Technique for Automated Recognition of Sunspots on Full-Disk Solar Images
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Technique for Automated Recognition of Sunspots on Full-Disk Solar Images S. Zharkov Department of Cybernetics, University of Bradford, Bradford, West Yorkshire BD7 1DP, UK Email: [email protected]
V. Zharkova Department of Cybernetics, University of Bradford, Bradford, West Yorkshire BD7 1DP, UK Email: [email protected]
S. Ipson Department of Cybernetics, University of Bradford, Bradford, West Yorkshire BD7 1DP, UK Email: [email protected]
A. Benkhalil Department of Cybernetics, University of Bradford, Bradford, West Yorkshire BD7 1DP, UK Email: [email protected] Received 31 May 2004; Revised 22 February 2005 A new robust technique is presented for automated identification of sunspots on full-disk white-light (WL) solar images obtained from SOHO/MDI instrument and Ca II K1 line images from the Meudon Observatory. Edge-detection methods are applied to find sunspot candidates followed by local thresholding using statistical properties of the region around sunspots. Possible initial oversegmentation of images is remedied with a median filter. The features are smoothed by using morphological closing operations and filled by applying watershed, followed by dilation operator to define regions of interest containing sunspots. A number of physical and geometrical parameters of detected sunspot features are extracted and stored in a relational database along with umbra-penumbra information in the form of pixel run-length data within a bounding rectangle. The detection results reveal very good agreement with the manual synoptic maps and a very high correlation (96%) with those produced manually by NOAA Observatory, USA. Keywords and phrases: digital solar image, sunspots, local threshold, edge-detection, morphological operators, sunspot area time series.
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INTRODUCTION
Sunspot identification and characterisation including location, lifetime, contrast, and so forth, are required for a quantitative study of the solar cycle. Sunspot studies also play an essential part in the modelling of the total solar irradiance during the solar cycle. As a component of solar active regions, sunspots and their behaviour are also used in the study of active region evolution and in the forecast of solar flare activity (Steinegger et al. [1]). This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Manual sunspot catalogues in diļ¬erent formats are produced at various locations all over the world such as the Meudon Observatory, France, the Locarno Solar Observatory, Switzerland, the Mount Wilson Observatory, USA and many others. The Zurich relative sunspot numbers (or since 1981 sunspot index data (SIDC)), compiled from these manual catalogues, are used as a primary indicator of solar activity (Hoyt and Schatten [2, 3] and Temmer et al. [4]). With the substantial increase in the size of solar image data archives, the automated detection and verification
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