The Growth and Decay of Sunspots

• Published in: Monthly notices of the Royal Astronomical Society 1974-10, Vol.169 (1), p.35-57
• Main Authors: Meyer, F., Schmidt, H. U., Weiss, N. O., Wilson, P. R.
• Format: Article
• Language: English
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/169.1.35

The evolution of a sunspot is related to supergranular convection. Magnetic flux is concentrated by converging supergranular flow to form the sunspot. Equilibrium requires a magnetic field strong enough to inhibit convection in the photosphere but small scale convection is still possible at depths greater than 2000 km below the spot. The region surrounding this flux rope is imperfectly cooled. As a result the direction of flow in adjacent super-granules may be reversed. If the flux rope is inclined or split it may be broken up and the spot will decay rapidly; if it is predominantly vertical the reversed flow forms an annular cell (corresponding to the moat) about the spot, which is preserved through a phase of slow decay. The strong field prevents large scale convection in this cell from penetrating into the flux rope beneath the spot but small flux tubes diffuse outwards at a rate which is determined by the modified small scale convection. They are then torn away from the penumbra and carried across the moat, appearing as moving magnetic features in the photosphere. A simple model of this decay process explains the observed linear decay of flux with time and is supported by some numerical experiments.