1D atmosphere models from inversion of Fe I 630 nm observations with an application to solar irradiance studies

Present-day semi-empirical models of solar irradiance (SI) variations employ spectra computed on one-dimensional atmosphere models (1D models) representative of various solar surface features to reconstruct SI changes measured on timescales greater than a day. Various recent studies have, however, pointed out that the spectra synthesized on 1D models do not reflect the radiative emission of the inhomogenous atmosphere revealed by high-resolution solar observations. We aimed to derive observational-based atmospheres from such observations and test their accuracy for SI estimates. We analysed spectro-polarimetric data of the Fe I 630 nm line pair on photospheric regions representative of the granular, quiet Sun pattern (QS) and of small- and large-scale magnetic features, both bright and dark with respect to the QS. The data were taken on 2011 August 6, with the CRISP at the Swedish Solar Telescope, under excellent seeing conditions. We derived atmosphere models of the observed regions from data inversion with the SIR code. We studied the sensitivity of results to spatial resolution and temporal evolution, and discussed the obtained atmospheres with respect to several 1D models. The atmospheres derived from our study agree well with most of the compared 1D models, both qualitatively and quantitatively (differences are within 10%), but for pore regions. Spectral synthesis computations on the atmosphere obtained from the QS observations return SI between 400 nm and 2400 nm that agrees, on average, within 2.2% with standard reference measurements, and within -0.14% with the SI computed on the quiet Sun atmosphere employed by the most advanced semi-empirical model of SI variations.Comment: Accepted for publication in The Astrophysical Journa

Analysis of full disc Ca II K spectroheliograms. II. Towards an accurate assessment of long-term variations in plage areas

Reconstructions of past irradiance variations require suitable data on solar activity. The longest direct proxy is the sunspot number, and it has been most widely employed for this purpose. These data, however, only provide information on the surface magnetic field emerging in sunspots, while a suitable proxy of the evolution of the bright magnetic features, specifically faculae/plage and network, is missing. This information can potentially be extracted from the historical full-disc observations in the Ca II K line. We have analysed over 100,000 historical images from 8 digitised photographic archives of the Arcetri, Kodaikanal, McMath-Hulbert, Meudon, Mitaka, Mt Wilson, Schauinsland, and Wendelstein observatories, as well as one archive of modern observations from the Rome/PSPT. The analysed data cover the period 1893--2018. We first performed careful photometric calibration and compensation for the centre-to-limb variation, and then segmented the images to identify plage regions. This has been consistently applied to both historical and modern observations. The plage series derived from different archives are generally in good agreement with each other. However, there are also clear deviations that most likely hint at intrinsic differences in the data and their digitisation. We showed that accurate image processing significantly reduces errors in the plage area estimates. Accurate photometric calibration also allows precise plage identification on images from different archives without the need to arbitrarily adjust the segmentation parameters. Finally, by comparing the plage area series from the various records, we found the conversion laws between them. This allowed us to produce a preliminary composite of the plage areas obtained from all the datasets studied here. This is a first step towards an accurate assessment of the long-term variation of plage regions.Comment: 30 pages, 22 figures, accepted in A&

The potential of Ca II K observations for solar activity and variability studies

Several observatories around the globe started regular full-disc imaging of the solar atmosphere in the Ca II K line in the early decades of the 20th century. These observations are continued today at a few sites with either old spectroheliographs or modern telescopes equipped with narrow-band filters. The Ca II K time series are unique in representing long-term variations of the Sun's chromospheric magnetic field. However, meaningful results from their analysis require accurate processing of the available data and robust merging of the information stored in different archives. This paper provides an overview of the historical and modern full-disc Ca II K observations, with focus on their quality and the main results obtained from their analysis over the last decade.Comment: 6 pages, 2 figure

Ca II K spectroheliograms for studies of long-term changes in solar irradiance

We address the importance of historical full disc Ca II K spectroheliograms for solar activity and irradiance reconstruction studies. We review our work on processing such data to enable them to be used in irradiance reconstructions. We also present our preliminary estimates of the plage areas from five of the longest available historical Ca II K archives.Comment: 5 pages, 3 figure

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

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. We analysed fulldisk 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 NLTE spectral synthesis of the Ca II K line integrated over the bandpass of PSPT filters. The synthesis was carried out by utilizing both the PRD and CRD with the most recent set of semi empirical atmosphere models in the literature and some earlier atmosphere models. We measured the CLV 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 widths 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. 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.Comment: 14 pages, 18 figures, accepted by A&

The Intensity Profile of the Solar Supergranulation

We have measured the average radial (cell center to network boundary) profile of the continuum intensity contrast associated with supergranular flows using data from the Precision Solar Photometric Telescope (PSPT) at the Mauna Loa Solar Observatory (MLSO). After removing the contribution of the network flux elements by the application of masks based on Ca II K intensity and averaging over more than 10^5 supergranular cells, we find a ~ 0.1% decrease in red and blue continuum intensity from the supergranular cell centers outward, corresponding to a ~ 1.0 K decrease in brightness temperature across the cells. The radial intensity profile may be caused either by the thermal signal associated with the supergranular flows or a variation in the packing density of unresolved magnetic flux elements. These are not unambiguously distinguished by the observations, and we raise the possibility that the network magnetic fields play an active role in supergranular scale selection by enhancing the radiative cooling of the deep photosphere at the cell boundaries.Comment: Accepted to Ap

Stray-light restoration of full-disk CaII K solar observations: a case study

AIMS: We investigate whether restoration techniques, such as those developed for application to current observations, can be used to remove stray-light degradation effects on archive CaII K full-disk observations. We analyze to what extent these techniques can recover homogeneous time series of data. METHODS:We develop a restoration algorithm based on a method presented by Walton & Preminger (1999). We apply this algorithm to data for both present-day and archive CaII K full-disk observations, which were acquired using the PSPT mounted at the Rome Observatory, or obtained by digitization of Mt Wilson photographic-archive spectroheliograms. RESULTS:We show that the restoring algorithm improves both spatial resolution and photometric contrast of the analyzed solar observations. We find that the improvement in spatial resolution is similar for analyzed recent and archive data. On the other hand, the improvement of photometric contrast is quite poor for the archive data, with respect to the one obtained for the present-day images. We show that the quality of restored archive data depends on the photographic calibration applied to the original observations. In particular, photometry can be recovered with a restoring algorithm if the photographic-calibration preserves the intensity information stored in the original data, principally outside the solar-disk observations.Comment: 10 pages; 9 figure