Are Coronal Loops Isothermal or Multithermal?

Schmelz, J. T.; Nasraoui, K.; Rightmire, Lisa; Kimble, J. A.; Zanna, G. Del; Cirtain, Jonathan; DeLuca, E. E.; Mason, H. E.

doi:10.1088/0004-637x/691/1/503

Cited by 30 publications

(36 citation statements)

References 28 publications

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“…This question is linked to the more basic question of whether there is flaring activity down to very small scales, that constitutes a major steady component of the heating, extending into the entire solar corona. Although very faint, such a component has been recently detected in active regions both with either filter ratios or differential emission measure (DEM) reconstruction performed by imaging broad-band instruments (Hinode/XRT, e.g., Reale et al 2009a,b;McTiernan 2009;Schmelz et al 2009) and spectroscopic line analyses (single-line or DEM reconstruction, e.g., Ko et al 2009;Patsourakos & Klimchuk 2009;Shestov et al 2010;Sylwester et al 2010). More solid confirmation has very recently been found (Testa & Reale 2012).…”

Section: Introductionmentioning

confidence: 99%

X-ray emitting hot plasma in solar active regions observed by the SphinX spectrometer

Miceli

Reale

Gburek

et al. 2012

A&A

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Aims. The detection of very hot plasma in the quiescent corona is important for diagnosing heating mechanisms. The presence and the amount of such hot plasma is currently debated. The SphinX instrument on-board the CORONAS-PHOTON mission is sensitive to X-ray emission of energies well above 1 keV and provides the opportunity to detect the hot plasma component. Methods. We analysed the X-ray spectra of the solar corona collected by the SphinX spectrometer in May 2009 (when two active regions were present). We modelled the spectrum extracted from the whole Sun over a time window of 17 days in the 1.34−7 keV energy band by adopting the latest release of the APED database. Results. The SphinX broadband spectrum cannot be modelled by a single isothermal component of optically thin plasma and two components are necessary. In particular, the high statistical significance of the count rates and the accurate calibration of the spectrometer allowed us to detect a very hot component at ∼7 million K with an emission measure of ∼2.7 × 10 44 cm −3 . The X-ray emission from the hot plasma dominates the solar X-ray spectrum above 4 keV. We checked that this hot component is invariably present in both the high and low emission regimes, i.e. even excluding resolvable microflares. We also present and discuss the possibility of a non-thermal origin (which would be compatible with a weak contribution from thick-target bremsstrahlung) for this hard emission component. Conclusions. Our results support the nanoflare scenario and might confirm that a minor flaring activity is ever-present in the quiescent corona, as also inferred for the coronae of other stars.

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Section: Introductionmentioning

confidence: 99%

X-ray emitting hot plasma in solar active regions observed by the SphinX spectrometer

Miceli

Reale

Gburek

et al. 2012

A&A

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“…Explaining the observations remains a challenge and a number of questions are currently under debate. For example, can loops be broadly classified as isothermal or multi-thermal (e.g., Schmelz et al 2009;Aschwanden & Boerner 2011)? Is heating impulsive or steady (e.g., Patsourakos & Klimchuk 2008;Tripathu et al 2010;Warren et al 2010)?…”

Section: Introductionmentioning

confidence: 99%

Heating of braided coronal loops

Wilmot-Smith

Pontin

Yeates

et al. 2011

A&A

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Aims. We investigate the relaxation of braided magnetic loops in order to find out how the type of braiding via footpoint motions affects resultant heating of the loop. Methods. Two magnetic loops, braided in different ways, are used as initial conditions in resistive MHD simulations and their subsequent evolution is studied. Results. The fields both undergo a resistive relaxation in which current sheets form and fragment and the system evolves towards a state of lower energy. In one case this relaxation is very efficient with current sheets filling the volume and homogeneous heating of the loop occurring. In the other case fewer current sheets develop, less magnetic energy is released in the process and a patchy heating of the loop results. The two cases, although very similar in their setup, can be distinguished by the mixing properties of the photospheric driver. The mixing can be measured by the topological entropy of the plasma flow, an observable quantity.

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“…Careful modeling of a high-resolution, high-quality spectrum would be needed to detect the κ-distributions from the Mg xi and Mg xii edges. This modeling would necessarily have to be accompanied by an EM-loci or DEM analysis (see e.g., Schmelz et al 2009Schmelz et al , 2011Warren et al 2011, for a recent application of these methods).…”

Section: Diagnostics Of the Distribution From The Continuummentioning

confidence: 99%

The non-Maxwellian continuum in the X-ray, UV, and radio range

Dudík

Kašparová

Dzifčáková

et al. 2012

A&A

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Aims. We investigate the X-ray, UV, and also the radio continuum arising from plasmas with a non-Maxwellian distribution of electron energies. The two investigated types of distributions are the κ-and n-distributions. Methods. We derived analytical expressions for the non-Maxwellian bremsstrahlung and free-bound continuum spectra. The spectra were calculated using available cross-sections. Then we compared the bremsstrahlung spectra arising from the different bremsstrahlung cross-sections that are routinely used in solar physics.Results. The behavior of the bremsstrahlung spectra for the non-Maxwellian distributions is highly dependent on the assumed type of the distribution. At flare temperatures and hard X-ray energies, the bremsstrahlung is greatly increased for κ-distributions and exhibits a strong high-energy tail. With decreasing κ, the maximum of the bremsstrahlung spectrum decreases and moves to higher wavelengths. In contrast, the maximum of the spectra for n-distributions increases with increasing n, and the spectrum then falls off very steeply with decreasing wavelength. In the millimeter radio range, the non-Maxwellian bremsstrahlung spectra are almost parallel to the thermal bremsstrahlung. Therefore, the non-Maxwellian distributions cannot be detected by off-limb observations made by the ALMA instrument. The free-bound continua are also highly dependent on the assumed type of the distribution. For n-distributions, the ionization edges disappear and a smooth continuum spectrum is formed for n 5. Opposite behavior occurs for κ-distributions where the ionization edges are in general significantly enhanced, with details depending on κ and T through the ionization equilibrium. We investigated how the non-Maxwellian κ-distributions can be determined from the observations of the continuum and conclude that one can sample the low-energy part of the distribution from the continuum.

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Are Coronal Loops Isothermal or Multithermal?

Cited by 30 publications

References 28 publications

X-ray emitting hot plasma in solar active regions observed by the SphinX spectrometer

X-ray emitting hot plasma in solar active regions observed by the SphinX spectrometer

Heating of braided coronal loops

The non-Maxwellian continuum in the X-ray, UV, and radio range

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