Statistical Properties of Superpenumbral Whorls around Sunspots

Abstract

We study properties of 897 superpenumbral fibrils using Hα Big Bear Solar Observatory (BBSO) and photospheric magnetic field National Solar Observatory/Kitt Peak (NSO/KP) data of 139 sunspots between 2000 July and 2001 April. From this low-resolution data, we find that about one-third of all superpenumbral fibrils begin inside the penumbra. The typical length of fibrils is 2.7 times the sunspot white-light penumbral radius. A majority of the fibrils are curved, i.e., 67% of them exhibit bow-extent/footpoint separation greater than 0.1. Both clockwise and counterclockwise fibrils are typically present within the same superpenumbra. We show that the topology of fibrils is clearly affected by distribution of magnetic fields around the sunspot.