Radiative emission of solar features in the Ca II K line: comparison of measurements and models⋆
INAF-Osservatorio Astronomico di Roma, via Frascati 33,
Monte Porzio Catone,
2 National Solar Observatory, Sacramento Peak, PO Box 62, Sunspot, NM 88349, USA
3 Laboratory for Atmospheric and Space Physics, Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO, 80303, USA
4 Max-Planck-Institut für Sonnensystemforshung, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany
5 School of Space Research, Kyung Hee University, Yongin, Gyeonggi 446-71, Korea
Accepted: 18 August 2010
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.
Key words: Sun: activity / Sun: photosphere / Sun: chromosphere / Sun: faculae, plages
Appendix A is only available in electronic form at http://www.aanda.org
© ESO, 2010