Intensity variations in EIT shutterless mode: Waves or flows?

Groof, A. De; Berghmans, D.; Driel-Gesztelyi, L. van; Poedts, Stefaan

doi:10.1051/0004-6361:20034252

Cited by 70 publications

(97 citation statements)

References 19 publications

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“…It is almost certainly responsible for prominences (Antiochos & Klimchuk 1991;Karpen et al 2006), and it may also explain "catastrophic cooling events," including coronal rain (Schrijver 2001;De Groof et al 2004;O'Shea et al 2007). During these events, a cold "blob" condenses out of the hot corona at the top of a loop.…”

Section: Discussionmentioning

confidence: 99%

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Can Thermal Nonequilibrium Explain Coronal Loops?

Klimchuk

Karpen

Antiochos

2010

ApJ

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Any successful model of coronal loops must explain a number of observed properties. For warm (∼1 MK) loops, these include (1) excess density, (2) flat temperature profile, (3) super-hydrostatic scale height, (4) unstructured intensity profile, and (5) 1000-5000 s lifetime. We examine whether thermal nonequilibrium can reproduce the observations by performing hydrodynamic simulations based on steady coronal heating that decreases exponentially with height. We consider both monolithic and multi-stranded loops. The simulations successfully reproduce certain aspects of the observations, including the excess density, but each of them fails in at least one critical way. Monolithic models have far too much intensity structure, while multi-strand models are either too structured or too long-lived. Our results appear to rule out the widespread existence of heating that is both highly concentrated low in the corona and steady or quasi-steady (slowly varying or impulsive with a rapid cadence). Active regions would have a very different appearance if the dominant heating mechanism had these properties. Thermal nonequilibrium may nonetheless play an important role in prominences and catastrophic cooling events (e.g., coronal rain) that occupy a small fraction of the coronal volume. However, apparent inconsistencies between the models and observations of cooling events have yet to be understood.

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Section: Discussionmentioning

confidence: 99%

“…This contrasts sharply with observed loops, which tend to have a far more uniform appearance. Falling bright knots are sometimes observed, but these are only detected at much cooler temperatures ( 0.1 MK) (Schrijver 2001;De Groof et al 2004;O'Shea et al 2007). We return to the subject of these knots later in Section 4.…”

Section: Intensitymentioning

confidence: 98%

Can Thermal Nonequilibrium Explain Coronal Loops?

Klimchuk

Karpen

Antiochos

2010

ApJ

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“…There is some observational evidence of nanoflares in the coronal X-ray emission (Katsukawa & Tsuneta 2001;Schmelz et al 2009). The bright points in coronal EUV lines (Madjarska et al 2003;Tian et al 2008) and the phenomenon of coronal rain (De Groof et al 2004;Antolin et al 2010) may also be associated with nanoflares. But the firm observational evidence of nanoflare heating of the solar corona is still absent.…”

Section: Introductionmentioning

confidence: 99%

“…Also, the hot plasma then can be quickly cooled down to the transition region or chromospheric temperatures owing to "catastrophic cooling" (Antiochos & Klimchuk 1991;Schrijver 2001;Müller et al 2003Müller et al , 2004Karpen et al 2006;Klimchuk et al 2010). The resulting dense and cool plasma blobs may then be attracted by gravity, slide down along the magnetic field lines, and result in "coronal rain" (De Groof et al 2004;Antolin et al 2010). In addition, such a compressible perturbation would generate an outgoing magnetoacoustic wave (Longcope & Priest 2007), which can be subject to steepening.…”

Section: Introductionmentioning

confidence: 99%

Entropy mode at a magnetic null point as a possible tool for indirect observation of nanoflares in the solar corona

Murawski

Zaqarashvili

Nakariakov

2011

A&A

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Aims. We aim to explore the dynamics of the entropy mode perturbation excited by an energy release in the vicinity of a magnetic null point that is embedded in a gravitationally stratified solar corona. Methods. We solve two-dimensional, time-dependent magnetohydrodynamic equations numerically to find spatial and temporal signatures of the entropy mode that is triggered impulsively by a spatially localized pulse of the gas pressure. Results. We find that the properties of the entropy mode are determined by the sign of the initial pressure pulse. The initial increase in the gas pressure creates, together with the magnetoacoustic-gravity waves, a stationary void of the rarefied plasma at the launching place, associated with the entropy mode. In contrast, an initial decrease in the gas pressure, which corresponds to a rapid (or catastrophic) cooling, forms a blob of the dense plasma at the launching place. Conclusions. The cool, dense blobs at magnetic null points may be observed in transition region and chromospheric spectral lines at coronal heights off the solar limb and may be associated with the places of nanoflare occurrence. Therefore, extensions of entropy mode studies may produce a diagnostic tool for indirect observations of nanoflares. The dense cool blobs may be affected by the gravity or carried by downflows, hence may initiate a coronal rain.

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“…In fact, thermal non-equilibrium may produce plasma flows, the formation of condensations, and the loss of connection between electron temperature and the ionization status of a plasma. Thermal non-equilibrium has been suggested to be responsible for the formation of prominences from coronal loops (e.g., Antiochos 1980; Karpen et al 2006, and references therein) and intensity variations traveling along the loop structure (Müller et al 2005) observed with the He ii SOHO/EIT band (De Groof et al 2004). Also, it has been recently suggested as a possible explanation of the main observational properties of coronal loops, including lifetime, temperature and density profiles, and filamentary structure (Klimchuk et al 2010).…”

Section: Introductionmentioning

confidence: 99%

Post-Flare Ultraviolet Light Curves Explained With Thermal Instability of Loop Plasma

Reale

Landi

Orlando

2012

ApJ

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In the present work, we study the C8 flare that occurred on 2000 September 26 at 19:49 UT and observed by the Solar and Heliospheric Observatory/Solar Ultraviolet Measurement of Emitted Radiation spectrometer from the beginning of the impulsive phase to well beyond the disappearance in the X-rays. The emission first decayed progressively through equilibrium states until the plasma reached 2-3 MK. Then, a series of cooler lines, i.e., Ca x, Ca vii, Ne vi, O iv, and Si iii (formed in the temperature range log T = 4.3-6.3 under equilibrium conditions), are emitted at the same time and all evolve in a similar way. Here, we show that the simultaneous emission of lines with such a different formation temperature is due to thermal instability occurring in the flaring plasma as soon as it has cooled below ∼2 MK. We can qualitatively reproduce the relative start time of the light curves of each line in the correct order with a simple (and standard) model of a single flaring loop. The agreement with the observed light curves is greatly improved, and a slower evolution of the line emission is predicted, if we assume that the model loop consists of an ensemble of subloops or strands heated at slightly different times. Our analysis can be useful for flare observations with the Solar Dynamics Observatory/Extreme ultraviolet Variability Experiment.

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Intensity variations in EIT shutterless mode: Waves or flows?

Cited by 70 publications

References 19 publications

Can Thermal Nonequilibrium Explain Coronal Loops?

Can Thermal Nonequilibrium Explain Coronal Loops?

Entropy mode at a magnetic null point as a possible tool for indirect observation of nanoflares in the solar corona

Post-Flare Ultraviolet Light Curves Explained With Thermal Instability of Loop Plasma

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