TY - JOUR A1 - Fuhrmann, Marcel A1 - Seehafer, Norbert A1 - Valori, Gherardo T1 - Preprocessing of solar vector magnetograms for force-free magnetic field extrapolation N2 - Context. Reliable measurements of the solar magnetic field are restricted to the phoptosphere. As an alternative to measurements, the field in the higher layers of the atmosphere is calculated from the measured photospheric field, mostly under the assumption that it is force-free. However, the magnetic field in the photosphere is not force-free. Moreover, most methods for the extrapolation of the photospheric magnetic field into the higher layers prescribe the magnetic vector on the whole boundary of the considered volume, which overdetermines the force-free field. Finally, the extrapolation methods are very sensitive to small-scale noise in the magnetograph data, which, however, if sufficienly resolved numerically, should affect the solution only in a thin boundary layer close to the photosphere. Aims. A new method for the preprocessing of solar photospheric vector magnetograms has been developed that, by improving their compatibility with the condition of force- freeness and removing small-scale noise, makes them more suitable for extrapolations into three- dimensional nonlinear force-free magnetic fields in the chromosphere and corona. Methods. A functional of the photospheric field values is minimized whereby the total magnetic force and the total magnetic torque on the considered volume above the photosphere, as well as a quantity measuring the degree of small-scale noise in the photospheric boundary data, are simultaneously made small. For the minimization, the method of simulated annealing is used and the smoothing of noisy magnetograph data is attained by windowed median averaging. Results. The method was applied to a magnetogram derived from a known nonlinear force-free test field to which an artificial noise had been added. The algorithm recovered all main structures of the magnetogram and removed small- scale noise. The main test was to extrapolate from the noisy photospheric vector magnetogram before and after the preprocessing. The preprocessing was found to significantly improve the agreement of the extrapolated with the exact field. Y1 - 2007 UR - http://www.aanda.org/ U6 - https://doi.org/10.1051/0004-6361:20078454 SN - 0004-6361 ER - TY - JOUR A1 - Fuhrmann, Marcel A1 - Seehafer, Norbert A1 - Valori, Gherardo A1 - Wiegelmann, Thomas T1 - A comparison of preprocessing methods for solar force-free magnetic field extrapolation Y1 - 2011 UR - http://www.aanda.org/index.php?option=com_article&access=standard&Itemid=129&url=/articles/aa/full_html/ 2011/02/aa15453-10/aa15453-10.html SN - 0004-6361 ER - TY - JOUR A1 - Fuhrmann, Marcel A1 - Seehafer, Norbert A1 - Valori, Gherardo A1 - Wiegelmann, T. T1 - A comparison of preprocessing methods for solar force-free magnetic field extrapolation JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Extrapolations of solar photospheric vector magnetograms into three-dimensional magnetic fields in the chromosphere and corona are usually done under the assumption that the fields are force-free. This condition is violated in the photosphere itself and a thin layer in the lower atmosphere above. The field calculations can be improved by preprocessing the photospheric magnetograms. The intention here is to remove a non-force-free component from the data. Aims. We compare two preprocessing methods presently in use, namely the methods of Wiegelmann et al. (2006, Sol. Phys., 233, 215) and Fuhrmann et al. (2007, A&A, 476, 349). Methods. The two preprocessing methods were applied to a vector magnetogram of the recently observed active region NOAA AR 10 953. We examine the changes in the magnetogram effected by the two preprocessing algorithms. Furthermore, the original magnetogram and the two preprocessed magnetograms were each used as input data for nonlinear force-free field extrapolations by means of two different methods, and we analyze the resulting fields. Results. Both preprocessing methods managed to significantly decrease the magnetic forces and magnetic torques that act through the magnetogram area and that can cause incompatibilities with the assumption of force-freeness in the solution domain. The force and torque decrease is stronger for the Fuhrmann et al. method. Both methods also reduced the amount of small-scale irregularities in the observed photospheric field, which can sharply worsen the quality of the solutions. For the chosen parameter set, the Wiegelmann et al. method led to greater changes in strong-field areas, leaving weak-field areas mostly unchanged, and thus providing an approximation of the magnetic field vector in the chromosphere, while the Fuhrmann et al. method weakly changed the whole magnetogram, thereby better preserving patterns present in the original magnetogram. Both preprocessing methods raised the magnetic energy content of the extrapolated fields to values above the minimum energy, corresponding to the potential field. Also, the fields calculated from the preprocessed magnetograms fulfill the solenoidal condition better than those calculated without preprocessing. KW - Sun: magnetic topology KW - Sun: atmosphere KW - magnetohydrodynamics (MHD) Y1 - 2011 U6 - https://doi.org/10.1051/0004-6361/201015453 SN - 0004-6361 VL - 526 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - DeRosa, Marc L. A1 - Schrijver, Carolus J. A1 - Barnes, Graham A1 - Leka, K. D. A1 - Lites, Bruce W. A1 - Aschwanden, Markus J. A1 - Amari, Tahar A1 - Canou, Aurélien A1 - McTiernan, James M. A1 - Régnier, Stéphane A1 - Thalmann, Julia K. A1 - Valori, Gherardo A1 - Wheatland, Michael S. A1 - Wiegelmann, Thomas A1 - Cheung, Mark C. M. A1 - Conlon, Paul A. A1 - Fuhrmann, Marcel A1 - Inhester, Bernd A1 - Tadesse, Tilaye T1 - A critical assessment of nonlinear force-free field modeling of the solar corona for active region 10953 N2 - Nonlinear force-free field (NLFFF) models are thought to be viable tools for investigating the structure, dynamics, and evolution of the coronae of solar active regions. In a series of NLFFF modeling studies, we have found that NLFFF models are successful in application to analytic test cases, and relatively successful when applied to numerically constructed Sun-like test cases, but they are less successful in application to real solar data. Different NLFFF models have been found to have markedly different field line configurations and to provide widely varying estimates of the magnetic free energy in the coronal volume, when applied to solar data. NLFFF models require consistent, force-free vector magnetic boundary data. However, vector magnetogram observations sampling the photosphere, which is dynamic and contains significant Lorentz and buoyancy forces, do not satisfy this requirement, thus creating several major problems for force-free coronal modeling efforts. In this paper, we discuss NLFFF modeling of NOAA Active Region 10953 using Hinode/SOT-SP, Hinode/XRT, STEREO/SECCHI-EUVI, and SOHO/MDI observations, and in the process illustrate three such issues we judge to be critical to the success of NLFFF modeling: (1) vector magnetic field data covering larger areas are needed so that more electric currents associated with the full active regions of interest are measured, (2) the modeling algorithms need a way to accommodate the various uncertainties in the boundary data, and (3) a more realistic physical model is needed to approximate the photosphere-to-corona interface in order to better transform the forced photospheric magnetograms into adequate approximations of nearly force-free fields at the base of the corona. We make recommendations for future modeling efforts to overcome these as yet unsolved problems. Y1 - 2009 UR - http://iopscience.iop.org/0004-637X/ U6 - https://doi.org/10.1088/0004-637x/696/2/1780 SN - 0004-637X ER - TY - THES A1 - Fuhrmann, Marcel T1 - Coronal magnetic field extrapolation of photospheric measurements Y1 - 2009 CY - Potsdam ER - TY - BOOK A1 - Fuhrmann, Michaela A1 - Schubarth, Wilfried A1 - Schulze-Reichelt, Friederike A1 - Mauermeister, Sylvi A1 - Seidel, Andreas A1 - Hartmann, Nina A1 - Erdmann, Melinda A1 - Apostolow, Benjamin A1 - Wagner, Laura A1 - Berndt, Sarah A1 - Wippermann, Melanie A1 - Ratzlaff, Olaf A1 - Lumpe, Matthias A1 - Kirjuchina, Ljuba A1 - Rost, Sophia A1 - Zurek, Peter Paul A1 - Faaß, Marcel A1 - Schellhorn, Sebastian A1 - Frank, Mario A1 - Kreitz, Christoph A1 - Wagner, Nelli A1 - Jenneck, Julia A1 - Kleemann, Katrin A1 - Vock, Miriam A1 - Schröder, Christian A1 - Erdmann, Kathrin A1 - Koziol, Matthias A1 - Meißner, Marlen A1 - Dibiasi, Anna A1 - Unger, Martin A1 - Piskunova, Elena V. A1 - Bahmutskiy, Andrey E. A1 - Bessonova, Ekatarina A. A1 - Borovik, Ludmila K. ED - Schubarth, Wilfried ED - Mauermeister, Sylvi ED - Schulze-Reichelt, Friederike ED - Seidel, Andreas T1 - Alles auf Anfang! BT - Befunde und Perspektiven zum Studieneingang T3 - Potsdamer Beiträge zur Hochschulforschung N2 - Im Zuge der Bologna-Reform ist an Hochschulen vieles in Bewegung gekommen. Studium und Lehre sind stärker ins Blickfeld gerückt. Dabei kommt der Studieneingangsphase besondere Bedeutung zu, werden doch hier die Weichen für ein erfolgreiches Studium gestellt. Deshalb ist es verständlich, dass die Hauptanstrengungen der Hochschulen auf den Studieneingang gerichtet sind – ganz nach dem Motto: „Auf den Anfang kommt es an!“. Konsens herrscht dahingehend, dass der Studieneingang neu zu gestalten ist, doch beim „Wie?“ gibt es unterschiedliche Antworten. Zugleich wird immer deutlicher, dass eine wirksame Neugestaltung der Eingangsphase nur mit einer umfassenden Reform des Studiums gelingen kann. Ziel des vierten Bandes der Potsdamer Beiträge zur Hochschulforschung ist es, eine Zwischenbilanz der Debatte zum Studieneingang zu ziehen. Auf der Basis empirischer Studien werden unterschiedliche Perspektiven auf den Studieneingang eingenommen und Empfehlungen zur Optimierung des Studieneingangs abgeleitet. Die zahlreichen Untersuchungsergebnisse Potsdamer Forschergruppen werden durch weitere nationale sowie internationale Perspektiven ergänzt. Der Band richtet sich an alle, die sich für die Entwicklung an Hochschulen interessieren. T3 - Potsdamer Beiträge zur Hochschulforschung - 4 Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-422965 SN - 978-3-86956-452-4 SN - 2192-1075 SN - 2192-1083 IS - 4 PB - Universitätsverlag Potsdam CY - Potsdam ER - TY - BOOK A1 - Schubarth, Wilfried A1 - Seidel, Andreas A1 - Mauermeister, Sylvi A1 - Fuhrmann, Michaela A1 - Faaß, Marcel A1 - Niproschke, Saskia A1 - Zylla, Birgitta A1 - Ulbricht, Juliane A1 - Schulze-Reichelt, Friederike A1 - Köhler, Anke A1 - Erdmann, Melinda A1 - Ratzlaff, Olaf A1 - Kottmann, Andrea A1 - Unger, Martin A1 - Dibiasi, Anna A1 - Grzywacz, Małgorzata A1 - Miłkowska, Grażyna A1 - Piorunek, Magdalena A1 - Sałaciński, Lech A1 - Grecmanová, Helena A1 - Dopita, Miroslav A1 - Kantorová, Jana A1 - Wippermann, Melanie A1 - Skopalová, Jitka A1 - V’junova, Natalja Ivanovna A1 - Ivanova, Olga Anatol’evna A1 - Apostolow, Benjamin ED - Schubarth, Wilfried ED - Mauermeister, Sylvi ED - Seidel, Andreas T1 - Studium nach Bologna BT - Befunde und Positionen T3 - Potsdamer Beiträge zur Hochschulforschung N2 - Ziel des vorliegenden dritten Bandes der Potsdamer Beiträge zur Hochschulforschung ist es, ausgewählte Aspekte der Hochschuldebatte um Studium und Lehre zu beleuchten und mit empirischen Befunden zu vertiefen. Im ­Fokus stehen solche aktuellen Debatten wie die Gestaltung des Studieneingangs, die Erhöhung der Beschäftigungsbefähigung, die Qualität der Praktika sowie Probleme der Lehrerbildung. Dabei wird die Hochschuldebatte in Deutschland durch einschlägige Beiträge aus anderen, west- und osteuropäischen Ländern erweitert. Die Reihe versteht sich als Forum verschiedener Akteure aus der Hochschulforschung, die die Diskussion zur Qualitätsentwicklung in Lehre und Studium mit ihren Impulsen aus Analysen und empirischen Ergebnissen bereichern sollen. Der Band richtet sich an alle, die sich für die Entwicklung an Hochschulen interessieren. T3 - Potsdamer Beiträge zur Hochschulforschung - 3 Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-103998 SN - 978-3-86956-399-2 SN - 2192-1075 SN - 2192-1083 IS - 3 PB - Universitätsverlag Potsdam CY - Potsdam ER - TY - JOUR A1 - Valori, Gherardo A1 - Green, Lucie M. A1 - Demoulin, Pascal A1 - Vargas Dominguez, S. A1 - van Driel-Gesztelyi, L. A1 - Wallace, A. A1 - Baker, Daniel N. A1 - Fuhrmann, Marcel T1 - Nonlinear force-free extrapolation of emerging flux with a global twist and serpentine fine structures JF - Solar physics : a journal for solar and solar-stellar research and the study of solar terrestrial physics N2 - We study the flux emergence process in NOAA active region 11024, between 29 June and 7 July 2009, by means of multi-wavelength observations and nonlinear force-free extrapolation. The main aim is to extend previous investigations by combining, as much as possible, high spatial resolution observations to test our present understanding of small-scale (undulatory) flux emergence, whilst putting these small-scale events in the context of the global evolution of the active region. The combination of these techniques allows us to follow the whole process, from the first appearance of the bipolar axial field on the east limb, until the buoyancy instability could set in and raise the main body of the twisted flux tube through the photosphere, forming magnetic tongues and signatures of serpentine field, until the simplification of the magnetic structure into a main bipole by the time the active region reaches the west limb. At the crucial time of the main emergence phase high spatial resolution spectropolarimetric measurements of the photospheric field are employed to reconstruct the three-dimensional structure of the nonlinear force-free coronal field, which is then used to test the current understanding of flux emergence processes. In particular, knowledge of the coronal connectivity confirms the identity of the magnetic tongues as seen in their photospheric signatures, and it exemplifies how the twisted flux, which is emerging on small scales in the form of a sea-serpent, is subsequently rearranged by reconnection into the large-scale field of the active region. In this way, the multi-wavelength observations combined with a nonlinear force-free extrapolation provide a coherent picture of the emergence process of small-scale magnetic bipoles, which subsequently reconnect to form a large-scale structure in the corona. KW - Active regions, magnetic fields KW - Magnetic field, photosphere, corona Y1 - 2012 U6 - https://doi.org/10.1007/s11207-011-9865-8 SN - 0038-0938 VL - 278 IS - 1 SP - 73 EP - 97 PB - Springer CY - Dordrecht ER -