Molecular absorption in transition region spectral lines

TL;DR

This study reports the detection of molecular and atomic absorption lines in the solar transition region, revealing unexpected cool plasma structures that challenge traditional models of the solar atmosphere.

Contribution

First observation of molecular absorption features in the solar transition region, indicating the presence of cool plasma above the chromosphere in a previously unrecognized stratification.

Findings

Detection of molecular lines not previously observed in solar spectra

Cool plasma exists above the transition region without association to filaments or coronal rain

Supports models with elevated pockets of cool gas in the solar atmosphere

Abstract

Aims: We present observations from the Interface Region Imaging Spectrograph (IRIS) of absorption features from a multitude of cool atomic and molecular lines within the profiles of Si IV transition region lines. Many of these spectral lines have not previously been detected in solar spectra. Methods: We examined spectra taken from deep exposures of plage on 12 October 2013. We observed unique absorption spectra over a magnetic element which is bright in transition region line emission and the ultraviolet continuum. We compared the absorption spectra with emission spectra that is likely related to fluorescence. Results: The absorption features require a population of sub-5000 K plasma to exist above the transition region. This peculiar stratification is an extreme deviation from the canonical structure of the chromosphere-corona boundary . The cool material is not associated with a filament or discernible coronal rain. This suggests that molecules may form in the upper solar atmosphere on small spatial scales and introduces a new complexity into our understanding of solar thermal structure. It lends credence to previous numerical studies that found evidence for elevated pockets of cool gas in the chromosphere.