Evolution of coronal magnetic fields
Final Report Abstract
We were able to enlarge the knowledge about the structure of the solar active-region corona. We further developed and tested a well-established method to reconstruct the nonlinear force-free coronal field from vector magnetic field data measured at photospheric levels. We were able to incorporate the handling of erroneous and/or missing boundary data in our reconstruction method and showed its positive effect on the model outcome. We also quantified the influence of changing boundary conditions (in the form of simultaneous data from different instruments) on the magnetic field connectivity physical quantities derived from the models. We were able to show that the magnetic field connectivity may be accurately determined from the models to within some megameters. Importantly, we were able to demonstrate that relative measures of the available free magnetic energy (in % of the total coronal magnetic energy content) may be actually very robust and relatively insensitive with respect to changing boundary conditions. Furthermore, steps have been undertaken to investigate the influence of reduced spatial resolution and limited field-of-views on the model outcome. From coronal force-free equilibria of unprecedented high-time cadence over a several-day period, we estimated the stored magnetic energy in the coronal volume above an active region. The fast build-up of strong magnetic energy and electric current concentrations before a large flare (due to the fast emergence of a sheared magnetic field configuration) and the subsequent release was analyzed in detail. We confirmed what has been suggested by earlier studies of lower temporal cadence and spatial resolution, namely that a substantial part of the available free magnetic energy in the solar atmosphere is stored and released at relatively low height in the solar corona, likely below some ten megameters above photospheric levels. Additionally, we were able to relate flare-associated changes of the model magnetic fields to a specific reconnection scenario and pictured a possible scenario for the associated footpoint exchange of involved fields. Moreover, we were able to find, in line with previous studies, further support for a coronal implosion due to the rapid release of magnetic energy and the consequent reduction of magnetic pressure in the active region. Another substantial part of our work was to demonstrate the important contribution of small-scale braided and twisted structures (hosting small flare events) for the release of free magnetic energy in order to heat the ambient plasma to the observed high coronal temperatures. We were able to show that a wast reservoir of free energy is available in the corona at a time and only a tiny fraction of it is actually liberated to contribute to coronal heating. And also twisted magnetic field configurations were shown to be of high importance for smaller-scale flaring processes and we were able to quantify the temporal change of the twist in coronal structures by using the geometrical information of the field lines based on nonlinear force-free reconstructions.
Publications
- 2011. Estimating the Relative Helicity of Coronal Magnetic Fields, Sol. Phys., 272, 243
Thalmann, J. K., Inhester, B. and Wiegelmann, T.
- 2012. Evolution of Magnetic Field and Energy in a Major Eruptive Active Region Based on SDO/HMI Observation, Astrophys. J.,748, 77
Sun, X., Hoeksema, J. T., Liu, Y., Wiegelmann, T.,Hayashi, K., Chen, Q., and Thalmann, J.
(See online at https://doi.org/10.1088/0004-637X/748/2/77) - 2012. How Should One Optimize Nonlinear Force-Free Coronal Magnetic Field Extrapolations from SDO/HMI Vector Magnetograms?, Sol. Phys., 281, 37
Wiegelmann, T., Thalmann, J. K., Inhester, B., Tadesse, T., Sun, X., and Hoeksema, J. T.
(See online at https://doi.org/10.1007/s11207-012-9966-z) - 2012. Nonlinear Force-free Field Modeling of a Solar Active Region Using SDO/HMI and SOLIS/VSM Data, Astronom. J., 144, 33
Thalmann, J. K., Pietarila, A., Sun, X., and Wiegelmann, T.
(See online at https://doi.org/10.1088/0004-6256/144/2/33) - 2013. Comparison of Force-free Coronal Magnetic Field Modeling Using Vector Fields from Hinode and Solar Dynamics Observatory, Astrophys. J., 769, 59
Thalmann, J. K., Tiwari, S. K., and Wiegelmann, T.
(See online at https://doi.org/10.1088/0004-637X/769/1/59) - 2014. Force-free Field Modeling of Twist and Braiding-induced Magnetic Energy in an Active-region Corona, Astrophys. J., 780, 102
Thalmann, J. K., Tiwari, S. K., and Wiegelmann, T.
(See online at https://doi.org/10.1088/0004-637X/780/1/102)
