Evolution of coronal magnetic fields

• Antragsteller: Dr. Thomas Wiegelmann
• Fachliche Zuordnung: Astrophysik und Astronomie
• Förderung: Förderung von 2006 bis 2010
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 26559079

• Erstellungsjahr: 2010

Zusammenfassung der Projektergebnisse

We were able to provide a substantial contribution to enlarge the knowledge about the structure of the coronal magnetic field. In particular, we were concentrating on the study of solar active regions. With the help of an optimization method, vector magnetograms measured at photospheric levels were extrapolated into the solar corona. The potential field configuration as well as the nonlinear force-free one were calculated for several active regions on the Sun. We were able to show that the consistency of the used boundary conditions (photospheric vector magnetograms) is influencing the accuracy of the reconstructions of coronal magnetic fields. For the first time, also chromospheric magnetic field information was incorporated to gain consistent boundary conditions for the nonlinear force-free extrapolations. From the potential and nonlinear force-free equilibria, we estimated the amount of magnetic energy stored in the coronal volumes. In particular, the slow build-up of magnetic energy before and subsequent release during and after simultaneously observed solar eruptions was confirmed. Furthermore, we showed that a substantial part of the available free magnetic energy (that is the excess energy of a nonlinear force-free field over that of a potential field) in the solar atmosphere is transformed into other forms of energy during these explosions. We also found that less violent explosions show a similar time evolution as strong ones, whereas the amount of released energy is one order of magnitude less. It was found that, in any case, not all of the available free magnetic energy was released during the investigated eruptions so that, in other words, the coronal magnetic field was not found to return (by relaxation) to a state of minimum energy (a potential field). By calculating the magnetic energy density as a function of the height in the solar corona it was clearly found that the locations of the major energy movements are restricted to low heights above the solar surface. Furthermore, a method was developed to estimate the magnetic helicity, based on the information about the three-dimensional magnetic field configuration within the solar corona. For this purpose, the necessary, iterative calculation of the associated magnetic vector potential was implemented and tested with the help of a realistic, force-free coronal magnetic field model. Consequently, the base for the successful application to real solar SDO-HMI data was established which enables future investigations of the role of the magnetic helicity and current helicity for the stability or probability for an instability/eruption to occur in the solar corona. In the press release “Tagebuch einer Sonneneruption” on August 11, 2008 the ability of calculating the solar magnetic field and its potential of forecasting solar eruptions was presented to a broad audience (http://www.mps.mpg.de/de/aktuelles/pressenotizen/pressenotiz_20080811.html). In the magazine of the Stadtwerke Passau, an article entitled “Die Sonne brennt” was published, describing the impact of the solar activity onto the Earth (http://www.swp-passau. de/pdf/tana4_2008/tana4_2008_daemmung.pdf).

Projektbezogene Publikationen (Auswahl)

• 2008. Can we improve the preprocessing of photospheric vector magnetograms by the inclusion of chromospheric observations?, Solar Phys., 247, 249-267
  Wiegelmann, T., Thalmann, J. K., Schrijver, C. J., Derosa, M. L. and Metcalf, T. R.
  
• 2008. Evolution of the flaring active region NOAA 10540 as a sequence of nonlinear force-free field extrapolations, A&A, 484, 495-502
  Thalmann, J. K. and Wiegelmann, T.
  
• 2008. First nonlinear forcefree field extrapolations of SOLIS/VSM data, A&A, 488, L71-L74
  Thalmann, J. K., Wiegelmann, T. and Raouafi, N.-E.
  
• 2008. Nonlinear force-free modeling of the solar coronal magnetic field, J. Geophys. Res., 113, A03S02
  Wiegelmann, T.
  
• 2008. Nonlinear forcefree field modeling of a solar active region around the time of a major flare and coronal mass ejection, APJ, 675, 1637-1644
  Schrijver, C. J., DeRosa, M. L., Metcalf, T., Barnes, G., Lites, B., Tarbell, T., McTiernan, J., Valori, G., Wiegelmann, T., Wheatland, M. S., Amari, T., Aulanier, G., Démoulin, P., Fuhrmann, M., Kusano, K., Régnier, S. and Thalmann, J. K.
  
• 2009. A critical assessment of nonlinear force-free field modeling of the solar corona for active region 10953, Astrophys. J., 696, 1780-1791
  DeRosa, M. L., Schrijver, C. J., Barnes, G., Leka, K. D., Lites, B. W., Aschwanden, M. J., Amari, T., Canou, A., McTiernan, J. M., Régnier, S., Thalmann, J. K., Valori, G., Wheatland, M. S., Wiegelmann, T., Cheung, M. C. M., Conlon, P. A., Fuhrmann, M., Inhester, B., Tadesse, T.
  
• 2010. Evolution of coronal magnetic fields, Ph.D.thesis, Berlin: uni-edition GmbH, ISBN: 978-3-942171-41-0
  Thalmann, J. K.