2.1 Sunspot time scales

Sunspots are long-lived (typically weeks) relative to a dynamical time scale of about one hour; one hour corresponds to the travel time of a magneto-acoustic wave across a sunspot in the photosphere. Even though sunspots are globally stable, high resolution observations readily demonstrate that they are not static on small scales. Down to the present-day resolution limit of some 0.1 arcsec, corresponding to 70 km on the solar surface, sunspots have dynamically evolving features that are known as umbral dots, light bridges, bright and dark penumbral filaments, penumbral grains, dark-cored bright filaments, and penumbral twists, just to mention the most prominent structures. Hence, a sunspot is a coherent phenomenon on large spatial and temporal scales. However, it seems crucial to realize that a sunspot is not static, but finds a dynamical equilibrium: A variety of small-scale features evolve on a dynamic time scale to produce a large scale coherent structure on long time scales.

In other words, the dynamic fine structure forms a globally stable sunspot and it is the goal of sunspot physics to understand how an ensemble of short-lived features with small scales is organized to form a coherent large and long-living sunspot. This “fine structure” is complex and is seen in white light images. This fine structure must be associated with magneto-convective motions on the small scales, and it is unquestioned that the small scale motions are the key to understand the energy transport within and the structure of sunspots. But it is a challenge to understand how the small-scale features form a stable sunspot.

  Go to previous page Go up Go to next page