Fine Structure of Penumbrae: Formation and Dynamics

Abstract

In this chapter we shall study amazing properties of sunspot penumbra —a beautiful “crown” surrounding mature sunspots and being one of the long-standing mysteries of solar magnetism. High-resolution observations with the Swedish 1-m Solar Telescope (SST) on La Palma revealed earlier unavailable features of penumbral filaments showing their fine substructure and new properties in their dynamics (Scharmer et al., Nature 420:151, 2002). We shall see that these properties are associated with the nature of umbra which itself is a dense conglomerate of twisted flux tubes. Being more or less vertical in the center of sunspots, twisted and interlaced flux tubes branch out at the periphery from the “trunk” due to an ongoing reconnection processes, and arc downward to the photosphere forming the penumbral “umbrella.” The twist of individual filaments, and resulted distribution of magnetic fields and temperature inside them are consistent with the onset of screw pinch instability, which in large explains the properties and behavior of penumbral filaments.