Properties and Modeling of Unresolved Fine Structure Loops Observed in the Solar Transition Region by IRIS

Abstract

Recent observations from the Interface Region Imaging Spectrograph (IRIS) have discovered a new class of numerous low-lying dynamic loop structures, and it has been argued that they are the long-postulated unresolved fine structures (UFS) that dominate the emission of the solar transition region. In this letter, we combine IRIS measurements of the properties of a sample of 108 UFS (intensities, lengths, widths, lifetimes) with 1-D non-equilibrium ionization simulations using the HYDRAD hydrodynamic model to examine whether the UFS are now truly spatially resolved in the sense of being individual structures rather than composed of multiple magnetic threads. We find that a simulation of an impulsively heated single strand can reproduce most of the observed properties suggesting that the UFS may be resolved, and the distribution of UFS widths implies that they are structured on a spatial scale of 133km on average. Spatial scales of a few hundred km appear to be typical for a range of chromospheric and coronal structures, and we conjecture that this could be an important clue to the coronal heating process.

arXiv commentThis is the version to be published

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• Observational Evidence of Magnetic Reconnection for Brightenings and Transition Region Arcades in IRIS observations
• The Duration of Energy Deposition on Unresolved Flaring Loops in the Solar Corona
• Effect of Coronal Loop Structure on Wave Heating by Phase Mixing
• Evidence for and Analysis of Multiple Hidden Coronal Strands in Cross-Sectional Emission Profiles: Further Results from NASA's High-resolution Solar Coronal Imager
• Observations of solar chromospheric heating at sub-arcsec spatial resolution
• Chromospheric counterparts of solar transition region unresolved fine structure loops
• Helical Twisting Number and Braiding Linkage Number of Solar Coronal Loops
• Small-scale loops heated to transition region temperatures and their chromospheric signatures in the simulated solar atmosphere
• Fundamental Transverse Vibrations of the Active Region Solar Corona
• The Equilibrium of Coronal Loops Near Separatrices
• A new view of the solar interface region from the Interface Region Imaging Spectrograph (IRIS)
• The cross-sectional shape and height expansion of coronal loops: High-resolution Coronal Imager (Hi-C) analysis of AR 12712