## 1 Introduction  
Forests are important terrestrial ecosystems which contribute more than 70% of the terrestrial carbon pool [1]. Monitoring the status of forests from space is important for forest management [2]. Remote sensing has provided many global vegetation indices which rely on canopy reflectance in different spectral channels and has been widely used in forest research [3, 4, 5, 6]. Over the past 30+ years, vegetation remote sensing based on \"greenness\" vegetation indices has greatly improved our understanding of the biosphere at a large scale. However, the vegetation indices (VIs) only explore the \"potential photosynthesis\" through \"greenness\". Conversely, chlorophyll fluorescence has a unique ability to provide information about actual photosynthesis. For this reason, remote sensing of vegetation chlorophyll fluorescence has been a key research topic in the last 15 years [7; 8; 9]. Solar-induced chlorophyll fluorescence (SIF) is emitted by chlorophyll molecules and has been shown to be a proxy of photosynthetic activity [8; 10; 11; 12]. As SIF is in competition with photochemical conversion, it may have the potential to estimate carbon cycles. During photosynthesis, the light absorbed by chlorophyll is used in the carbon fixation process, dissipated into heat, and re-emitted by chlorophyll a in the red and near infrared (NIR) spectral region [11; 13; 14]. The third part is called SIF, which is less than 5% of the total light absorbed by a plant; however, it offers a more direct measure of photosynthetic activity [15]. SIF provides a direct approach to diagnosing the actual functional status of vegetation, and the fluorescence signal can be quantified by passive (i.e., without artificial excitation sources) remote sensing systems exploiting the dark line of the solar and atmospheric spectra where irradiance is strongly reduced (Fraunhofer lines). In particular, two telluric O\({}_{2}\) absorption bands (O\({}_{2}\)-B band at 687 nm and O\({}_{2}\)-A band at 760.4 nm) have been extensively used to estimate SIF because of their spectral proximity to the peaks of the fluorescence emission spectrum positioned at about 685 and 740 nm [2; 16]. The recent advancement of satellite-based measurements of SIF provides tremendous potential for the global monitoring of terrestrial photosynthesis and the carbon cycle [11]. Global annual averaged SIF at 755 nm at a spatial resolution of \(2^{\circ}\times 2^{\circ}\) was derived from Greenhouse gases Observing SATellite (GOSAT) data for the first time by [PERSON], et al. [15]. SIF data have been derived from Global Ozone Monitoring Experiment 2 (GOME-2), Scanning Imaging Absorption spectrometer for Atmospheric CHartograhY (SCIAMACHY), the Orbiting Carbon Observatory-2 (OCO-2), and TROPOspheric Monitoring Instrument (TROPOMI) satellites to promote the development of SIF remote sensing [7; 10; 13; 17; 18; 19; 20]. The FLuorescence EXplorer (FLEX) is the 8 th ESA Earth Explorer mission that is planned for launch by 2024; this will provide the first 300 m resolution SIF imagery [21; 22; 23; 24; 25; 26]. Most of the atmospheric chemistry satellites (GOSAT, GOME-2, SCHIAMACHY, OCO-2, and TROPOMI) mainly offer far-red SIF observations, while red SIF is available only from GOME-2 and SCHIAMACHY [27; 28; 29; 14]. Different types of vegetation are usually characterized by different SIF values. In general, the highest SIF emissions are found in croplands, followed by broadleaf and then coniferous forests [30]. Due to the advantages of SIF in measuring vegetation photosynthesis, many researchers explored the potential of using SIF in modeling gross primary production (GPP). Some studies used MODIS products to simulate SIF at moderate spatial resolutions and obtained satisfactory results [11; 28; 31; 32; 33]. During the daytime, SIF has a positive correlation with photochemistry in leaves and thus could be used as a valid proxy for GPP. This implies the possibility of using vegetation indices to simulate SIF. [PERSON], et al.