Removal of Cloud Extinction for Hα Full-disk Solar Images

Hα full-disk solar images are important in research of solar science. Observations leading to such images may be affected by weather so that they show appreciable cloud extinction. This paper focuses on an approach to remove cloud extinction in Hα full-disk solar images. The approach can make details of solar activities easily recognizable in result images. We first evaluate and select a template image of Hα solar surface brightness. The template is constructed as an image modeling the Hα solar full disk with an isotropic surface brightness distribution following the direction-averaged Hα solar radial profile of a (cloud-extinction free) real image. We next subtract the template from a raw Hα full-disk solar image affected by cloud extinction, and perform median filtering on the difference image to obtain an image of cloud extinction. We finally subtract the raw image by the image of cloud extinction. The subtraction result is an Hα full-disk solar image with cloud extinction effectively removed. We have conducted some experiments, which validate our approach and show that it has little effect on image details of solar activities. Particularly, our experiments demonstrate that the performance of the approach is stable against patterns of cloud extinction. We also discuss shortcomings of our approach.