A Region-Growth Algorithm to Recognize Magnetic Bright Spots in the Solar Photosphere

Magnetic bright spots are the smallest magnetic structures in the solar photosphere. They are located in lanes between solar granules. Their sizes are about 100km to 300km, and their lifetimes range from several seconds to tens of minutes. It is important for solar physics to extensively study magnetic bright spots. For example, magnetic bright spots are considered as tracers of active regions whose flux ropes stretch into the solar corona. Motions of magnetic bright spots may have important impact on the heating of the solar chromosphere and corona. In addition, studies of magnetic bright spots can improve our knowledge about the solar sub-photosphere. Accurate recognitions of magnetic bright spots serve as the basis for all relevant important studies. The region-growth algorithm for recognizing magnetic bright spots is based on the image segmentation technique. The key steps of the algorithm are to select the seeds for the region growth and to define growth rules. In this paper we use certain data observed at the TiO wavelength by the 1m new vacuum solar telescope of the Yunnan Observatories. In applying the algorithm, we extract seeds as certain pixels in the convolution of a data image using a Laplacian mask. The pixels selected as seeds have post-convolution values passing a threshold. Our growth rule is that a pixel is included in a region for a spot if the gray value there passes a threshold. After processing with the algorithm we remove features falsely selected by the algorithm. We also apply the algorithm to some G-band data observed by the Solar Optical Telescope on the Hinode. We compare our results to those of Utz et al. We find that diameters of magnetic bright spots have an average 166.2km, which is consistent with the average given by Utz et al. 166km. This supports the reliability of our recognition approach.