Fast and robust segmentation of solar EUV images: algorithm and results for solar cycle 23

Abstract

Context. The study of the variability of the solar corona and the monitoring of coronal holes, quiet sun and active regions are of great importance in astrophysics as well as for space weather and space climate applications. Aims. In a previous work, we presented the spatial possibilistic clustering algorithm (SPoCA). This is a multi-channel unsupervised spatially-constrained fuzzy clustering method that automatically segments solar extreme ultraviolet (EUV) images into regions of interest. The results we reported on SoHO-EIT images taken from February 1997 to May 2005 were consistent with previous knowledge in terms of both areas and intensity estimations. However, they presented some artifacts due to the method itself. Methods. Herein, we propose a new algorithm, based on SPoCA, that removes these artifacts. We focus on two points: the definition of an optimal clustering with respect to the regions of interest, and the accurate definition of the cluster edges. We moreover propose methodological extensions to this method, and we illustrate these extensions with the automatic tracking of active regions. Results. The much improved algorithm can decompose the whole set of EIT solar images over the 23rd solar cycle into regions that can clearly be identified as quiet sun, coronal hole and active region. The variations of the parameters resulting from the segmentation, i.e. the area, mean intensity, and relative contribution to the solar irradiance, are consistent with previous results and thus validate the decomposition. Furthermore, we find indications for a small variation of the mean intensity of each region in correlation with the solar cycle. Conclusions. The method is generic enough to allow the introduction of other channels or data. New applications are now expected, e.g. related to SDO-AIA data.

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

“…The SPoCA-suite is a set of multichannel fuzzy clustering algorithms that automatically segment solar EUV images into a set of features (see Barra et al 2009;Verbeeck et al 2013b for a complete presentation). We have chosen SPoCA because of the maturity and flexibility of the program.…”

“…In this paper we use algorithms belonging to the set of Spatial Possibilistic Clustering Algorithm (SPoCA) initially developed by Barra et al (2008Barra et al ( , 2009 and later improved by Verbeeck et al (2013a,b). The algorithms within the SPoCAsuite are based on fuzzy clustering and allow separating the active regions (ARs), the coronal holes (CHs), and the quiet Sun (QS) in the best way possible.…”

Decay of Activity Complexes, Formation of Unipolar Magnetic Regions, and Coronal Holes in Their Causal Relation

“…After an appropriate smoothing and refinement, the resultant image detects CHs and low-intensity features in solar corona. Similar approaches are also used for automatic tracking of CHs (e.g., Barra et al, 2009;Krista and Gallagher, 2009).…”

Progress in space weather modeling in an operational environment

“…It can extract and track active regions as well as coronal holes. The algorithm was applied on the archive of SOHO-EIT data from 1997 till 2005 to get relative proportion of Active Regions, Coronal Holes, and Quiet Sun covering the Sun during the solar cycle 23 (Barra et al 2009). …”

“…The scientific progress in understanding and modeling space weather phenomena was presented in four topical reviews dealing with: (i) monitoring, modeling, and predicting solar weather ), (ii) the radiation environment of the Earth ), (iii) solar wind disturbances and their interaction with geospace , and (iv) the upper atmosphere's response to space weather events . One of the main recommendations of COST 724 pointed out the development and the online implementation of models for reliable space weather products (Belehaki & Lilensten 2008). In parallel, the European COST 296 action ''Mitigation of Ionospheric Effects on Radio Systems'', continuing the studies of the previous COST 238, COST 251, and COST 271 actions, worked systematically on the development of an increased knowledge of the effects imposed by the ionosphere on practical radio systems and for the development and implementation of techniques to mitigate the deleterious effects of the ionosphere on such systems.…”