Analysis of UV and EUV emission from impacts on the Sun after 2011 June 7 eruptive flare

Abstract

Context. On 2011 June 7 debris from a large filament eruption fell back to the Sun causing bright ultraviolet (UV) and extreme ultraviolet (EUV) splashes across the surface. These impacts may give clues on the process of stellar accretion. Aims. The aim is to investigate how the impact emission is influenced by structures in the falling ejecta and at the solar surface. Methods. We determine the UV and EUV light curves of a sample of impacts. The ballistic impact velocity is estimated from the ejection and landing times and, where possible, compared with the velocity derived by tracking the downflows in SDO/AIA and STEREO/EUVI images. Estimates of the column density before impact are made from the darkness of the falling plasma in the 193 Å channel. Results. The impact velocities were between 230 and 450 km s-1. All impacts produced bright EUV emission at the impact site but bright UV was only observed when the impacting fragments reached the chromosphere. There was no clear relation between EUV intensity and kinetic energy. Low UV to EUV intensity ratios (IUV/IEUV) were seen (i) from impacts of low column-density fragments; (ii) when splashes, produced by some impacts, prevented subsequent fragments from reaching the chromosphere; and (iii) from an impact in an active region. The earliest impacts with the lowest velocity (~250 km s-1) had the highest IUV/IEUV. Conclusions. The IUV/IEUV decreases with impact velocity, magnetic field at the impact site, and EUV ionising flux. Many of the infalling fragments dissipate above the chromosphere either due to ionisation and trapping in magnetic structures, or to them encountering a splash from an earlier impact. If the same happens in accreting stars then the reduced X-ray compared to optical emission that has been observed is more likely due to absorption by the trailing stream than locally at the impact site.