SUMER-Hinode observations of microflares: excitation of molecular hydrogen

Context: Concentrations of H2 have been detected by SUMER in active region plage. The H2 is excited by O VI line emission at 1031.94 Å which, although not observed, must be brightening along with the observed transition region line, Si 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 Si III 1113.24 Å and with XRT/Hinode X-ray and SOT/Hinode Ca 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 Si III plasma outflow. Conclusions: Microflare energy dissipation heats the chromosphere, reducing its opacity, so that O VI microflare emission is able to reach the lower layers of the chromosphere and excite the H2.