Deeper by the Dozen: Understanding the Cross-Field Temperature Distributions of Coronal Loops

Schmelz, J. T.; Pathak, S.; Jenkins, B. S.; Worley, B. T.

doi:10.1088/0004-637x/764/1/53

Cited by 18 publications

(8 citation statements)

References 32 publications

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“…Previous studies on the width of coronal loop cross-sections have been focused on the finest widths that could be spatially resolved with a particular telescope (i.e., Bray and Loughhead 1985;Aschwanden and Nightingale 2005;Winebarger et al 2013;Peter et al 2013), unresolved strands as substructures of loops (Patsourakos and Klimchuk 2007;Aschwanden and Boerner 2011;Schmelz et al 2013), spatial variations of loop widths w(s) along loops (Wang and Sakurai 1998;Klimchuk 2000;Watko and Klimchuk 2000;Lopez-Fuentes and Klimchuk 2010), temporal loop width variations , magnetic modeling of loop width variations (Petrie 2006;Schrijver 2007;Bellan 2003;Peter and Bingert 2012), or cross-field transport of energy (Winglee et al 1988;Amendt and Benford 1989;Chae 2002;Ruderman 2003;Georgoulis and LaBonte 2004;Vasquez and Hollweg 2004;Goossens 2004, 2005;Galloway et al 2006;Ruderman et al 2008;Erdelyi and Morton 2009;Pascoe et al 2009;Kontar et al 2011;Morton and Ruderman 2011;Arregui and Asensio-Ramos 2014;Arregui et al 2015;Kaneko et al 2015;Ruderman 2015). Summaries on the width of coronal loops are given in the textbooks of Bray et al (1991; Chapters 2 and 3) or Aschwanden (2004;Chapter 5.4.4).…”

Section: Introductionmentioning

confidence: 99%

The Width Distribution of Loops and Strands in the Solar Corona—Are We Hitting Rock Bottom?

Aschwanden

Peter

2017

ApJ

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In this study we analyze Atmospheric Imaging Assembly (AIA) and Hi-C images in order to investigate absolute limits for the finest loop strands. We develop a model of the occurrence-size distribution function of coronal loop widths, characterized by the lower limit of widths w min , the peak (or most frequent) width w p , the peak occurrence number n p , and a power law slope a. Our data analysis includes automated tracing of curvi-linear features with the OCCULT-2 code, automated sampling of the cross-sectional widths of coronal loops, and fitting of the theoretical size distribution to the observed distribution. With Monte-Carlo simulations and variable pixel sizes ∆x we derive a first diagnostic criterion to discriminate whether the loop widths are unresolved (w p /∆x ≈ 2.5 ± 0.2), or fully resolved (if w p /∆x > ∼ 2.7). For images with resolved loop widths we can apply a second diagnostic criterion that predicts the lower limit of loop widths as a function of the spatial resolution. We find that the loop widths are marginally resolved in AIA images, but are fully resolved in Hi-C images, where our model predicts a most frequent (peak) value at w p ≈ 550 km, in agreement with recent results of Brooks et al. This result agrees with the statistics of photospheric granulation sizes and thus supports coronal heating mechanisms operating on the macroscopic scale of photospheric magneto-convection, rather than nanoflare braiding models on unresolved microscopic scales.

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Section: Introductionmentioning

confidence: 99%

The Width Distribution of Loops and Strands in the Solar Corona—Are We Hitting Rock Bottom?

Aschwanden

Peter

2017

ApJ

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“…For a few examples, see e.g. Warren et al (2008), Tripathi et al (2009), Aschwanden & Boerner (2011), Schmelz et al (2013), although some loops show some departures (Schmelz et al 2009(Schmelz et al , 2010, and the issue is still debated.…”

Section: Introductionmentioning

confidence: 99%

The multi-thermal emission in solar active regions

Zanna¹

2013

A&A

159

110

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We present simultaneous SDO AIA and Hinode EIS observations of the hot cores of active regions (ARs) and assess the dominant contributions to the AIA EUV bands. This is an extension of our previous work. We find good agreement between SDO AIA, EVE and EIS observations, using our new EIS calibration and the latest EVE v.3 data. We find that all the AIA bands are multi-thermal, with the exception of the 171 and 335 Å, and provide ways to roughly estimate the main contributions directly from the AIA data. We present and discuss new atomic data for the AIA bands, showing that they are now sufficiently complete to obtain temperature information in the cores of ARs, with the exception of the 211 Å band. We found that the newly identified Fe xiv 93.61 Å line is the dominant contribution to the 94 Å band, whenever Fe xviii is not present. Three methods to estimate the Fe xviii emission in this band are presented, two using EIS and one directly from the AIA data. Fe xviii emission is often present in the cores of ARs, but we found cases where it is formed at 3 MK and not 7 MK, the temperature of peak ion abundance in equilibrium. The best EIS lines for elemental abundance determination and differential emission measure (DEM) analysis are discussed. A new set of abundances for many elements are obtained from EIS observations of hot 3 MK loops. The abundances of the elements with low first ionisation potential (FIP), relative to those of the high-FIP elements, are found to be enhanced by about a factor of three, compared to the photospheric values. A measurement of the path length implies that the absolute abundances of the low-FIP elements are higher than the photospheric values by at least a factor of three. We present a new DEM method customised for the AIA bands, to study the thermal structure of ARs at 1 resolution. This was tested on a few ARs, including one observed during the Hi-C rocket flight. We found excellent agreement between predicted and observed AIA count rates and EIS radiances. Overall we found few differences between the AIA and Hi-C 193 Å images of coronal structures, despite the higher Hi-C resolution (0.25 ). The Hi-C images and the AIA DEM modelling suggest that some of the cooler loops (below 1 MK) are already resolved by AIA, while the hotter (1.5-2.5 MK) "background" emission is in most places still unresolved even at the Hi-C resolution. This unresolved emission is significantly lower than previously observed with TRACE, the SOHO CDS, and Hinode EIS spectrometers. Its enhancement appears to be mostly due to increased iron abundance. We find an ubiquitous presence of emission at different temperatures that is not co-spatial, and suggest that future high-resolution imaging is carried out with isothermal bands.

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“…The procedure was also able to distinguish unresolved loops from loops near the resolution limit. The question whether loops are resolved, or consist of individual unresolved strands, has been extensively investigated (e.g., Del Zanna 2003;Del Zanna & Mason 2003;Aschwanden & Nightingale 2005;Patsourakos & Klimchuk 2007;Schmelz et al 2009Schmelz et al , 2011aSchmelz et al ,b, 2013Brooks et al 2012Brooks et al , 2013Peter et al 2013;Del Zanna 2013;Winebarger et al 2014). At least a portion of the observed loops seems to be well-resolved (Aschwanden & Nightingale 2005;Aschwanden & Boerner 2011;Brooks et al 2012Brooks et al , 2013Peter et al 2013;Winebarger et al 2014).…”

Section: Introductionmentioning

confidence: 99%

On the Area Expansion of Magnetic Flux Tubes in Solar Active Regions

Dudík

Dzifčáková

Cirtain

2014

ApJ

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We calculated the 3D distribution of the area expansion factors in a potential magnetic field extrapolated from the high-resolution Hinode/SOT magnetogram of a quiescent active region NOAA 11482. Retaining only closed loops within the computational box, we show that the distribution of area expansion factors show significant structure. Loop-like structures characterized by locally lower values of the expansion factor are embedded in a smooth background. These loop-like flux-tubes have squashed cross-sections and expand with height. The distribution of the expansion factors show overall increase with height, allowing an active region core characterized by low values of the expansion factor to be distinguished. The area expansion factors obtained from extrapolation of the SOT magnetogram are compared to those obtained from an approximation of the observed magnetogram by a series of 134 submerged charges. This approximation retains the general flux distribution in the observed magnetogram, but removes the small-scale structure in both the approximated magnetogram and the 3D distribution of the area expansion factors. We argue that the structuring of the expansion factor can be a significant ingredient in producing the observed structuring of the solar corona. However, due to the potential approximation used, these results may not be applicable to loops exhibiting twist neither to active regions producing significant flares.

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Deeper by the Dozen: Understanding the Cross-Field Temperature Distributions of Coronal Loops

Cited by 18 publications

References 32 publications

The Width Distribution of Loops and Strands in the Solar Corona—Are We Hitting Rock Bottom?

The Width Distribution of Loops and Strands in the Solar Corona—Are We Hitting Rock Bottom?

The multi-thermal emission in solar active regions

On the Area Expansion of Magnetic Flux Tubes in Solar Active Regions

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