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

TL;DR

This study provides observational evidence supporting the multi-thread model of solar coronal loops, demonstrating that fine thread expansion occurs below instrument resolution and may explain the discrepancy between expected and observed loop cross-sections.

Contribution

The paper presents high-resolution observations of expanding coronal loops, confirming the multi-thread structure hypothesis and addressing the long-standing loop expansion discrepancy.

Findings

Extended loops show measurable expansion related to length.

High-resolution observations capture expanding loops in active regions.

Thread separation is directly visible during loop evolution.

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.