Remote sensing is a key method for advancing our understanding of global photosynthesis and is thus critical to understanding terrestrial carbon uptake and climate change. Increasingly sophisticated spectral indices including solar‐induced florescence (SIF) and the photochemical reflectance index (PRI) are considered good proxies of canopy structure, biochemistry, and physiology. However, the relative influences of illumination intensity and angle on these measures are difficult to unravel, particularly at the scale of whole forest canopies. We exploit the solar dimming during the 2017 North American solar eclipse as well as a clear day before and cloudy day after the day of the eclipse. This novel approach allows us to assess changes in spectral vegetation indices due to illumination intensity independent of changes in illumination angle. Physiologically relevant spectral indices were most affected by dimming, with illumination level explaining 97% of variability in SIF and 99% of variability in PRI during the eclipse. The spectral change in reflectance through the eclipse period revealed changes in PRI were driven by reflectance differences at the 570 nm reference band rather than at the 531 nm signal band associated with xanthophyll pigment interconversions. This study refines our interpretation of vegetation properties from space with implications for our interpretation of signals related to terrestrial photosynthesis derived from sensors spanning a range of illumination conditions and angles.

中文翻译：

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在2017年北美日食前后的几天中，光谱植被指数和太阳诱发的荧光对照明强度和几何形状变化的响应

遥感是增进我们对全球光合作用的理解的关键方法，因此对于理解陆地碳吸收和气候变化至关重要。日益复杂的光谱指数，包括日光诱导荧光（SIF）和光化学反射指数（PRI），被认为是冠层结构，生物化学和生理学的良好代表。但是，很难阐明光照强度和角度对这些措施的相对影响，尤其是在整个森林冠层范围内。我们在2017年北美日食期间以及日食前后的晴天和阴天之后利用日光调节。这种新颖的方法使我们能够评估由于照明强度而与照明角度的变化无关的光谱植被指数的变化。与生理相关的光谱指数受调光的影响最大，光照水平解释了日食期间SIF的97％的变化和PRI的99％的变化。整个月蚀期间反射率的光谱变化表明PRI的变化是由与叶黄素色素互变相关的570 nm参考带而不是531 nm信号带的反射率差异驱动的。这项研究完善了我们对来自太空的植被特性的解释，并暗示了我们对与陆生光合作用相关信号的解释的意义，这些信号来自跨越一系列照明条件和角度的传感器。整个月蚀期间反射率的光谱变化表明PRI的变化是由与叶黄素色素互变相关的570 nm参考带而不是531 nm信号带的反射率差异驱动的。这项研究完善了我们对来自太空的植被特性的解释，这对我们对与陆生光合作用相关信号的解释具有深远的意义，这些信号来自跨越一系列照明条件和角度的传感器。整个月蚀期间反射率的光谱变化表明PRI的变化是由与叶黄素色素互变相关的570 nm参考带而不是531 nm信号带的反射率差异驱动的。这项研究完善了我们对来自太空的植被特性的解释，并暗示了我们对与陆生光合作用相关信号的解释的意义，这些信号来自跨越一系列照明条件和角度的传感器。