Thread Separation and Expansion Observed in Multi-Stranded Solar Coronal Loops

TL;DR

The paper addresses why coronal loops often show constant cross-sections despite magnetic field divergence by testing a sub-resolution multi-thread model using SDO/AIA and Solar Orbiter/EUI. It combines measurements of loop width along long post-flare, trans-equatorial, and active-region loops with high-resolution AR loop observations to detect expansion and thread separation. The results show measurable expansion in long loops and resolving evidence of expanding, separated threads in AR loops, supporting a model where loops are composed of a small number of finer threads. PSF-convolution modeling demonstrates how instrumental resolution can mask expansion, emphasizing the need for high-resolution imaging to properly interpret coronal loop physics and heating processes.

Abstract

The theoretical expectation that coronal loops should expand with height contrasts with observations that typically show constant cross-sections. We investigate the idea that this discrepancy results from loops being composed of fine threads whose expansion occurs below the resolution limits of instruments like SDO/AIA. In this paper, we present two significant findings: (1) several extended loops exhibit measurable expansion, suggesting length as a critical factor in detection capability, and (2) high-resolution Solar Orbiter/EUI observations have captured expanding loops in active regions. For both AIA and EUI data, we observe cases where thread separation is directly visible as the loops evolve. These findings complement our previous work indicating AIA loops may consist of relatively few threads. Collectively, these observations provide compeling evidence supporting the multi-thread model and offer a potential resolution to the long-standing loop expansion problem in solar coronal physics. However, the high densities and narrow temperature distributions of observed coronal loops remain unresolved.

Figures (9)

• Figure 1: Post-flare loop observed in the AIA 171 Å filter on 2012 July 19, at 12:03 UT. The solid box in the upper left panel outlines the segment of the loop that was traced. The dotted line follows the loop. The upper middle panel shows the straightened loop segment on a Mm spatial scale. The red arrow highlights the straightened loop. The wave-like pattern towards the end of the loop is a result of imperfect straightening over such large curved distances. These and subsequent images in other figures have been sharpened using the MGN procedures of Morgan2014. We used these images to aid loop identification, but all of the analysis was performed on the original images. The upper right panel shows measurements of the loop width (FWHM) as a function of distance along the post-flare loop. The data points correspond to the cross-field intensity profiles also shown as a function of distance in the lower two rows. In these plots, the raw data are shown by the solid black line. The gaussian fits to the cross-loop profiles are shown in blue. The solid straight line shows the fit between the selected background positions. The distance of the measurement from the start of the trace and the width of the loop at that position are shown in the legend.
• Figure 2: Same as Figure \ref{['fig1']} but showing a trans-equatorial loop observed in the AIA 171 Å filter on 2025 April 1, at 08:00 UT.
• Figure 3: Same as Figure \ref{['fig1']} but showing an active region loop observed in the EUI 174 Å filter on 2024 March 23, at 22:55 UT. The spatial resolution of the EUI observations on this date was 0.188$"$/pixel (see text). Note that for this shorter loop the measurements are made at intervals of 2.5 Mm. The dashed line in the top right panel is a polynomial fit to the loop widths measured in the lower panels. It is used in Section \ref{['model']} for an investigtion of the effect of the instrument PSF on the detection of the observed expansion.
• Figure 4: Same as Figure \ref{['fig1']} but showing another active region loop observed in the EUI 174 Å filter on 2024 March 23, at 23:15 UT. As in Figure \ref{['fig3']}, the measurements for this shorter loop are made at intervals of 2.5 Mm.
• Figure 5: AIA 171 Å images showing an example of a loop separating into two distinct threads. The solid box in the upper left panel shows the area encompassing the segment of the single loop that was traced. The blue dotted line follows the loop. The solid box in the middle panel of the second row outlines the area encompassing the segments of the two separated threads that were traced. The dotted lines follow the threads. They are color coded blue and sky blue for cross-referencing with the intensity profiles in the lower two rows. The cross-field intensity profiles as a function of distance along the separating threads are shown in the lower two rows. The raw data are shown by the solid black line. The gaussian fits to the cross-thread profiles are color coded blue and sky blue for cross-referencing with the traces in the images. The solid straight line shows the fit between the selected background positions. The distance of the measurement from the start of the trace and the width (FWHM) of the thread at that position are shown in the legend. The animation associated with this figure runs for 22 s and covers the period from 2011 October 14 21:00 UT to 2011 October 15 02:00 UT. It shows a single panel from the static figure with the same FOV. It is annotated with an arrow to indicate the loop that separates into two threads.