Contribution of chromospheric features to UV irradiance variability from spatially-resolved Ca II K spectroheliograms

Kariyappa, R.; Pap, J. M.

doi:10.1007/bf00146331

Cited by 25 publications

(25 citation statements)

References 12 publications

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“…Fredga 1971; Kariyappa & Sivaraman 1994;Kariyappa & Pap 1996;Worden et al 1998a;Giorgi et al 2005;Ermolli et al 2009a). However, in most cases acquisition of these exposures started many years after the start of solar observations.…”

Section: Introductionmentioning

confidence: 99%

Analysis of full disc Ca II K spectroheliograms

Chatzistergos

Ermolli

Solanki

et al. 2018

A&A

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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.

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Section: Introductionmentioning

confidence: 99%

Analysis of full disc Ca II K spectroheliograms

Chatzistergos

Ermolli

Solanki

et al. 2018

A&A

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“…It has been observed that the solar energy flux changes over a solar cycle. It has been shown that the long-term irradiance variations are attributable to the changing emission of bright magnetic elements (Foukal & Lean 1988;Kariyappa & Pap 1996;Worden et al 1998;Kariyappa 2000Kariyappa , 2008Veselovsky et al 2001;Kumara et al 2012), whereas the short-term irradiance variations are directly associated with active regions as they evolve and move across the solar disk (Lean 1987). To understand and clarify the difference between the observed and modelled irradiance variability, to know the underlying physical mechanisms of solar irradiance variability, and to estimate the contribution of various coronal features to EUV and UV irradiance, a detailed and accurate image analysis of spatially resolved observations from space is required.…”

Section: Introductionmentioning

confidence: 99%

Segmentation of coronal features to understand the solar EUV and UV irradiance variability

Kumara

Kariyappa

Zender

et al. 2013

A&A

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Context. The study of solar irradiance variability is of great importance in heliophysics, the Earth's climate, and space weather applications. These studies require careful identifying, tracking and monitoring of active regions (ARs), coronal holes (CHs), and the quiet Sun (QS). Aims. We studied the variability of solar irradiance for a period of two years (January 2011-December 2012) using the Large Yield Radiometer (LYRA), the Sun Watcher using APS and image Processing (SWAP) on board PROBA2, and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). Methods. We used the spatial possibilistic clustering algorithm (SPoCA) to identify and segment coronal features from the EUV observations of AIA. The AIA segmentation maps were then applied on SWAP images, and parameters such as the intensity, fractional area, and contribution of ARs/CHs/QS features were computed and compared with the full-disk integrated intensity and LYRA irradiance measurements. Results. We report the results obtained from SDO/AIA and PROBA2/SWAP images taken from January 2011 to December 2012 and compare the resulting integrated full-disk intensity with PROBA2/LYRA irradiance. We determine the contributions of the segmented features to EUV and UV irradiance variations. The variations of the parameters resulting from the segmentation, namely the area, integrated intensity, and relative contribution to the solar irradiance, are compared with LYRA irradiance. We find that the active regions have a great impact on the irradiance fluctuations. In the EUV passbands considered in this study, the QS is the greatest contributor to the solar irradiance, with up to 63% of total intensity values. Active regions, on the other hand, contribute to about 10%, and off-limb structures to about 24%. We also find that the area of the features is highly variable suggesting that their area has to be taken into account in irradiance models, in addition to their intensity variations. Conclusions. We successfully show that the feature extraction allows us to use EUV telescopes to measure irradiance fluctuations and to quantify the contribution of each part to the EUV spectral solar irradiance observed with a calibrated radiometer. This study also shows that SPoCA is viable, and that the segmentation of images can be a useful tool. We also provide the measurement correlation between SWAP and AIA during this analysis.

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“…For instance, Arcetri, Kodaikanal, and Mt Wilson Ca ii K historic observations have recently been digitized (Ulrich et al 2004;Makarov et al 2004;Marchei et al 2006), and other similar series are now being processed as well. Some Ca ii K series already underwent digitization and analysis in the 1990s, when selected (regularly spaced) spectroheliograms or almost entire series were converted at 2 pixel −1 (e.g., Ribes & Mein 1985 (p. 282); Kariyappa & Pap 1996;Caccin et al 1998;Worden et al 1998) or 5 pixel −1 (Foukal 1996) resolution, respectively. Defects in and decay of spectroheliogram photographic plates, missing photographic calibration, and undocumented changes of the instrumentation lead to various artifacts in and problems with the historic data (Zharkova et al 2003;Fuller et al 2005;, which are avoided in the full-disk images taken by the most recent synoptic observing programs.…”

Section: Introductionmentioning

confidence: 99%

COMPARISON AMONG Ca II K SPECTROHELIOGRAM TIME SERIES WITH AN APPLICATION TO SOLAR ACTIVITY STUDIES

Ermolli

Solanki

Tlatov

et al. 2009

ApJ

103

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Various observatories around the globe started regular full-disk imaging of the solar atmosphere in the Ca ii K line in the early decades of the 20th century. The archives made by these observations have the potential of providing far more detailed information on solar magnetism than just the sunspot number and area records to which most studies of solar activity and irradiance changes are restricted. We evaluate the image quality and contents of three Ca ii K spectroheliogram time series, specifically those obtained by the digitization of the Arcetri, Kodaikanal, and Mt Wilson photographic archives, in order to estimate their value for studies focusing on timescales longer than the solar cycle. We analyze the quality of these data and compare the results obtained with those achieved for similar present-day observations taken with the Meudon spectroheliograph and with the Rome-PSPT. We also investigate whether image-segmentation techniques, such as those developed for identification of plage regions on present-day Ca ii K observations, can be used to process historic series. We show that historic data suffer from stronger geometrical distortions and photometric uncertainties than similar presentday observations. The latter uncertainties mostly originate from the photographic calibration of the original data and from stray-light effects. We also show that the image contents of the three analyzed series vary in time. These variations are probably due to instrument changes and aging of the spectrographs used, as well as changes of the observing programs. The segmentation technique tested in this study gives reasonably consistent results for the three analyzed series after application of a simple photographic calibration. Although the plage areas measured from the three analyzed series differ somewhat, the difference to previously published results is larger.

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Contribution of chromospheric features to UV irradiance variability from spatially-resolved Ca II K spectroheliograms

Cited by 25 publications

References 12 publications

Analysis of full disc Ca II K spectroheliograms

Analysis of full disc Ca II K spectroheliograms

Segmentation of coronal features to understand the solar EUV and UV irradiance variability

COMPARISON AMONG Ca II K SPECTROHELIOGRAM TIME SERIES WITH AN APPLICATION TO SOLAR ACTIVITY STUDIES

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