Subject will be restored when possible 

Karel Schrijver Submitted: 20071203 09:28
We compare a variety of nonlinear forcefree field (NLFFF)
extrapolation algorithms, including optimization, magnetofrictional,
and GradRubinlike codes, applied to a solarlike reference model.
The model used to test the algorithms includes realistic photospheric
Lorentz forces and a complex field including a weakly twisted, right
helical flux bundle. The codes were applied to both forced
``photospheric'' and more forcefree ``chromospheric'' vector magnetic
field boundary data derived from the model. When applied to the
chromospheric boundary data, the codes are able to recover the
presence of the flux bundle and the field's free energy, though some
details of the field connectivity are lost. When the codes are
applied to the forced photospheric boundary data, the reference model
field is not well recovered, indicating that the combination of
Lorentz forces and small spatial scale structure at the photosphere
severely impact the extrapolation of the field. Preprocessing of the
forced photospheric boundary does improve the extrapolations
considerably for the layers above the chromosphere, but the
extrapolations are sensitive to the details of the numerical codes and
neither the field connectivity nor the free magnetic energy in the
full volume are well recovered. The magnetic virial theorem gives a
rapid measure of the total magnetic energy without extrapolation,
though, like the NLFFF codes, it is sensitive to the Lorentz forces in
the coronal volume. Both the magnetic virial theorem and the
Wiegelmann extrapolation, when applied to the preprocessed
photospheric boundary, give a magnetic energy which is nearly
equivalent to the value derived from the chromospheric boundary, but
both underestimate the free energy above the photosphere by at least a
factor of two. We discuss the interpretation of the preprocessed
field in this context. When applying the NLFFF codes to solar data,
the problems associated with Lorentz forces present in the low solar
atmosphere must be recognized: the various codes will not necessarily
converge to the correct, or even the same, solution.
Authors: T.R. Metcalf, M.L. DeRosa, C.J. Schrijver, G. Barnes, A.A. van Ballegooijen, T. Wiegelmann, M.S. Wheatland, G. Valori, and J.M. McTiernan
Projects: None

Publication Status: Solar Physics, in press.
Last Modified: 20071203 14:51


