Can Thermal Nonequilibrium Explain Coronal Loops?

Klimchuk, J. A.; Karpen, J. T.; Antiochos, S. K.

doi:10.1088/0004-637x/714/2/1239

Cited by 91 publications

(106 citation statements)

References 42 publications

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“…The most basic structures are coronal loops, which are thin, elongated, arch-like, bent cylinders delineating magnetic field lines, rooted in the solar chromosphere on one or both ends. Coronal loops commonly occur above photospheric magnetic flux concentrations, i.e., in active regions, with the hot X-ray loops (>2 MK; e.g., Klimchuk et al 2010) constituting the active region core, and the warm (∼1 MK) EUV loops located on the periphery. Apart from the coronal loops, coronal emission can originate from bright points located above small bipolar photospheric regions, and in the EUV spectral domain from the moss (e.g., Peres et al 1994;Berger et al 1999;Fletcher & De Pontieu 1999;Schrijver et al 1999;Martens et al 2000), i.e., upper transition region of hot, high-pressure loops.…”

Section: Introductionmentioning

confidence: 99%

Is it possible to model observed active region coronal emission simultaneously in EUV and X-ray filters?

Dudík

Dzifčáková

Karlický

et al. 2011

A&A

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Aims. We investigate the possibility of modeling the active region coronal emission in the EUV and X-ray filters using one, universal, steady heating function, tied to the properties of the magnetic field. Methods. We employ a simple, static model to compute the temperature and density distributions in the active region corona. The model allows us to explore a wide range of parameters of the heating function. The predicted EUV and X-ray emission in the filters of EIT/SOHO and XRT/Hinode are calculated and compared with observations. Using the combined improved filter-ratio (CIFR) method, a temperature diagnostic is employed to compare the modeled temperature structure of the active region with the temperature structure derived from the observations. Results. The global properties of the observations are most closely matched for heating functions scaling as B 0.7−0.8 0 /L 0.5 0 that depend on the spatially variable heating scale-length. The modeled X-ray emission originates from locations where large heating scale-lengths are found. However, the majority of the loops observed in the 171 and 195 filters can be modeled only by loops with very short heating scale-lengths. These loops are known to be thermally unstable. We are unable to find a model that both matches the observations in all EUV and X-ray filters, and contains only stable loops. As a result, although our model with a steady heating function can explain some of the emission properties of the 171 and 195 loops, it cannot explain their observed lifetimes. Thus, the model does not lead to a self-consistent solution. The performance of the CIFR method is evaluated and we find that the diagnosed temperature can be approximated with a geometric mean of the emission-measure weighted and maximum temperature along the line of sight. Conclusions. We conclude that if one universal heating function exists, it should be at least partially time-dependent.

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Section: Introductionmentioning

confidence: 99%

Is it possible to model observed active region coronal emission simultaneously in EUV and X-ray filters?

Dudík

Dzifčáková

Karlický

et al. 2011

A&A

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“…Heating at both footpoints of the loops can also produce flows if it is significantly concentrated toward the footpoints (Klimchuk et al 2010, and references therein). The source of the heating could be either truly steady or the frequency of the impulsive heating could be sufficiently high that a steady approximation would be valid.…”

Section: Introductionmentioning

confidence: 99%

Doppler shift of hot coronal lines in a moss area of an active region

Dadashi

Teriaca

Tripathi

et al. 2012

A&A

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The moss is the area at the footpoint of the hot (3 to 5 MK) loops forming the core of the active region where emission is believed to result from the heat flux conducted down to the transition region from the hot loops. Studying the variation of Doppler shift as a function of line formation temperatures over the moss area can give clues on the heating mechanism in the hot loops in the core of the active regions. We investigate the absolute Doppler shift of lines formed at temperatures between 1 MK and 2 MK in a moss area within active region NOAA 11243 using a novel technique that allows determining the absolute Doppler shift of EUV lines by combining observations from the SUMER and EIS spectrometers. The inner (brighter and denser) part of the moss area shows roughly constant blue shift (upward motions) of 5 km s −1 in the temperature range of 1 MK to 1.6 MK. For hotter lines the blue shift decreases and reaches 1 km s −1 for Fe xv 284 Å (∼2 MK). The measurements are discussed in relation to models of the heating of hot loops.The results for the hot coronal lines seem to support the quasi-steady heating models for nonsymmetric hot loops in the core of active regions.

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“…Consequently, Klimchuk et al (2010) argue that the heat input for active region loops is probably not Movies are available in electronic form at http://www.aanda.org concentrated near the loop's footpoints alone. The authors furthermore suggest that the heating is probably not steady.…”

Section: Introductionmentioning

confidence: 99%

“…When the loop cross-section is allowed to vary, and especially when the heating profile is made asymmetric, the nature of catastrophic cooling may be significantly different than concluded by Klimchuk et al (2010). It may be premature to rule out thermal non-equilibrium for explaining some of the observed properties of coronal loops (Mikić, priv.…”

Section: Introductionmentioning

confidence: 99%

Catastrophic cooling and cessation of heating in the solar corona

Peter

Bingert

Kamio

2012

A&A

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Context. Condensations in the more than 10 6 K hot corona of the Sun are commonly observed in the extreme ultraviolet (EUV). While their contribution to the total solar EUV radiation is still a matter of debate, these condensations certainly provide a valuable tool for studying the dynamic response of the corona to the heating processes. Aims. We investigate different distributions of energy input in time and space to investigate which process is most relevant for understanding these coronal condensations. Methods. For a comparison to observations we synthesize EUV emission from a time-dependent, one-dimensional model for coronal loops, where we employ two heating scenarios: simply shutting down the heating and a model where the heating is very concentrated at the loop footpoints, while keeping the total heat input constant. Results. The heating off/on model does not lead to significant EUV count rates that one observes with SDO/AIA. In contrast, the concentration of the heating near the footpoints leads to thermal non-equilibrium near the loop top resulting in the well-known catastrophic cooling. This process gives a good match to observations of coronal condensations. Conclusions. This shows that the corona needs a steady supply of energy to support the coronal plasma, even during coronal condensations. Otherwise the corona would drain very fast, too fast to even form a condensation.

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Can Thermal Nonequilibrium Explain Coronal Loops?

Cited by 91 publications

References 42 publications

Is it possible to model observed active region coronal emission simultaneously in EUV and X-ray filters?

Is it possible to model observed active region coronal emission simultaneously in EUV and X-ray filters?

Doppler shift of hot coronal lines in a moss area of an active region

Catastrophic cooling and cessation of heating in the solar corona

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