The Mount Wilson Observatory S-index of the Sun

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'_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 We present previously unpublished measurements of the reflected sunlight from the Moon using the second-generation MWO HK photometer during solar cycle 23 and determine cycle minimum S_min,23 = 0.1634 +/- 0.0008, amplitude Delta S_23 = 0.0143 +/- 0.0012, and mean <S_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_min> = 0.1621 +/- 0.0008, <Delta S_cyc> = 0.0145 +/- 0.0012, <S_cyc> = 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.

17 pages, 7 figures, 3 tables. Accepted for publication in the Astrophysical Journal

References in corpus (5)

• Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics
• Toroidal versus poloidal magnetic fields in Sun-like stars: a rotation threshold
• Patterns of photometric and chromospheric variation among Sun-like stars: A 20-year perspective
• Sun-as-a-Star Spectrum Variations 1974-2006
• Solar cycle variation in solar irradiance

Cited by in corpus (10)

• The flare catalog and the flare activity in the Kepler mission
• The Starspots of HAT-P-11: Evidence for a Solar-like Dynamo
• The influence of metallicity on stellar differential rotation and magnetic activity
• The discovery and mass measurement of a new ultra-short-period planet: EPIC~228732031b
• Chromospheric Activity of HAT-P-11: an Unusually Active Planet-Hosting K Star
• Far beyond the Sun: I. The beating magnetic heart in Horologium
• No "Maunder minimum" candidates in M67: mitigating interstellar contamination of chromospheric emission lines
• The Effect of Magnetic Variability on Stellar Angular Momentum Loss II: The Sun, 61 Cygni A, $\epsilon$ Eridani, $\xi$ Bootis A and $\tau$ Bootis A
• Carrington cycle 24: The solar chromospheric emission in a historical and stellar perspective
• The magnetic future of the Sun