# SIPNet & SAHI: Multiscale Sunspot Extraction for High-Resolution Full Solar Images
## 1 Introduction  
Sunspots, dark regions that appear on the surface of the solar photosphere, are concentrated areas of magnetic fields. They appear as objects with reduced brightness, such as pores without penumbra and sunspots consisting of dark umbra surrounded by lighter penumbra. Some of them are isolated structures and are often found in groups. Hereafter, we will use the term sunspots to denote all objects described above. They cover various sizes over the high-resolution, full-disk, solar continuum images. Sunspots with different scales have been discussed insofar as they correspond to different magnetic field strengths [1; 2] and have different consequences of interactions between magnetic fields and moving plasma [3]. It is well known that small sunspots are more common than large ones [4; 5; 6; 7]. High-resolution observations make it possible to detect multiscale sunspots, especially small sunspots. Figure 1 shows a high-resolution, full-disk continuum image observed by SDO/HMI on 24 October 2014 at 4:00 UT. It can be seen there are sunspots of multiple scales; some representative sunspots are labeled in boxes from a to f. The regions marked as d, e, and f are very small, so they are magnified six times in dashed boxes. The projected area of sunspots in millionths of the solar hemisphere (msh) is used to evaluate their size. Corresponding to an image resolution of 4096 \(\times\) 4096 pixels2, a sunspot occupying 12 pixels has an area of about 1 msh, which corresponds to a circular feature of about 4 pixels in diameter (equivalent diameter) and approximates 0.7 millionths of the image; a sunspot with an area of about 10 msh occupies 116 pixels, corresponding to an equivalent diameter of 12 pixels and 7 millionths of the image, and so on. In Figure 1, the sunspot group in region c has a very large size with over 8000 msh. Both the sunspot marked in region b and the biggest sunspot marked in region f have similar sizes of about200 msh. The sunspot marked in region a and the sunspot marked in the bottom-left corner of region f have similar sizes of about 15 msh. The sunspot marked in region d and the sunspot marked on the left side of region f have a similar size of about 5 msh. The sunspot marked in region e has a size of only about 1 msh. This is how multi-scale sunspots appear in a high-resolution full-disk image. Many automated sunspot detection methods have been proposed in recent years. Traditional image-processing methods for detecting sunspots mainly depend on the intensity of sunspots because sunspots appear to be darker in comparison to their surroundings; these include edge detection [8], watershed [9], morphological operations [9; 10; 11; 12], region growing [13], level-set [14; 15], and so on. Whichever method is used, the intensity threshold is essential because almost all of these methods require thresholds to segment sunspots from the background. Some assistive technologies were adopted to set thresholds, such as the statistical Bayesian method [16], fuzzy-sets [17], and the simulated annealing genetic method [18]. Recently, an adaptive method was adopted to process images under various conditions, while processing multiple images taken within a short time to eliminate the seeing effect [19], and a localized sunspot detection scheme was proposed using the fractional-order derivative mask [20]. The threshold is very critical for sunspot detection, in which a larger threshold could miss some of the pixels that are part of the sunspots while a smaller threshold could increase noise. Besides that, removing solar limb-darkening and smoothing operations are inevitable during image preprocessing, because the solar atmosphere's line-on-sight thickness that changes from the disk center to limb leads to limb-darkening in solar full-disk images. These preprocesses are obviously good at extracting larger and darker sunspots; however, tiny, faint sunspots are generally removed by the above operations, especially threshold, smoothing, and morphological operations.