The Evolution of Sunspots

Abstract

The evolution of sunspots is described in the context of the evolution of active regions. The nesting tendency in formation of sunspots is pointed out. Birth and growth of an active region are attributed to an Ω-shaped loop emerging from a deep-seated toroidal flux strand. It is argued that the field strength in the toroidal strand exceeds the equipartition value; the top of the Ω-loop, however frayed, reaches the upper boundary of the convection zone at intrinsic field strengths close to the local equipartition value. The formation of pores is preceded by the convective collapse of flux tubes. The coalescence of the buoyant tops of flux tubes leads to the growth of large sunspots. Inclusions of bright structure in umbrae, and the formation of dark cores are discussed. Penumbrae form after the spot has reached a critical flux level. Most sunspots decay fast by fragmentation. Spots decaying gradually are surrounded by moat cells, across which magnetic features of both polarities are streaming away from the spot; the decay rate appears to be approximately constant. The removal of magnetic flux from the atmosphere, including the seeming disappearance in situ, is addressed. Finally, the complementary roles of Ω-loops and U-loops in the magnetic structure in convection zone and atmosphere are discussed.