Astronomy & Astrophysics

Free access article

A&A 481, L41-L44 (2008)
DOI: 10.1051/0004-6361:20078977

Letter

SUMER-Hinode observations of microflares: excitation of molecular hydrogen

D. E. Innes

Max-Planck Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany
e-mail: innes@mps.mpg.de

(Received 1 November 2007 / Accepted 20 November 2007)

Abstract
Context.Concentrations of H₂ have been detected by SUMER in active region plage. The H₂ is excited by O $\scriptstyle{\rm VI}$ line emission at 1031.94 Å which, although not observed, must be brightening along with the observed transition region line, $\ion{iii}$ 1113.24 Å.
Aims.We investigate the excitation of H₂ and demonstrate the association between the observed H₂ emission and footpoints of X-ray microflares.
Methods.We have made co-ordinated observations of active region plage with the spectrometer SUMER/SoHO in lines of H₂ 1119.10 Å and $\ion{iii}$ 1113.24 Å and with XRT/Hinode X-ray and SOT/Hinode Ca $\scriptstyle{\rm II}$ filters.
Results.In six hours of observation, six of the seven H₂ events seen occurred near a footpoint of a brightening X-ray loop. The seventh is associated with an unusually strong $\ion{iii}$ plasma outflow.
Conclusions.Microflare energy dissipation heats the chromosphere, reducing its opacity, so that O $\scriptstyle{\rm VI}$ microflare emission is able to reach the lower layers of the chromosphere and excite the H₂.

Key words: molecular processes -- Sun: activity -- Sun: flares -- Sun: UV radiation