Dynamic response of the solar corona to heating driven by photospheric motions
Final Report Abstract
The corona of the Sun contains a plasma at temperatures of a million degrees or more, hundred times hotter than the surface of the Sun. How the corona sustains this hot outer atmosphere is the central question of the coronal heating problem. Observations show that the corona is a highly structured and dynamic environment that is closely related to the magnetic field on the Sun. Most prominent are coronal loops, arced structures that are best seen in extreme UV and X-ray observations. This calls a for three-dimensional magneto-hydrodynamic model (3D MHD) to describe the observed phenomena. In this project we employ a 3D MHD model in which the photospheric motions drive the magnetic field that is anchored in the photosphere, which leads to a braiding of the magnetic field lines. In the upper atmosphere these changes of the magnetic field induce currents that are then dissipated and heat the plasma. This Ohmic heating is, within the approximations of the model, capable to keep the corona at the observed high temperatures and produces plasma densities that are comparable to the observed ones. The central part of this project was to investigate the dynamic nature of the corona in the framework of 3D MHD models, in particular the transient nature of the energy deposition and the response of the coronal emission emerging from the modeled corona to this dynamic heating. We find that the coronal loops forming in the 3D MHD model are heated by the Ohmic dissipation in a non-symmetric transient fashion. While beforehand one-dimensional models basically assumed a distribution of the energy input in time and space, our 3D model provides the energy input to heat the corona as a function of space and time. Following individual magnetic field lines, we can isolate the variability of the heat input and the resulting thermal structure and coronal emission. This shows that at some places the heat input is highly transient, while at other places (and times) the variability is quite smooth. This shows that the discussion if the heating of the corona is transient or steady has a twofold answer, in that both of the two extremes are found — sometimes even with the same loop, one leg being heated transient, the other in a steady fashion. The investigation of the patterns of the synthesized emission showed a peculiar result for emission lines formed in the transition region from the chromopshere to the corona at around 100.000 K. These show loop-like structures that do not follow the magnetic field. This is in contrast to the emission formed at a million K, which forms loops outlining the magnetic field lines. The reason for the cool loop-like structures is to be found in the source region being sort of half shells, like up-turned egg-boxes, and the projection then leads to an apparent looplike structure. This would be a new interpretation of the small network loops on the Sun, and future observations will have to show if this interpretation will hold.
Publications
- (2009), Diplomarbeit. Einfluss photosphärischer Bewegungen auf die Struktur der solaren Korona. Kiepenheuer-Institut & Universität Freiburg
Warnecke, Jörn
- (2009), Doktorarbeit. Heating of the Corona in a 3D MHD Forward Model Approach. Kiepenheuer-Institut & Universität Freiburg
Bingert, Sven
- (2010). Advances in Space Research 45, 310. On the nature of coronal loops above the quiet sun network
Bingert S., Zacharias P., Peter, H., Gudiksen, B.V.
- (2011). Astronomy & Astrophysics 530, A112. Intermittent heating in the solar corona employing a 3D MHD model
Bingert S., Peter H.
