Dynamics of solar and stellar granulation

This is a project jointly proposed by H. Muthsam, Faculty of Mathematics, University of Vienna, and A. Hanslmeier, Institute for Geophysics, Astronomy and Meteorology, University of Graz. The researchers have been active in modelling and observations of solar granulation, respectively. They intend to combine their efforts in order to tackle the phenomenon from the modelling and observing side. Solar granulation is the outermost manifestation of solar convection. It is accessible directly to observations and differs, in this respect, favourably from so many phenomena in the Sun or in stars which are hidden below the surface. It is a laboratory for flows in the stellar environment, and a large numbers of observations have revealed many properties of these flows. Much pain is taken (and money invested) to obtain high resolution observations, for example with continuous upgrade of the solar observatories at the Canarian islands. We can obtain or access such observational material. - On the other hand, we have developed a package for numerical radiation-magneto- hydrodynamics, ANTARES. Together with our visualization and analysis tool VIVAT it is the basis for our modelling work. ANTARES has high-resolution numerics built in, and it is the only software package used in this area which allows for local grid refinement, so that we have reached unprecedented resolution in regions of interest. It will be the purpose of the project to analyse such material and to confront it with observations. In addition to the models we have already, new models with even higher resolution will be calculated during the life span of the project. These calculations will benefit, among others, from being granted resources on European supercomputer centres via DEISA. We plan to investigate, on this modelling basis and observationally, the generation of sound waves, of magnetohydrodynamics waves, p-mode oscillations and other phenomena caused by solar granulation. Such studies are important, among others for an understanding the heating of the solar corona, the tenuous shell surrounding the Sun with a temperature of ~1 million degrees. Besides high resolution runs and observations, we will also concentrate on long term runs. Observations show, for example, that there are granules which live, unlike others, via their descendants for hours. Probably the reason for this must be sought below the solar surface where our runs can be expected to tell us a story. In addition, we plan to make a start to calculate convection models of A-stars. Up to now, these stars pose great difficulties to the modellers to match the shapes of their spectral lines. Such a match would give confidence that the models are correct. Possibly a reason for this difficulty is the lack of resolution, where our grid refinement strategy might conceivably help.

Energy released by nuclear reactions near the center of the Sun (or similar stars) is, in the interior, transported upwards by radiation. Higher up, in a spherical shell (of thickness of one third of the solar radius in the case of the Sun) it is transported outwards by huge convective flow systems. In a very shallow layer, quite at the top of the solar body, it is mainly converted into just the light in which we see the Sun shine. Solar granulation is the outermost and therefore directly observable manifestation of the convective flows inside the Sun. Scales of granules (elements of hot upwelling gas, bounded by downflows of cool gas) are, by solar standards, quite small (~1.000 km). Besides their interest for the structure of the solar (or stellar) atmospheric and subatmospheric layers, granules can also serve as a testbed for flows under astrophysical conditions because they offer unique observability due to the nearness of the Sun.At the universities of Vienna and Graz, groups lead by H. Muthsam and A. Hanslmeier, respectively, work with different, but complementary methodology on problems of granulation. The Vienna group is active in modelling of astrophysical flows (in this case: solar granulation), whereas the group in Graz leans more on the observational side. We constructed models of solar granulation which are the highest resolved ones worldwide. While it was known for many years that some vortex tubes, roughly resembling tornadoes, existed in downflows of solar granulation, our simulations with their resolution demonstrated their true ubiquity: where originally one or a few were seen, hundreds are now discerned and a study of the true turbulent state of the atmosphere can be undertaken. The scale of most of these vortices is much less than what is observable with solar telescopes. So, there is no substitute for computational models when investigating the fine-structure of solar turbulence. Other calculations made direct contact with observations. Software was developed in Graz for automatic detection of granules, be they simulated or observed on the Sun. This allows then an automated study of properties of granules. On the observational side, work was carried out to establish a deepened knowledge of magnetic bright points (MBPs) by making use of the HINODE satellite mission's data. MBPs are small scale magnetic features in the higher solar atmosphere which may play a role for heating the solar corona. The corona is the extended tenuous layer outside of the Sun seen during eclipses of the Sun. It is heated to 1 million degrees by processes still not fully understood. MBPs are also of interest as a tracer of magnetic fields which have, in more recent years, turned out to be present in areas which previously were thought to be essentially field-free and which are actively studied by observational means (observations with satellites and solar telescopes) and theoretical models by a number of research groups worldwide in order to understand those complicated physical structures. Within the project, a number of basic parameters of these physical systems could be derived.