The Mount Wilson Observatory S-index of the sun

Journal Article · Fri Jan 20 04:00:00 EST 2017 · Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/835/1/25· OSTI ID:22869568

Egeland, Ricky ^[1]; Soon, Willie ^[2]; Baliunas, Sallie; Hall, Jeffrey C. ^[3]; Pevtsov, Alexei A. ^[4]; Bertello, Luca ^[5]

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1. High Altitude Observatory, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000 (United States)
2. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 (United States)
3. Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001 (United States)
4. National Solar Observatory, Sunspot, NM 88349 (United States)
5. National Solar Observatory, Boulder, CO 80303 (United States)

The most commonly used index of stellar magnetic activity is the instrumental flux scale of singly ionized calcium H and K line core emission, S, developed by the Mount Wilson Observatory (MWO) HK Project, or the derivative index R{sub HK}{sup ′}. 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 We present previously unpublished measurements of the reflected sunlight from the Moon using the second-generation MWO HK photometer during solar cycle 23 and determine cycle minimum S{sub 23,min}=0.1634±0.0008, amplitude ΔS{sub 23}=0.0143±0.0012, and mean 〈S{sub 23}〉=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 〈S{sub min}〉=0.1621±0.0008, 〈ΔS{sub cyc}〉=0.0145±0.0012, 〈S{sub cyc}〉=0.1694±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.

• OSTI ID: 22869568
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 835; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
CALCIUM
CHROMOSPHERE
COMPARATIVE EVALUATIONS
EMISSION
MOON
PHOTOMETERS
SOLAR CYCLE
SUN