Sunspots, and in particular their umbra (the darker inner part of a sunspot), can be populated by bright substructures of different shapes and sizes that evolve in time. These substructures are caused by convection in the presence of magnetic fields. A post written by Dr. Ada Ortiz, from Expert Analytics (Oslo, Norway).
Sunspot magnetic fields are strong enough to suppress the convective motions that fill the surface of the Sun (a.k.a. the photosphere). We are talking big here, as the magnetic fields measured in sunspots have values around 2000 Gauss (for comparison, the Earth magnetic field has a strength of 0.5 Gauss). Therefore, one can get the idea that a sunspot is nothing but a dark structure filled with a huge magnetic field. Sort of moles in the Sun’s face.
But is this totally true? Well, no. Sunspots, and in particular their umbra (the darker inner part of a sunspot), can be populated by bright substructures of different shapes and sizes that evolve in time. These substructures are caused by convection in the presence of magnetic fields. In other words, they are bubbles of hot material that rise and sink due to heating and cooling processes within the sunspot umbra.
Umbral dots (which look like coffee beans, due to their shape) and light bridges are among the convective substructures that have been observed in sunspots. They host regular convective motions, with upflows occurring in the central part and downflows at the edges. Theoretical models had predicted the existence of such motions, but an observational confirmation was missing.
Measurements taken at the Swedish Solar Telescope (SST) on La Palma (Spain) proved that umbral dots and light bridges are more complex that previously thought, and that some even present a dark lane traversing them (an example of a substructure inside another substructure; you know, nature is quite complex). Such dark lanes are clearly seen in the accompanying figure.
These observations discovered for example the presence of downflows at the edges of umbral dots. This finding was possible thanks to the high spatial resolution of the SST. In addition, it was realised that umbral dots exhibit fields 500 Gauss weaker than the surrounding umbra and that the dark lanes have even weaker fields. The big challenge when observing these features is that they are really small, around 300 km in diameter. Their very small size and rapid evolution also complicates the determination of their velocities and magnetic fields. This is an example of astrophysics at the edge of what is observable with the present observational capabilities.
The 4-meter primary mirror of the European Solar Telescope will allow us to observe details of the surface of the Sun we have never seen before. It will also allow us to make the most precise measurements of magnetic fields and velocity fields ever, and study their temporal evolution. Then, small substructures like umbral dots and light bridges will not remain at the edge of what is observable any more. Bigger is sometimes better!
For more information see Ortiz et al., 2010, ApJ, 713, 1282 and Rouppe van der Voort et al., 2010, ApJ, 718, L78.