The three-dimensional structure of the magnetic field of a sunspot
Balthasar H.1, Gömöry P.2,
1 Astrophysikalishes Institut Potsdam, An der Sternwarte 16,
14482 Potsdam, Germany
2 Astronomical Institute of the Slovak Academy of Sciences,
SK-05960 Tatranská Lomnica, Slovakia
Abstract:
Spectro-polarimetric observations in several spectral lines allow to determine the
height variation of the magnetic field of a small sunspot throughout the solar photosphere.
The full Stokes-vector is measured with high spatial resolution. From these data we derive
the magnetic field vector. The magnetic fieeld strength decreases with height everywhere in
the spot, even in the outer penumbra where some other authors have reported the opposite.
The precise value of this decrease depends on the exact position in the spot. Values vary
between 0.5 and 2.2 G/km when they are determined from an iron and a silicon line in the
near infrared. The magnetic field is less inclined in the higher layers where the silicon
line is formed. Once the magnetic vector field is known, it is straight forward to determine
current densities and helicities. Current densities exhibit a radial structure in the penumbra,
although it is still dificult to correlate this with the structure seen in the intensity
continuum. In spite of this, current densities have a potential to serve as diagnostic tools
to understand the penumbra, at least with the spatial resolution of the upcoming telescopes.
The mean infered helicity is negative, as expected for a spot in the northern hemisphere.
Nevertheless, there are locations inside the spot with positive helicity.