[Paper Review] A Comparison of EIT and TRACE Loop Widths

S. I. Chastain, J. T. Schmelz|arXiv (Cornell University)|May 18, 2017

Non-Destructive Testing Techniques3 citations

TL;DR

This study compares coronal loop widths in EIT and TRACE solar imaging data to assess resolution effects on loop detectability. It finds that EIT loops are universally unresolved (wider than one pixel but not resolved), while many TRACE loops are also unresolved or poorly resolved, challenging assumptions about loop structure and highlighting the need for higher-resolution instruments to accurately define and analyze coronal loops.

ABSTRACT

In this study we have compared coronal loops in the Extreme ultraviolet Imaging Telescope (EIT) on-board the Solar and Heliospheric Observatory (SOHO) with coronal loops from the Transition Region and Coronal Explorer (TRACE). The purpose of which is to quantitatively and qualitatively examine the effects of spatial resolution on the width of coronal loops and implications for how a coronal loop is defined. Out of twenty-two loop sections analyzed, we find that none of them were resolved in EIT and none of them were close to the width of the TRACE loops. These findings suggest that coronal loops are unresolved in EIT. We also find examples of how unresolved loops can be quite misleading. We have also found that many of the TRACE loops that we have analyzed may be unresolved as well. Our findings emphasize the importance of studying loop width in order to better understand coronal loops and also emphasize the need for instruments with higher spatial resolution.

Motivation & Objective

• Investigate how spatial resolution affects the observed width and detectability of coronal loops in solar imaging data.
• Address the inconsistency in loop identification across instruments with different resolutions, such as EIT and TRACE.
• Assess whether observed loops in EIT and TRACE are resolved or unresolved, and evaluate implications for loop definition and analysis.
• Highlight the limitations of using unresolved loops in quantitative studies of coronal loop properties.
• Provide empirical evidence for a preferred spatial scale of coronal loops based on width measurements across instruments.

Proposed method

• Acquired and analyzed loop segments from EIT (2.6 arcsec/pixel) and TRACE (0.5 arcsec/pixel) instruments using the 171 Å filter.
• Measured loop width profiles along selected loop segments to compare widths across instruments.
• Plotted loop width versus position along the loop to assess consistency and resolution effects.
• Used visual inspection and width measurements to determine if loops were resolved (≥2 pixels wide) or unresolved (≤1 pixel wide).
• Compared results with prior studies (e.g., Klimchuk 2000, 2015; Peter et al. 2013; Schmelz et al. 2016) to contextualize findings.
• Evaluated the implications of unresolved loops for loop definition and the reliability of loop-based models in solar physics.

Experimental results

Research questions

• RQ1 To what extent are coronal loops resolved in EIT and TRACE, and how does this affect their observed width?
• RQ2 Why do the same loops appear differently in EIT versus TRACE, and what does this imply about loop definition across instruments?
• RQ3 Can loop width measurements alone indicate whether a loop is resolved or unresolved without higher-resolution data?
• RQ4 What is the upper limit of coronal loop width, and does it align with the spatial resolution of EIT and TRACE?
• RQ5 How do unresolved loops in EIT and TRACE affect the reliability of quantitative loop analysis and modeling?

Key findings

• None of the 22 loop sections analyzed in EIT were resolved, as all were narrower than one pixel width in EIT (2.6 arcsec/pixel), indicating they are unresolved.
• All EIT loop widths were less than one pixel, suggesting that coronal loops are fundamentally unresolved in EIT and should be explicitly labeled as such.
• Many TRACE loops were also unresolved or poorly resolved, with several measuring less than two pixels in width (≈3000 km), indicating they may be below the resolution threshold.
• The upper limit for loop width in TRACE is approximately 3000 km (two pixels), suggesting a preferred spatial scale for coronal loops.
• Some TRACE loops were clearly resolved (wider than two pixels), but a significant number were not, indicating that resolution varies across loops even within the same instrument.
• The study confirms that unresolved loops can be misleading in analysis, and that loop identity and width depend critically on instrument resolution, challenging the assumption that a loop in one instrument is the same in another.