Fine structure of sunspot umbrae
Final Report Abstract
Sunspots are rather large, objects on the Sun, with horizontal sizes of typically 20,000 km, or more. Above the surface, they are visible over a height range of about 1500 km. Despite these large scales, physical processes, which are relevant for the formation, the evolution, and for the disappearance of spots, occur on very small length scales, down to a few dozen kilometers, at the resolution limit of modern telescopes. On large scales, the main constituents of sunspots are umbrae, penumbrae, and light bridges. All of them also show distinct, and very different, small-scale structures, where magnetic field and material flow interact. To reliably determine the magnetic field configuration of the observed area on the Sun, a proper treatment of systematic error sources is needed. Observational data often suffer from scattered light, which adds information from outside the observed area to the measurements. If not properly treated, the scattered-light contribution can easily be misinterpreted. Within this project, we test a deconvolution technique based on a principal-component analysis and show that is works well for our ground-based data. In one part of this project, we focus on the fine structure of light bridges in the photosphere, and their properties in higher layers of the solar atmosphere. Using data with excellent spatial resolution, we find very short-lived structures, which we interpret as signs of magneto-convective flows. Observations of the vertical extent of light bridges show a rather constant horizontal size even in the Transition Region, i.e., they exist over a large range of temperatures, from 6000 K up to 100,000 K or so. There must be several heating mechanisms operating at different heights. The observations in the higher layers of the solar atmosphere were made with the space-borne Interface Region Imaging Spectrograph. Material flows in and around sunspot penumbrae are the second focus of this project. Our analysis is based both on ground-based observations and numerical simulations. We investigated the flow field of decaying sunspots. We find two distinct flow systems, one driven by the predominantly horizontal Evershed flow in the penumbra, and a second one, just outside the spot, the moat flow, driven by convection, dominates. Moving magnetic features (MMFs) are magnetic flux concentrations that are observed to radially move away from sunspots. They are also related to the decay of sunspots. We compare findings on observed MMFs with the predictions of realistic magneto-hydrodynamical simulations of a sunspot and its surroundings. These simulations allow to study the behavior of the magnetic field below the visible surface. The analysis is still ongoing, with the following intermediate results, based on a few MMFs: While the MMF is moving away from the spot, the magnetic field is still connected to the sunspot penumbra. The motion is driven by the sunspot’s moat flow, and is directed outward at all depths, until the MMF reaches a certain distance to the penumbra, given by the magnetic network surrounding the spot. The magnetic field connecting the MMF to the spot submerges either inside the penumbra and re-emerges again outside, or submerges outside the penumbra and re-emerges again further out. In the first case, the MMF has the same polarity as the sunspot, in the second case, both polarities are visible. In either case, the MMF does not transport net flux away from the spot. The planned comparison between these simulations and the available MMF observations should help to answer the question, to which degree MMS contribute to the decay of sunspots.
Publications
- Active region fine structure observed at 0.08 arcsec resolution. A&A, 596:A7, 2016
R. Schlichenmaier, O. von der Lühe, S. Hoch, D. Soltau, T. Berkefeld, D. Schmidt, W. Schmidt, C. Denker, H. Balthasar, A. Hofmann, K. G. Strassmeier, J. Staude, A. Feller, A. Lagg, S. K. Solanki, M. Collados, M. Sigwarth, R. Volkmer, T. Waldmann, F. Kneer, H. Nicklas, and M. Sobotka
(See online at https://doi.org/10.1051/0004-6361/201628561) - Evolution of the flow field in decaying active regions. Transition from a moat flow to a supergranular flow. A&A, 620:A122, Dec. 2018
H. Strecker and N. Bello González
(See online at https://doi.org/10.1051/0004-6361/201732164) - Magnetic properties of a long-lived sunspot. A&A, 620: A104, 2018
R. Schmassmann, M., Schlichenmaier and N. Bello Gonzalez
(See online at https://doi.org/10.1051/0004-6361/201833441) - Structure of sunspot light bridges in the chromosphere and transition region. A&A, 609:A73, 2018
R. Rezaei
(See online at https://doi.org/10.1051/0004-6361/201629828)
