Modeling the three-dimensional structure and dynamics of the corona of the Sun and stars
Final Report Abstract
Cool stars like the Sun are surrounded by a hot outer atmosphere, the corona. This corona is about a hundred times hotter than the solar surface, and the processes leading to these high temperatures are enigmatic, still. In this project an analyses of three-dimensional magnetohydrodynamics (MHD) models of the outer solar atmosphere was conducted. In these models the heating mechanism is based on the braiding of magnetic field lines by plasma motions on the solar surface. The subsequent induction of currents in the corona and their dissipation results in a multitude of small-scale heating events that eventually provide the energy to sustain the hot corona To compare the models to real observations, spectra of optically thin emission lines in the extreme ultraviolet have been synthesized from the simulation data. Average properties of these spectra, for example the Doppler shifts, were found to be comparable to those of observed spectra. To understand the physical cause of the net Doppler shifts, we related the density and flow structures in regions of interest to the synthesized spectra. From this we found (at least) two processes that can produce the net Doppler shifts. In some places of the simulation box we find that at reconnection sites in the corona magnetic field lines of a coronal loop are connected to different places on the surface, depending on the driving at the surface. Being connected to upflow or downflow regions, the particular loop is either filled with plasma or draining, resulting in blue- or redshifts. Besides these changes of the Doppler shifts as a function of time, we also see that plasma is fed into the corona through “cold fingers” that reach high up into the corona. The plasma is then heated and eventually will fall back to the Sun after cooling again. Because the plasma is still cool and filling only a small faction of the space when feeding the corona, these upflows are not visible in (average) spectra, and only the redshifts as an indication of the down-falling plasma are seen. As a surprise, we found a strong ejection of cool material into the corona in one simulation run. While at first sight one might identify some sort of plasmoid ejection due to magnetic reconnection as the underlying process, this is not the case here. Prior to the rise of the ejection, an enhanced heating causes a strong rise of the pressure that eventually leads to the onset of the eruption, similar to an explosion. So in fact, this is basically a hydrodynamic phenomenon with the ejection being guided by the magnetic field. It is only the heating process at the beginning which is directly related to the magnetic field. This raises the question if other (small-scale) ejection phenomena seen on the Sun could also be related to the phenomenon we found, instead of being a plasmoid-type phenomenon as usually assumed.
Publications
- (2009). Advances in Space Research, 43(9), 1451. Spectral analysis of 3D MHD models of coronal structures
Zacharias P., Bingert S., Peter H.
- (2009). In: Memorie della Società Astronomica Italiana (Mem. S.A.It.), Vol. 80, p. 654. Doppler shifts in the transition region and corona
Zacharias P., Bingert S., Peter H.
- (2010), Dissertation. Spectral Analysis of 3D MHD Models of the Solar Corona. Kiepenheuer-Institut & Universität Freiburg
Zacharias, Pia
- (2011). Astronomy & Astrophysics: 531, A97. Investigation of mass flows in the transition region and corona in a three-dimensional numerical model approach
Zacharias P., Bingert S., Peter H.
- (2011). Astronomy & Astrophysics: 532, A112. Ejection of cool plasma into the hot corona
Zacharias P., Bingert S., Peter H.
