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Published online by Cambridge University Press: 30 March 2016
The UV and X-ray wavelength ranges are rich in emission unes from the transition region and the corona, formed at temperatures from 104 K to 107 K. Fortunately, these emission lines, absorbed by the earth upper atmosphere, can be observed by some solar space observatories. In particular, the NASA SMM satellite, from 1980 to 1990, has obtained very interesting spectra from which important solar results were derived. In a close future, they will be again observed but with improved spectral, spatial and temporal resolution by SOLAR-A and SOHO.