J/A+A/625/A69 Plage area composite series (Chatzistergos+, 2019)
Analysis of full disc Ca II K spectroheliograms.
II. Towards an accurate assessment of long-term variations in plage areas.
Chatzistergos T., Ermolli I., Krivova N.A., Solanki S.K.
<Astron. Astrophys. 625, A69 (2019)>
=2019A&A...625A..69C 2019A&A...625A..69C (SIMBAD/NED BibCode)
ADC_Keywords: Solar system ; Sun
Keywords: Sun: activity - Sun: photosphere - Sun: chromosphere -
Sun: faculae - plages
Abstract:
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 use several historical archives of full-disc Ca II K observations
to derive plage areas over more than a century. Employment of different
datasets allows the identification of systematic effects in the images,
such as changes in instruments and procedures, as well as an assessment
of the uncertainties in the results.
We have analysed over 100000 historical images from eight digitised
photographic archives of the Arcetri, Kodaikanal, McMath-Hulbert, Meudon,
Mitaka, Mt Wilson, Schauinsland, and Wendelstein observatories, and
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.
Description:
Annual, monthly, and daily values of the plage area composite series
produced in the paper.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
comp_a.dat 20 126 Plage series, annual values
comp_m.dat 25 1512 Plage series, monthly values
comp_d.dat 30 46872 Plage series, daily values
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See also:
VI/138 : Sunspots catalogues, 1853-1870 (Casas+, 2013)
J/A+A/390/707 : Hemispheric Sunspot Numbers 1975-2000 (Temmer+, 2002)
J/A+A/447/735 : Hemispheric Sunspot Numbers 1945-2004 (Temmer+, 2006)
J/A+A/584/A73 : Sunspot areas and tilt angles (Senthamizh Pavai+, 2015)
J/A+A/595/A104 : Scheiner drawing sunspot areas and tilt angles (Arlt+, 2016)
J/A+A/602/A69 : Group sunspot number series since 1739 (Chatzistergos+, 2017)
Byte-by-byte Description of file (#): comp_a.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 4 I4 yr Year Year of observation
6- 12 F7.3 --- plagedf ?=-99 Projected plage areas in disc fraction (1)
14- 20 F7.3 --- plagemh ?=-99 Corrected for foreshortening fractional
plage areas (1)
--------------------------------------------------------------------------------
Note (1): missing values are denoted with -99.
-------------------------------------------------------------------------------
Byte-by-byte Description of file (#): comp_m.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 4 I4 yr Year Year of observation
8- 9 I2 "month" Month Month of observation
11- 17 F7.3 --- plagedf ?=-99 Projected plage areas in disc fraction (1)
19- 25 F7.3 --- plagemh ?=-99 Corrected for foreshortening fractional
plage areas (1)
--------------------------------------------------------------------------------
Note (1): missing values are denoted with -99.
-------------------------------------------------------------------------------
Byte-by-byte Description of file (#): comp_d.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 4 I4 yr Year Year of observation
8- 9 I2 "month" Month Month of observation
13- 14 I2 d Day Day of observation
16- 22 F7.3 --- plagedf ?=-99 Projected plage areas in disc fraction (1)
24- 30 F7.3 --- plagemh ?=-99 Corrected for foreshortening fractional
plage areas (1)
--------------------------------------------------------------------------------
Note (1): missing values are denoted with -99.
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Acknowledgements:
Theodosios Chatzistergos, chatzistergos(at)mps.mpg.de
References:
Chatzistergos et al., Paper I 2018A&A...609A..92C 2018A&A...609A..92C
(End) Theodosios Chatzistergos [MPS], Patricia Vannier [CDS] 26-Mar-2019