parameters of both thermal and nonthermal HXR sources with respect to length scale and flare importance, and we characterize their temporal evolution. This is required for further studies involving parameters such as thermal energies, plasma densities, and current densities. Methods. In a previous paper, we obtained time series of the geometric HXR source sizes (thermal and nonthermal) for 24 flares from GOES class C3.4 to X17.2 using the RHESSI instrument. Here, we investigate how themal volumes, nonthermal footpoint areas, and footpoint separations depend on the flare length scale and GOES importance. In addition,we study the time evolution of these geometric parameters.Results. The increase in the thermal source volume with length scale is slightly below a Euclidian scaling, but not too far from it. The thermal source volume is correlated with GOES class, which contrasts with what was found for RHESSI microflares. The nonthermal footpoint areas, on the other hand, are not well-correlated with either length scale or GOES class. With regard to temporal evolution, the RHESSI thermal source volumes tend to show a more complex behavior than the simple growth-decline pattern typical of flares observed in EUV. In most events, the thermal volume shows a rapid initial rise to a peak in the early impulsive phase. This peak is correlated with the initial downward motion of the coronal source, and is consistent with the notion of initially contracting magnetic field lines. Conclusions. The behavior of the geometric parameters of thermal and nonthermal HXR sources is generally consistent with the standard model of eruptive solar flares: a quasi-Euclidian scaling of volume with length scale, an initially decreasing thermal volume due to field line shrinking, followed by an increase in both volume and footpoint separation with time as the arcade of reconnected flare loops grows. However, the thermal volume is not the dominant factor in determining thermal energy, as well as thermal SXR and HXR flux. Electron density and/or temperature seem to be more important parameters in this respect.

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The influence of albedo on the size of hard X-ray flare sources

Cited by 12 publications

References 43 publications

Thermal and nonthermal hard X-ray source sizes in solar flares obtained from RHESSI observations

Thermal and nonthermal hard X-ray source sizes in solar flares obtained from RHESSI observations

Radiative hydrodynamic modelling and observations of the X-class solar flare on 2011 March 9

Thermal and nonthermal hard X-ray source sizes in solar flares obtained from RHESSI observations

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