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 singlyionized 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 We present previously unpublished measurements of the reflected sunlight from the Moon using the second-generation MWO HK photometer during solar cycle 23 and determine cycle minimum S 23,min = 0.1634 ± 0.0008, amplitude ∆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, ∆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.

Common dynamo scaling in slowly rotating young and evolved stars

“…The Ca II H&K emission ratios, R HK = L HK /L bol = F HK /σT eff , were calculated from the averaged MWO S -index observations 24 after removing sections of data with apparent calibration issues and outliers more than 4σ away from the sample mean 25 . The conversion is colour dependent and defined separately for the main sequence and evolved stars 26 .…”

The Influence of Metallicity on Stellar Differential Rotation and Magnetic Activity

“…For the solar S indices, observations of the daily Kindex K SP from the Evans Coronal Facility at Sacramento Peak (Keil et al 1998) were transformed to the MWO S-index scale using the calibration of Egeland et al (2017). This calibration was developed using overlapping observations (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003) of reflected sunlight from the Moon through the MWO HKP-2 spectrophotometer.…”

Evolution of Long Term Variability in Solar Analogs

“…Figure 1 shows the solar S-index record and three other stars from our sample on the same scale, illustrating the range of mean activity levels and amplitudes. The youngest, most active star in our sample is HD 20630 (κ 1 Ceti), which was discussed at this symposium by Dr. Dias do Nascimento, Jr. Not shown are HD 30495 and HD 146233 (18 Sco), the former which is studied in detail in , and the latter which is a solar twin (Porto de Mello and da Silva 1997; Meléndez et al 2014) and has a mean activity and amplitude very similar to the Sun (Hall et al 2007;Egeland et al 2017). Table 1 shows the properties of the sample.…”

“…Some initial results from this study were presented in and . A similarly long Sun-as-a-star Ca ii H & K record was developed in Egeland et al (2017), which accurately placed the long NSO Sacramento Peak Ca ii K-line record on the S-index scale using coincident observations of the Moon from the MWO HKP-2 instrument. Figure 1 shows the solar S-index record and three other stars from our sample on the same scale, illustrating the range of mean activity levels and amplitudes.…”