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This PhD project is devoted to investigation of solar activity as observed by LYRA, the Large Yield Radiometer on-board PROBA2 mission (launched by ESA in November 2009). LYRA measures solar irradiance in four UV and EUV channels. Its high-cadence data (up to 20 Hz) present an opportunity to investigate short-term (minutes and seconds) variations of solar UV and EUV flux, namely during solar flares.The PhD project is divided in four parts:Part 1: Analysis of the instrument performances, calibration of the dataPart 2: Cross-calibration with GOES X-ray sensor(the reference in flare monitoring) and SDO-EVE (a high-performance spectrometer that was launched a few months after PROBA2, and of which the spectral coverage partly overlap the one of LYRA)Part 3: Multi-instrumental analysis of the flare timeline as a function of the observed spectral range using at least LYRA, SDO/EVE, and GOES. A model, based on CHIANTI database, predicting the spectral output of a theory-flare will bedeveloped and its predictions confronted to the observations.Part 4: Investigation of short-timescale phenomena during flares observed with LYRA (e.g. quasi-periodic pulsations)

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The physics of the solar corona is an important field of study in Astrophysics. It is currently understood that its structure and dynamics are dominated by the magnetic field. The corona is seen as the source region of the solar wind that travels through interplanetary space and even has effects on Earth. The corona is also the source region of violent eruptions such as solar flares and coronal mass ejections (CMEs). The largest structures observed in the solar corona, especially during solar maximum, are the ray-like coronal helmet streamers. In the lower solar corona, helmet streamers consist of closed magnetic loop-like arcades connecting to the solar surface. In the outer solar corona, they extend to a radial stalk connecting to the out-flowing solar wind.The dynamic nature of the solar corona enables a wide variety of coronal waves. They are found in almost every coronal structure and show a great range of spatial and temporal scales. Most important for this work are the transverse oscillations that were discovered in coronal streamers in the past decade. These waves are a propagating disturbance caused by a CME-driven shock moving the streamer sideways. The area of study of these waves is gaining momentum following these observations, but theory and numerical models are lagging behind.This work concentrates on the characterization of coronal streamers and their oscillations. For this, we aim to use observations from different points of view. This is possible thanks to the multiple spacecraft with coronagraphs orbiting at different positions around the Sun. With this study of coronal streamers and the streamer waves, we gain a better understanding of the plasma properties in these magnetic structures. The combination of observations and theoretical models of the streamer waves sets the stage for the technique of coronal seismology. Observable parameters of the waves, such as the period and wavelength, are substituted in the equations of theoretical models to obtain estimates for physical parameters which are difficult to observe directly in the solar corona, such as the magnetic field strength.