Egeland, R., Soon, W., Baliunas, S., Hall, J., Pevtsov, A., Bertello, L. (2017) The Mount Wilson Observatory S-index of the Sun. Astrophysical Journal, 835 (1), 25. https://doi.org/10.3847/1538-4357/835/1/25

### The Mount Wilson Observatory S-index of the Sun

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Author: Egeland, Ricky1,2; Soon, Willie3; Baliunas, Sallie4;
Organizations: 1 High Altitude Observatory, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000, USA
2 Department of Physics, Montana State University, Bozeman, MT 59717-3840, USA
3Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
4No affiliation
5Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA
6National Solar Observatory, Sunspot, NM 88349, USA
7 ReSoLVE Centre of Excellence, Space Climate Research Unit, 90014 University of Oulu, Finland
8National Solar Observatory, Boulder, CO 80303, USA
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 12.6 MB)
Language: English
Published: IOP Publishing, 2017
Publish Date: 2019-03-05
Description:

# Abstract

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 $${R}_{\mathrm{HK}}^{\prime }$$. 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 We present previously unpublished measurements of the reflected sunlight from the Moon using the second-generation MWO HK photometer during solar cycle 23 and determine cycle minimum $${S}_{23,\min }=0.1634\pm 0.0008$$, amplitude $${\rm{\Delta }}{S}_{23}=0.0143\pm 0.0012$$, and mean $$\langle {S}_{23}\rangle =0.1701\pm 0.0005$$. 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 $$\langle {S}_{\min }\rangle =0.1621\pm 0.0008$$, $$\langle {\rm{\Delta }}{S}_{\mathrm{cyc}}\rangle =0.0145\pm 0.0012$$, $$\langle {S}_{\mathrm{cyc}}\rangle =0.1694\pm 0.0005$$. Our measurements represent an improvement over previous estimates that 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.

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Series: Astrophysical journal
ISSN: 0004-637X
ISSN-E: 1538-4357
ISSN-L: 0004-637X
Volume: 835
Article number: 25
DOI: 10.3847/1538-4357/835/1/25