Fig. 20.
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Differential variability of absorption in Na I D1 and D2 lines. The observing seasons of 2016, 2017, and 2018 are marked in red, cyan, and gray. The identity relation is dashed while linear fits are dotted blue. The weaker Na I D1 line is more variable than the more saturated D2, while their ratio mirrors the chromospheric activity index.
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