Modeling Realistic Heating Profiles of Transition Region Hot Loops on the Sun: Evidence for Impulsive Heating and Non-equilibrium Ionization

TL;DR

This paper demonstrates that hot solar transition region loops are heated impulsively with complex, multi-scale profiles, and that non-equilibrium ionization significantly influences observed emission spectra, challenging simplistic heating models.

Contribution

It introduces a forward modeling approach incorporating non-equilibrium ionization and density-dependent recombination to better understand impulsive heating in solar loops.

Findings

Heating is impulsive and multi-scaled.

Non-equilibrium ionization affects emission spectra.

Density-dependent recombination is essential for accurate modeling.

Abstract

The study examines the heating profile of hot solar transition region loops, particularly focusing on transient brightenings observed in IRIS 1400{\AA} slit-jaw images. The findings challenge the adequacy of simplistic, singular heating mechanisms, revealing that the heating is temporally impulsive and requires a spatially complex profile with multiple heating scales. A forward modeling code is utilized to generate synthetic IRIS emission spectra of these loops based on HYDRAD output, confirming that emitting ions are out of equilibrium. The modeling further indicates that density-dependent dielectronic recombination rates must be included to reproduce the observed line ratios. Collectively, this evidence substantiates that the loops are subject to impulsive heating and that the components of the transiently brightened plasma are driven far from thermal equilibrium. Heating events such as these are ubiquitous in the transition region and the analysis described above provides a robust observational diagnostic tool for characterizing the plasma.