Formation of the helium extreme-UV resonance lines

¹ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern 0315 Oslo Norway
e-mail: thomas.golding@astro.uio.no; mats.carlsson@astro.uio.no
² Institute for Solar Physics, Department of Astronomy, Stockholm University, AlbaNova University Centre, 106 91 Stockholm, Sweden
e-mail: jorrit.leenaarts@astro.su.se

Received: 2 August 2016
Accepted: 30 September 2016

Abstract

Context. While classical models successfully reproduce intensities of many transition region lines, they predict helium extreme-UV (EUV) line intensities roughly an order of magnitude lower than the observed value.

Aims. Our aim is to determine the relevant formation mechanism(s) of the helium EUV resonance lines capable of explaining the high intensities under quiet Sun conditions.

Methods. We synthesised and studied the emergent spectra from a 3D radiation-magnetohydrodynamics simulation model. The effects of coronal illumination and non-equilibrium ionisation of hydrogen and helium are included self-consistently in the numerical simulation.

Results. Radiative transfer calculations result in helium EUV line intensities that are an order of magnitude larger than the intensities calculated under the classical assumptions. The enhanced intensity of He iλ584 is primarily caused by He ii recombination cascades. The enhanced intensity of He iiλ304 and He iiλ256 is caused primarily by non-equilibrium helium ionisation.

Conclusions. The analysis shows that the long standing problem of the high helium EUV line intensities disappears when taking into account optically thick radiative transfer and non-equilibrium ionisation effects.