# UVSQ-SAT, a Pathfinder CubeSat Mission for Observing Essential Climate Variables
## 2 Scientific Rationale of the UVSQ-SAT Mission
### Solar Spectral Irradiance in the Herzberg Continuum  
The role of solar variability in climate variability remains a topic of considerable scientific and societal importance. Solar radiation is the energy source and is important for the climate. The incoming solar flux or/and its spectral distribution at the top of the atmosphere (due to changes in solar activity or in the Earth's orbital parameters) fluctuate over a wide range of temporal scales, from the 27 day rotational cycle to thousands of years. It also includes 11 y solar cycles and cycles of the order of hundreds of years, called \"grand solar minima\" and \"grand solar maxima\". The solar spectrum [17] and its variability represent key inputs not only for solar physics, but also for climate physics. Climate models require time varying solar spectra as forcing with the available information often based on solar reconstructions and solar models. There are multiples lines of evidence [18; 19] showing that solar variability has been a key forcing in the history of the Earth's climate. Correlations between solar proxies and atmospheric/climate indicators have been established in present day datasets and in sedimentary and ice core archives. However, most of the apparent correlations and associated solar signals tend to be very variable and intermittent. Some are also very difficult to reproduce in climate models. Establishing a quantitative forcing-response relationship for the Sun-Earth link is problematic without a clear understanding of the key mechanisms engaged in the action of solar variability on the atmosphere and climate, notably at regional scales. There is no general consensus on those mechanisms. The overall response of the atmosphere and surface climate to solar variability involves a wide range of coupled chemical, dynamical, and radiative processes and the interactions between different atmospheric layers and between the atmosphere and the ocean. It is worth stressing that the issue of the solar impacts is not just critical for the paleoclimate. It is also highly relevant for the present day climate evolution, which is driven by the GHG rising concentrations. Climate change is a major and growing threat to natural, managed, and human systems. There is already growing evidence for its adverse impacts on the natural environment and human societies (e.g., ecosystems, biological diversity, water resources, and the economy) [20]. There are several sources of uncertainties in climate simulations, in particular in the projections that are used by decision makers to design differentiated mitigation and adaptation strategies [21]. Some of the uncertainties originate from the difficulty to separate the anthropogenic contribution from the natural variability. Quantifying accurately the anthropic contribution and projecting future changes requires understanding and quantifying the natural climate variability including the solar driven variations. It has even been suggested that a new grand solar minimum might occur in the 21 st Century [22] and even last until the end of the 22 nd Century [23]. The uncertainties are not limited to the mechanisms [18]. They also pertain to the solar variability itself, especially the spectral variations [24]. Indeed, solar forcing is not simply limited to a change in total energy flux. Spectral variations are also important. The relative variations in incoming Solar Spectral Irradiance (SSI) increase very rapidly with decreasing wavelength in the UV range and below. For instance, over an 11 y cycle, the Total Solar Irradiance (TSI) fluctuates by about 0.1% (\(\sim\)1.4 Wm\({}^{-2}\)), whereas, in contrast, the radiative flux in the 200 nm region, a key spectral window for stratospheric ozone photochemistry, varies by several %. This has important implications for the way variations in incoming solar energy are redistributed among the different atmospheric layers. The choice of solar UV irradiance variability used to force the models is critical for the solar perturbations of the middle atmosphere [24; 25; 26].