Radiative emission of solar features in the Ca II K line: comparison of measurements and models

Abstract

Context. The intensity of the Ca II K resonance line observed with spectrographs and Lyot-type filters has long served as a diagnostic of the solar chromosphere. However, the literature contains a relative lack of photometric measurements of solar features observed at this spectral range. Aims. We study the radiative emission of various types of solar features, such as quiet Sun, enhanced network, plage, and bright plage regions, identified on filtergrams taken in the Ca II K line. Methods. We analysed full-disk images obtained with the PSPT, by using three interference filters that sample the Ca II K line with different bandpasses. We studied the dependence of the radiative emission of disk features on the filter bandpass. We also performed a non-local thermal equilibrium (NLTE) spectral synthesis of the Ca II K line integrated over the bandpass of PSPT filters. The synthesis was carried out by utilizing the partial frequency redistribution (PRD) with the most recent set of semi-empirical atmosphere models in the literature and some earlier atmosphere models. As the studied models were computed by assuming the complete redistribution formalism (CRD), we also performed simulations with this approximation for comparison. Results. We measured the center-to-limb variation of intensity values for various solar features identified on PSPT images and compared the results obtained with those derived from the synthesis. We find that CRD calculations derived using the most recent quiet Sun model, on average, reproduce the measured values of the quiet Sun regions slightly more accurately than PRD computations with the same model. This may reflect that the utilized atmospheric model was computed assuming CRD. Calculations with PRD on earlier quiet Sun model atmospheres reproduce measured quantities with a similar accuracy as to that achieved here by applying CRD to the recent model. We also find that the median contrast values measured for most of the identified bright features, disk positions, and filter bandpasses are, on average, a factor ≈1.9 lower than those derived from PRD simulations performed using the recent bright feature models. The discrepancy between measured and modeled values decreases by ≈12% after taking into account straylight effects on PSPT images. When moving towards the limb, PRD computations display closer agreement with the data than performed in CRD. Moreover, PRD computations on either the most recent or the earlier atmosphere models of bright features reproduce measurements from plage and bright plage regions with a similar accuracy.

Variability of Sun-like stars: reproducing observed photometric trends

“…Skumanich et al (1984) showed that the center-to-limb variation of the Ca II K emission (which hereafter is used as a representation of the summed emission in the H and K lines, see Radick et al 1998) is the same for the quiet Sun and faculae. This was confirmed by later measurements (see, e.g., Ermolli et al 2007Ermolli et al , 2010. Thus one can write…”

“…Functions C HK (μ) and C Q cont (μ) describe the CLV of the HK and continuum fluxes. The HK flux and its CLV are strongly affected by the effects of non-local thermodynamical equilibrium (see, e.g., Ermolli et al 2010), which are not taken into account in the spectra employed by the SATIRE model (Unruh et al 1999). Therefore, instead of taking the theoretical CLV resulting from SATIRE, we employed quadratic polynomial parameterizations (as a function of the heliocentric angle cosine μ) of the CLV from Skumanich et al (1984), who employed several observational datasets to define them.…”

Recent variability of the solar spectral irradiance and its impact on climate modelling

“…Images taken between September 1997 and January 2012 have been processed and segmented to identify different features on the solar disc as described by Ermolli et al (2010). The seven classes of atmospheric features proposed by Fontenla et al (2009) are considered.…”

Solar Cycle Indices from the Photosphere to the Corona: Measurements and Underlying Physics

“…Therefore, the Ca II K line emission can be used as a good proxy of the LOS magnetic flux density over the whole solar disk (e.g. Ermolli et al 2010, and references therein). The H α line at 656.28 nm has been also widely employed for studying the solar chromosphere, which was defined as what is seen in this line (Lockyer 1868).…”