Estimating Active Region Free Energy and Helicity from the Minimum Current Corona Model

Details Estimating Active Region Free Energy and Helicity from the Minimum Current Corona Model Estimating Active Region Free Energy and Helicity from the Minimum Current Corona Model

Aug 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006iaujd...3e..80b&link_type=abstract

Solar Active Regions and 3D Magnetic Structure, 26th meeting of the IAU, Joint Discussion 3, 16-17 August, 2006, Prague, Czech R

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We employ the Minimum Current Corona (MCC) model to estimate the amount of magnetic free energy and helicity injected into the coronal magnetic field of an active region. In the MCC model, each concentration of photospheric magnetic flux is represented by a point source, greatly simplifying the magnetic topology. Advecting an initial partitioning of the flux through a long time series of magnetograms results in a persistent set of sources. We show that the centroid velocity of a partition compares well with the flux-weighted average over the partition of the local correlation tracking velocities. Flux domains, bundles of field lines interconnecting pairs of sources, are surrounded by separatrix surfaces. The intersection of two separatrices is a separator field line, which is the site of reconnection in this model. The evolution of the photospheric field causes the sources to also evolve, which would lead to changes in the domain fluxes to maintain a potential field configuration if reconnection could proceed rapidly. However, in the absence of reconnection, currents begin to flow to maintain the initial distribution of domain fluxes. The minimum energy state occurs when currents flow along the separators. The magnitude of the separator currents can be estimated and combined with geometrical properties of the separators to give a lower bound to the magnetic free energy of the system. The motion of sources about one another adds braiding helicity to the system, while the internal rotation of a partition adds spin helicity. Starting from an initial potential field configuration, changes in the free energy are presented for a time series of data for NOAA AR 8210 on 1 May 1998. This work was supported by AFOSR, NSF and NASA.

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