## 1 Introduction
Another important photoproduction process is chlorophyll fluorescence, where excessive energy is expelled in order to prevent harmful photo-oxidation [3,18,28,32]. Decades of laboratory studies and recent field-based solar-induced fluorescence (SIF) studies have shown that fluorescence provides a direct indicator of plant photosynthetic function (e.g., carbon fixation), enabling early detection of environmentally induced stress [52,53,54,55,56,57,58,59,60]. However, retrieving the SIF signal from passive remote sensing spectral observations over vegetation canopies under ambient solar illumination has been challenging because it is a weak signal added to the reflected radiance. Nevertheless, recent studies have shown success for SIF retrievals at leaf, canopy, and even satellite levels. Previously, since the SIF signal is small and requires very high spectral resolution for retrieval, attempts were made to develop optical reflectance-based approaches for SIF [61,62,63,64,65]. In contrast to these relative indices, radiance-based algorithms enable direct estimates of SIF in physical or arbitrary units [54,55,66,67,68,69,70]. The chlorophyll fluorescence emission spectrum occurs across the red and near-infrared spectral region (650-800 nm) with two peaks near 685 nm and 740 nm. Radiance-based SIF retrieval approaches utilize measurements within and around atmospheric Fraunhofer lines that overlay the emission window to decouple SIF from reflected radiance. This is possible because the SIF signal contributes more to reflected radiance from vegetation canopies within these narrow dark lines where irradiance is low because of strong atmospheric absorption [54,55,57]. The two telluric oxygen bands, O\({}_{2}\)-A (centered at 760 nm, \(\sim\)7 nm width) and O\({}_{2}\)-B (688 nm, \(\sim\)4 nm width) bands, are the most exploited absorption features for SIF retrievals. Many studies have reported that SIF can be detected using this passive remote sensing based concept [26,54,69,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95]. The successful SIF retrievals to date have utilized the far-red (O\({}_{2}\)-A) retrieval to improve estimates and/or reduce uncertainty of photosynthetic activities, LUE and GPP, at leaf [76] and canopy levels [26,60,73,74,77]. However, few studies have successfully retrieved red fluorescence from the O\({}_{2}\)-B feature, nor have they examined red fluorescence in conjunction with canopy GPP measurements. The fluorescence emission and NPQ energy dissipation are important components in the carbon fixation machinery of plants. The two processes are considered to be photosynthetic stress indices under sub-optimal environmental conditions and direct probes of plants' physiological and photosynthetic status [29, 32, 53, 54, 55, 56, 78, 79, 80, 81]. In recent years, advances in remote sensing technology, especially those providing higher spectral resolution and signal-to-noise ratio, have enabled retrievals from high spectral resolution optical observations that correlate to the two dissipative pathways. Since both NPQ and chlorophyll fluorescence provide information relevant to photosynthetic activities (e.g., LUE and GPP) at the canopy scale [54, 82, 83], further investigations are warranted. In particular, the relationships among photosynthesis, PRI, and SIF derived from remote sensing observations, need further investigation, including examination of the red SIF. Several recent studies investigated the use of PRI and SIF to model LUE and GPP with various success [26, 73, 74, 78], but the mechanism linking PRI and SIF to each other and to GPP is not yet well-described. In the current study, we introduce a collaborative field campaign conducted in a cornfield during four years. Hyperspectral observations and eddy covariance measurements were acquired during each year of the campaign. The study aims to investigate a greater range, both diurnal and seasonal changes, than previous publications.