Solar radiation variability spans a wide range in time, ranging from seconds to decadal and longer. The nearly 40 years of measurements of solar irradiance from space established that the total solar irradiance varies by ≈ 0.1 % $\approx 0.1\%$ in phase with the Sun’s magnetic cycle. Specific intervals of the solar spectrum, e.g., ultraviolet (UV), vary by orders of magnitude more. These variations can affect the Earth’s climate in a complex non-linear way. Specifically, some of the processes of interaction between solar UV radiation and the Earth’s atmosphere involve threshold processes and do not require a detailed reconstruction of the solar spectrum. For this reason a spectral UV index based on the (FUV-MUV) color has been recently introduced. This color is calculated using SORCE SOLSTICE integrated fluxes in the FUV and MUV bands. We present in this work the reconstructions of the solar (FUV-MUV) color and Ca ii K and Mg ii indices, from 1749–2015, using a semi-empirical approach based on the reconstruction of the area coverage of different solar magnetic features, i.e., sunspot, faculae and network. We remark that our results are in noteworthy agreement with latest solar UV proxy reconstructions that exploit more sophisticated techniques requiring historical full-disk observations. This makes us confident that our technique can represent an alternative approach which can complement classical solar reconstruction efforts. Moreover, this technique, based on broad-band observations, can be utilized to estimate the activity on Sun-like stars, that cannot be resolved spatially, hosting extra-solar planetary systems.

中文翻译：

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太阳紫外光谱指数的长期（1749-2015）变化

太阳辐射的变化范围很广，从几秒到十年甚至更长。近 40 年来对太空太阳辐照度的测量表明，总太阳辐照度随太阳磁循环的相位变化约 0.1 % $\大约 0.1\%$。太阳光谱的特定间隔，例如紫外线 (UV)，变化幅度更大。这些变化会以复杂的非线性方式影响地球气候。具体来说，太阳紫外线辐射与地球大气之间的一些相互作用过程涉及阈值过程，不需要对太阳光谱进行详细的重建。为此，最近引入了基于 (FUV-MUV) 颜色的光谱 UV 指数。该颜色是使用 FUV 和 MUV 波段中的 SORCE SOLSTICE 积分通量计算的。我们在这项工作中介绍了 1749-2015 年间太阳 (FUV-MUV) 颜色和 Ca ii K 和 Mg ii 指数的重建，使用基于不同太阳磁特征区域覆盖重建的半经验方法，即，太阳黑子、光斑和网络。我们注意到，我们的结果与最新的太阳紫外线代理重建非常一致，这些重建利用了需要历史全盘观测的更复杂技术。这使我们相信我们的技术可以代表一种替代方法，可以补充经典的太阳能重建工作。此外，这种基于宽带观测的技术可用于估计无法在空间上解析的类太阳恒星的活动，这些恒星承载着太阳系外行星系统。