Thread Separation and Expansion Observed in Multistranded Solar Coronal Loops

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 Solar Dynamics Observatory/Atmospheric Imaging Assembly (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/Extreme Ultraviolet Imager (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 compelling evidence supporting the multithread 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.