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Abstract

Solar spectral irradiance (SSI) is the wavelength-dependent energy input to the top of the Earth’s atmosphere. Among it, the ultraviolet (UV) part of the irradiance represents the primary forcing mechanism for the photochemistry, heating, and dynamics of the Earth’s atmosphere. Both temporal and spectral variations in UV irradiance represent crucial inputs to the modelling and understanding of the behaviour of the Earth’s atmosphere. Therefore, measuring long-term UV irradiance variations over the 11-year solar activity cycle (and even over longer timescales) is fundamental. The new solar spectral irradiance dataset based on long-term solar UV irradiance observations (165-300 nm) of the SOLAR/SOLSPEC space-based instrument, provides measurements of the full-disk SSI during solar cycle 24 (from April 5, 2008 to February 10, 2017) and is used for further investigations of the long-term trend of solar activity and the construction of a homogeneous solar spectral irradiance record. A new method based on an adaptation of the Multiple Same-Irradiance-Level (MuSIL) technique was used to separate solar variability from any uncorrected instrumental trends in the UV irradiance measurements, allowing to correct degradation of the UV irradiance records to provide new solar cycle variability results during most of solar cycle 24. In particular, in the important 165-242 nm spectral region (including the Herzberg continuum), the SOLAR/SOLSPEC data agrees with the observations (SORCE/SOLSTICE) and models (SATIRE-S, NRLSSI 2) to within a 1-sigma error envelope while, between 242 and 300 nm, SOLAR/SOLSPEC agrees only with the models. New measurements are required: perspectives for new instrumentation will also be presented.