Diagnostic of the solar transition region and corona from VUV spectroscopy and imaging

Imaging and spectroscopy at vacuum ultra-violet (VUV: 100 to 2000 Å) wavelengths are the key tools to study the outer solar atmosphere. This is a very dynamic environment where structures and their changes can be observed down to very small scales. However, it also shows average properties that must be accounted for by any reliable modelling effort. The average Doppler shift shown by spectral lines formed from chromosphere to corona, in particular, reveals important information on the mass and energy balance of the solar atmosphere, providing a tight observational constraint on any model of the solar corona. So far, spectroscopic studies revealed a persistent average red shift of the lines formed at temperatures below 0.5 MK that revert to blue shift at higher temperatures. However, for temperatures above 1 MK the behaviour was, so far, unknown. Since VUV radiation is entirely absorbed by the atmosphere of the Earth, it can only be detected with instruments operating in space. In this thesis I have used simultaneous and co-spatial VUV spectra recorded by two spectrometers: SUMER (Solar Ultraviolet Measurements of Emitted Radiation)/SoHO (Solar and Heliospheric Observatory) and EIS (Extreme ultraviolet Imaging Spectrometer)/Hinode. The behaviour of the absolute Doppler shift versus temperature (up to 2 MK) is measured by using a novel technique developed during this thesis work and applied to the quiet Sun near disk center and to the moss area of an active region. The results are compared with different coronal models present in the literature.