Analysis of full disc Ca II K spectroheliograms

Chatzistergos, Theodosios; Ermolli, I.; Krivova, N. A.; Solanki, S. K.

doi:10.1051/0004-6361/201834402

Cited by 54 publications

(82 citation statements)

References 62 publications

(179 reference statements)

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“…In addition, at greater height, the emitted radiation is more strongly affected by shock waves and local heating events, which reduce the agreement even more. Lack of photometric calibration of the historical images and potential not very accurate methods applied for processing can distort the relation too (see Chatzistergos 2017;Chatzistergos et al 2018bChatzistergos et al , 2019b.…”

Section: Comparison To Results From the Literaturementioning

confidence: 99%

“…to derive information on the structure, activity, and variability of the Sun, or for related applications such as Earth's climate response to solar irradiance variability. Regular magnetograms are, however, available only for the last four solar cycles, while synoptic Ca II K solar observations have been carried out for more than 120 years (Chatzistergos 2017;Chatzistergos et al 2019b). In recent years, following the availability of a number of digitized series of historical Ca II K observations, attempts have been made to reconstruct magnetograms from Ca II K observations, based on the relation between the Ca II K intensity and the magnetic field strength.…”

Section: Introductionmentioning

confidence: 99%

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Recovering the unsigned photospheric magnetic field from Ca II K observations

Chatzistergos

Ermolli

Solanki

et al. 2019

A&A

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Context.A number of studies have aimed at defining the exact form of the relation between magnetic field strength and Ca II H and K core brightness. All previous studies have, however, been restricted to isolated regions on the solar disc or to a limited set of observations. Aims. We reassess the relationship between the photospheric magnetic field strength and the Ca II K intensity for a variety of surface features as a function of the position on the disc and the solar activity level. This relationship can be used to recover the unsigned photospheric magnetic field from images recorded in the core of Ca II K line. Methods. We have analysed 131 pairs of high-quality, full-disc, near-co-temporal observations from the Helioseismic and Magnetic Imager (SDO/HMI) and Precision Solar Photometric Telescope (Rome/PSPT) spanning half a solar cycle. To analytically describe the observationally-determined relation, we considered three different functions: a power law with an offset, a logarithmic function, and a power law function of the logarithm of the magnetic flux density. We used the obtained relations to reconstruct maps of the line-of-sight component of the unsigned magnetic field (unsigned magnetograms) from Ca II K observations, which were then compared to the original magnetograms. Results. We find that both power-law functions represent the data well, while the logarithmic function is good only for quiet periods. We see no significant variation over the solar cycle or over the disc in the derived fit parameters, independently of the function used. We find that errors in the independent variable, usually not accounted for, introduce attenuation bias. To address this, we binned the data with respect to the magnetic field strength and Ca II K contrast separately and derived the relation for the bisector of the two binned curves. The reconstructed unsigned magnetograms show good agreement with the original ones. RMS differences are less than 90 G. The results were unaffected by the stray-light correction of the SDO/HMI and Rome/PSPT data. Conclusions. Our results imply that Ca II K observations, accurately processed and calibrated, can be used to reconstruct unsigned magnetograms by using the relations derived in our study.

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Section: Comparison To Results From the Literaturementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Recovering the unsigned photospheric magnetic field from Ca II K observations

Chatzistergos

Ermolli

Solanki

et al. 2019

A&A

Self Cite

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“…Images with severe artifacts (e.g. missing parts of disc) were excluded from analysis as was done by Chatzistergos et al (2018bChatzistergos et al ( , 2019b.…”

Section: Methodsmentioning

confidence: 99%

“…They also agree on the overall increase of solar activity over the first half of the 20th century and the decrease over recent decades. However, they also show significant inconsistencies, such as large differences in absolute value and in the detected short-and long-term trends (Ermolli et al, 2018;Chatzistergos et al, 2019b). Such differences are seen both between the results derived from different archives and between the same archives but processed and analyzed with different methodologies.…”

Section: Introductionmentioning

confidence: 98%

“…The Kodaikanal photographic archive underwent three digitizations over the last three decades (Kariyappa and Sivaraman, 1994;Makarov et al, 2004;Priyal et al, 2014), although only the last two digitizations included the entire solar disc. In Chatzistergos et al (2019b) we consistently analyzed with the same method the data from the former digitization of the Kodaikanal archive along with the archives from Arcetri, McMath-Hulbert, Meudon, Mitaka, Mt. Wilson, Schauinsland, and Wendelstein to derive plage areas.…”

Section: Introductionmentioning

confidence: 99%

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Delving into the Historical Ca ii K Archive from the Kodaikanal Observatory: The Potential of the Most Recent Digitized Series

et al. 2019

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Full-disc Ca II K photographic observations of the Sun carry direct information as regards the evolution of solar-plage regions for more than a century and are therefore a unique dataset for solar-activity studies. For a long time Ca II K observations were barely explored, but recent digitizations of multiple archives have allowed their extensive analysis. However, various studies have reported diverse results partly due to the insufficient quality of the digitized data. Furthermore, inhomogeneities have been identified within the individual archives, which, at least partly, could be due to the digitization. As a result, some of the archives, e.g. that from the Kodaikanal observatory, were re-digitized. The results obtained by different authors who analyzed the data from the new digitization of the Kodaikanal archive differ from each other and from those derived from the old digitization. Since the data were processed and analyzed using different techniques, it is not clear, however, whether the differences are due to the digitization or the processing of the data. To understand the reasons for such discrepancies, we analyze here the data from the two most recent digitizations of this archive. We use the same techniques to consistently process the images from both archives and to derive the plage areas from them. Some issues have been identified in both digitizations, implying that they are intrinsic characteristics of the data. Moreover, errors in timing of the observations plague both digitizations. Overall, the most recent 16-bit digitization offers an improvement over the earlier 8-bit one. It also includes considerably more data and should be preferred.

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