# 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 exceptionally weak Solar Cycle 24 and the future Solar Cycle 25 (expected to begin in late 2019) are interesting periods in this context as they might possibly imply the beginning of a general negative solar forcing which would be expected to be vastly outweighed by the global anthropogenic positive forcing [27]. It is also time to clarify better the mechanisms involved in the solar forcing and atmospheric response. The idea is to investigate carefully processes affecting several atmospheric layers. Historically, the impact of solar variability on surface climate has often been seen as resulting only from the direct radiative effects on the Earth's surface and the lower atmosphere. In this framework, the drivers are variations in incoming TSI in wavelength ranges where the middle atmosphere is more or less transparent, i.e., wavelengths longer than 320 nm, corresponding mostly soft UV (UVA), visible, and near-infrared (IR) ranges. They directly cause changes in the heating rate of the Earth's surface and the lower atmosphere, modifying surface temperatures and climate. At first order, the change in global temperature is essentially due to this direct effect. However, there is also an indirect effect, the so-called \"top-down\" mechanism (in opposition to the direct effect referred as the \"bottom-up\" mechanism). In that case, the drivers are variations in the incoming UV flux (below 320 nm) and energetic particles whose energies are almost entirely absorbed by the middle atmosphere. They cause photochemical and dynamic perturbations of the middle atmosphere, which then propagate to the troposphere via stratospheric-tropospheric couplings and result in modifications of surface climate, notably on regional patterns [28; 29; 30]. Both mechanisms (top-down, bottom-up) operate at the same time in reality and influence the middle atmosphere and surface climate. An additional complication in studying the top-down mechanism is the fact that UV variations impact the middle atmosphere not only directly via changes in radiative heating, but also indirectly via photochemically driven changes in ozone, the key chemical species and UV absorber in the stratosphere. As a result, the stratospheric temperature response to UV changes is amplified by about a factor of two in a chemistry-climate model (with ozone calculated interactively) compared to the response in the climate model with specified constant ozone [31]. Clearly, the ozone response to solar variability needs to be accounted for in models by treating ozone like temperature, as a variable of the system instead of an input of the model. Only chemistry-climate models can simulate this interaction. Thus, it is necessary to have continuous measurements of the UV solar spectral irradiance with a good accuracy, and particularly in the Herzberg continuum (expected SSI uncertainty at 1\(\sigma\) of \(\pm\)0.1% per decade) for its influence on stratospheric ozone chemistry. Several missions (Compact Spectral Irradiance Monitor (CSIM) [32], UVSQ-SAT) aim to test the efficacy of a CubeSat making accurate SSI measurements of a much bigger and more expensive satellite. Indeed, the new UVSQ-SAT concept is designed to see if it is possible to achieve the SSI required accuracies using DEep uv INnovative detector technologies for Space observations (DEVINS) for monitoring UV irradiance variability in the Herzberg continuum. The first step of this strategy is to demonstrate the ability to build the DEVINS sensor, which is a compact/robust/radiation resistant solid-state photodetector that does not require cooling. DEVINS needs to be able to have a functional lifetime higher than classical space based UV sensors [33], which have a limited scientific operating lifetime in weeks rather than months or years due to contaminant trapping by their cooled surfaces [34].