Solar Ca K-line emission as a function of the photospheric magnetic proxy, Sph, ⊙ (in ppm), of the Sun during the solar cycle 23 between 1996 and 2008 (solid line) and smoothed over 1 yr. The first year of cycle 24 is also represented in dashed line. The start of each year is indicated: red for the rising phase and blue for the falling phase of the activity cycle. The individual data points before smoothing are shown in gray.
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