University of Oulu

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.

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)
Persistent link:
Language: English
Published: IOP Publishing, 2017
Publish Date: 2019-03-05


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 We present previously unpublished measurements of the reflected sunlight from the Moon using the second-generation MWO HK photometer during solar cycle 23 and determine cycle minimum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
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Copyright information: © 2017. The American Astronomical Society. All rights reserved.