Coronal Magnetic Field Extrapolation from Photospheric Measurements Applied to an Active Region

Details Coronal Magnetic Field Extrapolation from Photospheric Measurements Applied to an Active Region Coronal Magnetic Field Extrapolation from Photospheric Measurements Applied to an Active Region

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008espm...12.3.37f&link_type=abstract

"12th European Solar Physics Meeting, Freiburg, Germany, held September, 8-12, 2008. Online at http://espm.kis.uni-freiburg.de/,

Physics

Scientific paper

We outline an MHD relaxation method that permits to extrapolate photospheric vector magnetograms into coronal nonlinear force-free fields. The method is applied to a magnetogram taken before an eruptive event in NOAA AR 7792 on 25 October 1994. The event produced a coronal mass ejection (CME) and eruptive flare with a prominent sigmoidal soft X-ray source. Multiwavelength observations as well as theoretical modeling indicate the importance of twisted magnetic configurations in solar active regions(ARs) in the initiation of such events. Manoharan et al. (1996) proposed a model for this event that included the merging of two slightly twisted flux bundles near the polarity inversion line into a flux rope of larger total twist and an overlying flux system much closer to a potential-field state. For the extrapolation we use a vector magnetogram taken at the Mees Solar Observatory 16 hours before the actual event. The magnetic field extrapolation is able to recover main parts of the structures suggested in the model by Manoharan et al. We find the overlying nearly potential flux and part of the sigmoidal field, i.e., one of the suggested weakly twisted flux bundles, in the location observed. This supports the notion that sigmoids are coronal manifestations of twisted magnetic flux tubes which start expanding in eruptive events and may exist even before the onset of such events. We tentatively attribute the incomplete reconstruction of the sigmoidal field structure to the strong evolution of the photospheric field at the suggested location of flux tube merging between the time of the magnetogram and the eruption, as indicated by a magnetogram on the following day.

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