The solar interface region from the photosphere to the chromosphere and beyond to the lower corona has been best studied for a long time on the basis of flash spectra obtained during solar total eclipses. Eclipses are very favourable for this type of observation as the occultation takes place outside the Earth atmosphere and are therefore free of parasitic scattered light coming from the "occulting disk" (i.e., the Moon) a property of paramount importance when observing near the solar limb. The resolution of past flash spectra did not allow resolving the low layers of the transition region and were further hampered by non-linear effects affecting photographic films. Independently, EUV filtergrams of the limb region obtained in space were analyzed using one dimensional hydrostatic models like the VAL models but this method ignores the ubiquitous magnetic field emergence phenomenon associated with the chromospheric network and responsible for: i) spicules and interspicular regions, ii) coronal jets and macrospicules, and iii) the chromospheric prolateness. The components of the solar interface region are dynamical and different type of waves and magnetic reconnections are suggested to be at work. A jump of temperature from 0.01 to 1 MK is observed near the 2 Mm heights and higher, further producing a permanent solar wind flow. The heating processes responsible for this temperature jump and for the flow are not yet fully understood. In this thesis, we reconsider these problems on the basis of original, superior flash spectra which benefit from present technology such as CCD detectors, white light (W-L) eclipse images and new EUV images obtained with space-borne instruments. We illustrate the mechanisms of low First Ionisation Potential (FIP) emission lines present in the low layers of the solar atmosphere. We identify more precisely low FIP lines both inside and nearby prominences. We characterize in detail the He shells and the solar interface region. Method: 1) High cadence CCD slitless flash spectra made during the total solar eclipses of 2008, 2009, 2010 and slit spectra made in 2012. 2) Analysis of the continuum between the myriad of faint emission lines outside the solar limb and construction of the light curves of some low FIP line emissions. 3) Abell inversion evaluations to deduce the scales heights and discussion of the variations of temperature and density. 4) Analysis of simultaneously obtained EUV high resolution images from the Trace, AIA/SDO and SOT/Hinode space missions and W-L eclipse images to discuss the coronal components. Main results: i) The solar edge and the temperature bifurcation: structuration and discussion of the spectral behaviour of the analysed low FIP lines and the true continuum in the 400 to 600 km heights above the limb in the frame of the solar diameter measurements, emission mechanisms; ii) Visible mesospheric and chromospheric emission lines in the interface regions including the He I lines and especially the Pα HeII line starting at 800 km above the limb, produced by photo-ionisation showing shells around the Sun and probing the prominence-corona interface; iii) The contribution of small scale dynamical structures like spicules and macro-spicules starting at 1 Mm above the limb showing that the 1D hydrostatic stratified models are not adapted to the upper layers. Conclusion: We show that low FIP lines are over abundant in the photosphere- chromosphere interface , that the solar corona is supplied permanently by these elements. The titanium is an abundant element in the interspicular medium and an analogy about the temperature gradients between the photosphere- chromosphere and prominence-corona interfaces can be established.
