Observations of a solar flare and filament eruption in Lyman α and X-rays

Context. Lα is a strong chromospheric emission line, which has been relatively rarely observed in flares. The Transition Region and Coronal Explorer (TRACE) has a broad “Lyman α” channel centered at 1216 Å used primarily at the beginning of the mission. A small number of flares were observed in this channel. Aims. We aim to characterise the appearance and behaviour of a flare and filament ejection which occurred on 8th September 1999 and was observed by TRACE in Lα, as well as by the Yohkoh Soft and Hard X-ray telescopes. We explore the flare energetics and its spatial and temporal evolution. We have in mind the fact that the Lα line is a target for the Extreme Ultraviolet Imaging telescope (EUI) which has been selected for the Solar Orbiter mission, as well as the LYOT telescope on the proposed SMESE mission. Methods. We use imaging data from the TRACE 1216 Å, 1600 Å and 171 Å channels, and the Yohkoh hard and soft X-ray telescopes. A correction is applied to the TRACE data to obtain a better estimate of the pure Lα signature. The Lα power is obtained from a knowledge of the TRACE response function, and the flare electron energy budget is estimated by interpreting Yohkoh/HXT emission in the context of the collisional thick target model. Results. We find that the Lα flare is characterised by strong, compact footpoints (smaller than the UV ribbons) which correlate well with HXR footpoints. The Lα power radiated by the flare footpoints can be estimated, and is found to be on the order of 10 erg s at the peak. This is less than 10% of the power inferred for the electrons which generate the co-spatial HXR emission, and can thus readily be provided by them. The early stages of the filament eruption that accompany the flare are also visible, and show a diffuse, roughly circular spreading sheet-like morphology, with embedded denser blobs. Conclusions. On the basis of this observation, we conclude that flare and filament observations in the Lα line with the planned EUI and LYOT telescopes will provide valuable insight into solar flare evolution and energetics, especially when accompanied by HXR imaging and spectroscopy.