Solar impulsive EUV and UV brightenings in flare footpoints and their connection with X-ray emission

Abstract

Context. The thick-target model of the impulsive phase of solar flares describes the propagation of non-thermal electron beams into the chromosphere. The stopping depth of the electron is related to its energy. Thus, the thermal reaction at different levels should be connected to the parameters of the electron spectrum. Aims. We investigate the impulsive thermal reaction produced by non-thermal electron beams at different heights of the solar atmosphere observed in solar flare footpoints. Methods. We used methods developed for the investigation of the impulsive SXR brightenings. The EUV and UV images obtained by TRACE in the 171 A and 1600 A filter were used as the basic data set for this analysis. For the first time, EUV images have been desaturated using diffraction pattern properties. Results. We find that the thermal impulsive reaction observed in solar flare footpoints is different for UV and EUV brightenings. Results are compared to those obtained for the impulsive SXR brightenings. We find that observed dissimilarities are connected to the properties of the non-thermal electron spectrum and to the depth of their penetration into the chromosphere. Moreover, we show that the lower levels of the chromosphere are excited by more energetic electrons.