Basilio Ruiz Cobo, Klaus G. Puschmann
The aim of this work is the determination of the twist, writhe, and self
magnetic helicity of penumbral filaments located in an inner Sunspot penumbra.
To this extent, we inverted data taken with the spectropolarimeter (SP) aboard
{\it Hinode} with the SIR (Stokes Inversion based on Response function) code.
For the construction of a 3D geometrical model we applied a genetic algorithm
minimizing the divergence of $\vec{B}$ and the net magnetohydrodynamic force,
consequently a force-free solution would be reached if possible.
We estimated two proxies to the magnetic helicity frequently used in
literature: the force-free parameter $\alpha_z$ and the current helicity term
$h_{c_{z}}$. We show that both proxies are only qualitative indicators of the
local twist as the magnetic field in the area under study significantly
departures from a force-free configuration. The local twist shows significant
values only at the borders of bright penumbral filaments with opposite signs on
each side. These locations are precisely correlated to large electric currents.
The average twist (and writhe) of penumbral structures is very small. The
spines (dark filaments in the background) show a nearly zero writhe. The writhe
per unit length of the intraspines diminishes with increasing length of the
tube axes. Thus, the axes of tubes related to intraspines are less wrung when
the tubes are more horizontal. As the writhe of the spines is very small, we
can conclude that the writhe reaches only significant values when the tube
includes the border of a intraspine.
View original:
http://arxiv.org/abs/1111.3881
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