2016/11/15 by Ricky Egeland, Willie Soon, Sallie Baliunas +3 · 1 citation
Physics and Astronomy · #astro-ph.SR
paper · pdf · doi:10.3847/1538-4357/835/1/25
17 pages, 7 figures, 3 tables. Accepted for publication in the Astrophysical Journal
arxiv created 2016/11/15 · arxiv updated 2017/01/17
The most commonly used index of stellar magnetic activity is the instrumental flux scale of singly-ionized calcium H & K line core emission, S, developed by the Mount Wilson Observatory (MWO) HK Project, or the derivative index R'HK. Accurately placing the Sun on the S scale is important for comparing solar activity to that of the Sun-like stars. We present previously unpublished measurements of the reflected sunlight from the Moon using the second-generation MWO HK photometer during solar cycle 23 and determine cycle minimum Smin,23 = 0.1634 +/- 0.0008, amplitude Delta S23 = 0.0143 +/- 0.0012, and mean <S23> = 0.1701 +/- 0.0005. By establishing a proxy relationship with the closely related National Solar Observatory Sacramento Peak calcium K emission index, itself well-correlated with the Kodaikanal Observatory plage index, we extend the MWO S time series to cover cycles 15-24 and find on average <Smin> = 0.1621 +/- 0.0008, <Delta Scyc> = 0.0145 +/- 0.0012, <Scyc> = 0.1694 +/- 0.0005. Our measurements represent an improvement over previous estimates which relied on stellar measurements or solar proxies with non-overlapping time series. We find good agreement from these results with measurements by the Solar-Stellar Spectrograph at Lowell Observatory, an independently calibrated instrument, which gives us additional confidence that we have accurately placed the Sun on the S-index flux scale.