Molecular absorption in transition region spectral lines^⋆

¹ Max Planck Institute for Solar System Resarch, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: schmit@mps.mpg.de
² Center for Astrophysics and Space Astronomy, University of Colorado, 389 UCB, Boulder, Colorado 80309-0389, USA
³ Department of Physics, Montana State University, PO Box 173148, Bozeman MT 59717-3148, USA

Received: 19 June 2014
Accepted: 31 August 2014

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.