Proposal of an instrument design to observe annual changes in Spectral Outgoing Radiation

Abstract. From the wide range of possibilities, we propose an instrument capable of measuring annual changes in global Spectral Outgoing Radiation (SORa) from the entire Earth's surface between 200 nm and 1100 nm with a stability of 0.1 Wm^-2 over a period of one solar cycle or beyond. Photomultiplier tubes (PMTs) as detectors provide data with a cadence of one second and high dynamic range. Based on Total Solar Radiation TSI(t) data with a stability of 0.01 Wm^-2 per year, Spectral Solar Irradiance SSI(t) can be derived and normalized to ΣSSI(t)=TSI(t) for using the Sun as a referenced radiation source supported by solar modeling. Calibrated by SSI(t), a set of 12 spectrometers with 60 PMTs in total and 16 photometers simultaneously detect SOR(t). This database can also be provided to calibrate other space instruments to allow improved comparison of results. In previous missions in space, it has already been shown that the spectrometer design can detect both solar and terrestrial radiation with high dynamic range. The established measurement technique compensates for degradation through repeated calibration. The instrument also enables the determination of the global green Earth coverage and its annual changes by measuring chlorophyll absorption from 350 nm to 490 nm and 620 nm to 690 nm and green backscatter from 500 nm to 600 nm. Mapping the Earth will also make it possible to track annual local changes in green coverage and to assess the impact of different climate policies and climate engineering actions. Another aspect is the derivation of a correction parameter for the Earth Energy Imbalance derived from changes in green areas. Data evaluation can also include determining further parameters such as the Normalized Difference Vegetation Index, the Enhanced Vegetation Index, and the Global Leaf Area Index.