Analysis of full disc Ca II K spectroheliograms

Abstract

Context. Historical Ca II K spectroheliograms (SHG) are unique in representing long-term variations of the solar chromospheric magnetic field. They usually suffer from numerous problems and lack photometric calibration. Thus accurate processing of these data is required to get meaningful results from their analysis. Aims. In this paper we aim at developing an automatic processing and photometric calibration method that provides precise and consistent results when applied to historical SHG. Methods. The proposed method is based on the assumption that the centre-to-limb variation of the intensity in quiet Sun regions does not vary with time. We tested the accuracy of the proposed method on various sets of synthetic images that mimic problems encountered in historical observations. We also tested our approach on a large sample of images randomly extracted from seven different SHG archives. Results. The tests carried out on the synthetic data show that the maximum relative errors of the method are generally <6.5%, while the average error is <1%, even if rather poor quality observations are considered. In the absence of strong artefacts the method returns images that differ from the ideal ones by <2% in any pixel. The method gives consistent values for both plage and network areas. We also show that our method returns consistent results for images from different SHG archives. Conclusions. Our tests show that the proposed method is more accurate than other methods presented in the literature. Our method can also be applied to process images from photographic archives of solar observations at other wavelengths than Ca II K.

Time‐Latitude Distribution of Prominences for 10 Solar Cycles: A Study Using Kodaikanal, Meudon, and Kanzelhohe Data

“…The 8-bit version was presented in Ermolli et al (2009) as the most uniform data set in terms of resolution, large-scale inhomogeneity, and contrast among different available long-term data sets. An accurate photometric calibration of this series was made by Chatzistergos et al (2018) to make the data set ready for irradiance calculation and also detection of plages . Recently, the 16-bit series was studied by confirming its superiority over the 8-bit series in terms of uniformity.…”

“…It should be noted that just median filtering is not an accurate method for background normalization when the objective is to make the data ready for irradiance study (Chatzistergos et al, 2018) and may affect detection of features, measurement of contrast. We did not need the data to be photometrically calibrated as we opted for only detection of prominence locations avoiding measurement of individual feature areas.…”

Recovering the unsigned photospheric magnetic field from Ca II K observations

“…In particular, for each image pixel i, we defined its contrast C i as C i = I i /I QS i , where I i is the measured intensity of pixel i, and I QS i is the intensity of the quiet Sun (QS, hereafter) at the same position. The latter was derived with the iterative procedure described by Chatzistergos et al (2018b), which returns contrast images Article number, page 3 of 18 A&A proofs: manuscript no. b_vs_cak_arxiv with an average error in contrast values lower than 0.6% (see Chatzistergos et al 2018b, for more details).…”

“…The latter was derived with the iterative procedure described by Chatzistergos et al (2018b), which returns contrast images Article number, page 3 of 18 A&A proofs: manuscript no. b_vs_cak_arxiv with an average error in contrast values lower than 0.6% (see Chatzistergos et al 2018b, for more details).…”

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

“…It is worth noting that most of the results presented in the literature lack error estimates. Chatzistergos et al (2018a) showed that with proper methods it is possible to derive consistent and accurate results e.g. of the plage coverage for images from different Ca II K archives including both historical and modern observations.…”