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

TL;DR

IRIS observations combined with simulations suggest that unresolved fine structures in the solar transition region are likely spatially resolved at scales around 133 km, providing insights into solar atmospheric dynamics and heating.

Contribution

This study integrates IRIS measurements with 1-D non-equilibrium ionization simulations to assess whether UFS are truly resolved structures.

Findings

Most observed UFS properties are reproduced by single-strand simulations.

UFS are structured on an average spatial scale of 133 km.

Spatial scales of a few hundred km are common in solar atmospheric structures.

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.