The thermal structure of the solar upper atmosphere

In the present work we review the thermal structure of the solar atmosphere. We first discuss the main diagnostic techniques used to measure it, outlining their pitfalls and limitations. Then, we review the recent measurements of the thermal structure of the solar atmosphere carried out with the SOHO spectrometers CDS and SUMER. The review shows that the solar upper atmosphere is made by an ensemble of few, nearly isothermal plasmas with fixed temperatures, disconnected from the colder, thermally continuous lower atmosphere.