Dr. Jan Jurcák, from the Astronomical Institute of the Academy of Sciences of the Czech Republic, writes about granules, convective cells on the surface of the Sun.
Observations of an active region by the Swedish Solar Telescope (La Palma). Data provided by Dr. Michiel van Noort (MPS, Germany).
The solar surface is constantly heated from deeper layers. The heating is provided in the form of bubbles of hot material that rise convectively through the sub-surface layers. These convective cells are called granules and appear as bright dots on the solar surface. Their typical size is around 1000 km.
Granules are sensitive to the presence of magnetic field. Their shapes, sizes and lifetimes are highly dependent on the magnetic field properties. With increasing field strength, the granules become smaller. In the extreme cases of sunspots, the magnetic field is strong enough to suppress convective motions, the solar surface cools down considerably and appears dark compared to its surroundings.
Not only do the granules react to the magnetic field properties. The convective cells are also able to change the magnetic field configuration. They can bend and twist the magnetic field lines, drag them down to sub-surface layers or push them to the higher layers of the solar atmosphere. The European Solar Telescope will allow us to study these processes in greater detail and simultaneously in multiple layers of the solar atmosphere.
The movie shows a two-hour long evolution of an active region, this is, a region where the magnetic field is penetrating the solar surface. The magnetic field is strongest in the darkest regions, but its presence can be recognised also in other areas, where the granules are smaller, brighter, or very elongated, and in the myriad of tiny bright points sitting in between the granules.