The Effect of Background Subtraction on the Temperature of EIT Coronal Loops

Schmelz, J. T.; Beene, J. E.; Nasraoui, K.; Blevins, H. T.; Martens, Petrus C.; Cirtain, Jonathan

doi:10.1086/379212

Cited by 36 publications

(43 citation statements)

References 37 publications

(53 reference statements)

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“…One of the curious results from Schmelz et al (2003Schmelz et al ( , 2007 was that the 171-195 ratio always seemed A to produce a loop temperature of 1.2 MK (within uncertainties), even though the ratio should have been sensitive to a much broader temperature range. The work of Testa et al (2002) and Martens et al (2002) suggested that the answer could be attributed to multithermal plasma along the line of sight.…”

Section: Discussionmentioning

confidence: 95%

“…This second-generation analysis technique found results similar to those described above-a single temperature with no significant variations along the loop length. Schmelz et al (2003Schmelz et al ( , 2007 used several background subtraction methods to produce loops with a uniform temperature, but the 171-195 and 195-284 ratios resulted in statistically different temperature values A (1.2 and 1.8 MK, respectively) for the same pixel, indicating that the plasma along the line of sight might not be isothermal. Their temperature results were the same with and without background subtraction, and more suspiciously, the results were similar for loop pixels and for randomly selected pixels in the region, indicating that ratio analysis might not generate a physically meaningful value of temperature.…”

Section: Introductionmentioning

confidence: 99%

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Coronal Heat: Solar Loop Temperatures from TRACE Triple-Filter Data

Schmelz

Kashyap

Weber

2007

ApJ

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The Transition Region and Coronal Explorer (TRACE) has state-of-the-art spatial resolution and shows the most detailed images of coronal loops ever observed. The temperatures of these loops are primarily derived from the 171 to 195 filter ratio, with data from the third filter at 284 used by several authors to improve theÅ A precision of the derived temperatures. Most of these studies assume that the plasma is isothermal and model the loops primarily as uniform temperature structures with footpoint-dominated heating. However, these triple-filter data are insufficient to constrain the plasma temperature and cannot be used to determine the isothermality or otherwise of coronal loop structures. We show this explicitly by constructing differential emission measures with these same triple-filter data using a sophisticated Markov-chain Monte Carlo-based reconstruction algorithm. We find that these TRACE data cannot, in general, limit the temperature distribution for coronal loop plasma. In other words, many different temperature distributions (isothermal, broad, sloped, etc.) can reproduce the observed fluxes, and the TRACE coronal data alone cannot determine which of these distributions represents the actual coronal plasma.

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Section: Discussionmentioning

confidence: 95%

Section: Introductionmentioning

confidence: 99%

Coronal Heat: Solar Loop Temperatures from TRACE Triple-Filter Data

Schmelz

Kashyap

Weber

2007

ApJ

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“…The implied assumption is that in the volume of consideration the plasma is isothermal, which is only an approximation and not always true in coronal structures. While the significance of temperature estimates from narrowband instruments like the Extreme-Ultraviolet Imaging Telescope, on board SOHO, or TRACE has already been questioned (Martens et al 2002;Schmelz et al 2003;Del Zanna & Mason 2003), the estimates from SXT have been generally accepted in the literature. However, they can also present problems if the plasma is multithermal (e.g., Aschwanden & Nitta 2000).…”

Section: Temperaturesmentioning

confidence: 99%

An Investigation into the Variability of Heating in a Solar Active Region

Ugarte-Urra

Winebarger

Warren

2006

ApJ

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Previous studies have indicated that both steady and impulsive heating mechanisms play a role in active region heating. In this paper, we present a study of 20 hours of soft X-ray and EUV observations of solar active region NOAA AR 8731. We examine the evolution of six representative loop structures that brighten and fade first from X-ray images and subsequently from the EUV images. We determine their lifetime and the delay between their appearance in the different filters. We find that the lifetime in the EUV filters is much longer than expected for a single cooling loop. We also notice that the delay in the loops' appearance in the X-ray and EUV filters is proportional to the loop length. We model one of the loops using a hydrodynamic model with both impulsive and quasisteady heating functions and find that neither of these simple heating functions can well reproduce the observed loop characteristics in both the X-ray and EUV images. Hence, although this active region is dominated by variable emission and the characteristics of the observed loops are qualitatively consistent with a cooling loop, the timescale of the heating in this active region remains unknown.

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“…Aschwanden et al 2003;Schmelz et al 2003;Terzo & Reale 2010). By combining the theories for the propagation and damping of the transverse wave it is possible to constrain the unknown parameters in the problem (Verwichte et al 2006) self-consistently.…”

Section: Introductionmentioning

confidence: 99%

Statistical seismology of transverse waves in the solar corona

Verwichte

Doorsselaere

White

et al. 2013

A&A

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Context. Observations show that transverse oscillations commonly occur in solar coronal loops. The rapid damping of these waves has been attributed to resonant absorption. The oscillation characteristics carries information of the structuring of the corona. However, self-consistent seismological methods that extract information from individual oscillations are limited because there are fewer observables than unknown parameters in the model, and the problem is underdetermined. Furthermore, it has been shown that one-to-one comparisons of the observed scaling of period and damping times with wave damping theories are misleading. Aims. We aim to investigate whether seismological information can be gained from the observed scaling laws in a statistical sense. Methods. A statistical approach is used whereby scaling laws are produced by forward modelling using distributions of values for key loop cross-sectional structuring parameters. We study two types of observations: 1) transverse loops oscillations as seen mainly with TRACE and SDO and 2) running transverse waves seen with the Coronal Multichannel Polarimeter (CoMP). Results. We demonstrate that the observed period-damping time scaling law does provide information about the physical damping mechanism, if observations are collected from as wide range of periods as possible and a comparison with theory is performed in a statistical sense. The distribution of the ratio of damping time over period, i.e. the quality factor, has been derived analytically and fitted to the observations. A minimum value for the quality factor of 0.65 has been found. From this, a constraint linking the ranges of possible values for the density contrast and inhomogeneity layer thickness is obtained for transverse loop oscillations. If the layer thickness is not constrained, then the density contrast is at most equal to 3. For transverse waves seen by CoMP, it is found that the ratio of maximum to minimum values for these two parameters has to be less than 2.06; i.e., the sampled values for the layer thickness and Alfvén travel time come from a relatively narrow distribution. Conclusions. Now that more and more transverse loop oscillations have been analysed, a statistical approach to coronal seismology becomes possible. Using the observed data cloud, we have found restrictions to the loop's density contrast and inhomogeneity layer thickness. Surprisingly, for running waves, narrow distributions for loop parameters have been found.

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The Effect of Background Subtraction on the Temperature of EIT Coronal Loops

Cited by 36 publications

References 37 publications

Coronal Heat: Solar Loop Temperatures from TRACE Triple-Filter Data

Coronal Heat: Solar Loop Temperatures from TRACE Triple-Filter Data

An Investigation into the Variability of Heating in a Solar Active Region

Statistical seismology of transverse waves in the solar corona

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