Probing the dynamics in sequences of solar atmospheric images : algorithms and results

(eng) The dynamic nature of the solar atmosphere has been widely observed since the first telescopic observations of the Sun that began around 1610 with Galileo Galilei. The last two decades have highlighted the unsuitability of static assumptions in studying the physics of the chromosphere, the transition region, and the corona. In this work we investigate the dynamics of the solar atmosphere as observed by imaging telescopes in extreme ultraviolet (EUV) passbands. The dynamics is concerned with the study of fluid motion: we present algorithms of motion analysis specifically adapted to solar EUV images as produced by the EUV imaging telescope (EIT) on board the SoHO space mission, and other imagers onboard the TRACE and STEREO space missions. We present a multiscale optical-flow algorithm based on the Lucas-Kanade method and we apply it to the processing of EIT and TRACE series of EUV images. It can reliably retrieve motion from two successive images. We will demonstrate that this algorithm can be used for the detection and the analysis of coronal loop oscillations over long time periods via a tracking method over more than two images. The tracking technique is extended to the analysis of the solar rotation over the 23rd solar cycle that is now covered by the EIT archive. Using STEREO EUVI images, we introduce a stereoscopic reconstruction method achieving 3D reconstructions of filaments using the STEREO data. We also develop a spatio-temporal method of motion estimation based on a motion-oriented continuous wavelet transform, and we present its application on EUV images of the solar atmosphere. The results presented in this work illustrate how these novel methods are able to improve the existing observational methods of motion measurement: it represents a breakthrough that enables to quantify, in a reproducible way, measurements that are habitually achieved via manual methods and human observers. These developments also have a potential for chromospheric or coronagraphic image sequences, and for future observations from EUV solar imagers such as ESA PROBA2-SWAP and NASA SDO-AIA.