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last change 2008 June 27, R. Arlt
Cosmic magnetic fields
A 3-D look into the solar photosphere
Zeeman-Tomography with Hinode

A new method of inverting and analyzing spectropolarimetric observations allows us to reconstruct 3-dimensional structures in the solar photosphere. The inversion of spectro-polarimetric line profiles is based on supervised machine learning algorithms i.e. artificial neural networks (ANN, Carroll & Staude 2001) and utilizes recent high-resolution, mixed-polarity magnetoconvection simulations. The ANN based Zeeman-Tomography is being developed in the stellar activity program of the Astrophysical Institute Potsdam and provides a unique opportunity to incorporate the results of magnetohydrodynamic simulations into the inversion process, and moreover allows us to infer the depth stratification of various atmospheric quantities like the temperature, line-of-sight velocity, and the line-of-sight magnetic field on a geometric height scale. Having retrieved the run of these quantities on a common height scale facilitates the subsequent combination of all individual stratifications to obtain a complete 3-dimensional reconstruction (tomography) of the atmospheric parameters. This is in contrast to conventional magnetic flux maps or one-dimensional inversions that only allow to estimate the magnetic field in one surface layer.

We have applied this technique to very recent spectropolarimetric observations of a quiet solar region taken with the Solar Optical Telescope (SOT) aboard the Hinode satellite. The 3-dimensional reconstruction is made for a photospheric surface layer of 500 km in height and a field of view of 12,000 by 12,000 km. Each reconstructed volume, for the temperature, velocity and magnetic field, consists of 370,000 individual data points.

The figures on the right show the tomographic reconstructions as retrived from the spectropolarimetric observations. Many of the magnetic structures seen in the figures do not seem to reach the upper photosphere (a strong decline of the magnetic flux and topological coherence from the bottom to the top photosphere). It will be of particular interest to see in upcoming investigations if these small-scale magnetic fields are "connected" to the upper atmosphere (chromosphere and corona) and to assess their significance for the total magnetic energy budget of the solar atmosphere.



Dr. Thorsten Carroll
Astrophysikalisches Institut Potsdam
An der Sternwarte 16
D-14482 Potsdam
(0331) 7499 539

Markus Kopf
(0331) 7499 207


[Stellar activity pages]

[AIP home page]


Intensity Flux density
Figure 1: continuum intensity (left) of the analyzed quiet photospheric region and the apparent flux density (right) obtained in a conventional manner from the measured polarimetric profiles. The circle (lower right) indicates a strong magnetic field, see also below.

Temperature Line-of-sight velocity
Figure 2 & 3: 3-dimensional reconstruction of the temperature (left) for the observed FOV in a bottom-up-view (deepest layer on top) to emphasize the rich structuring in the lower photosphere. Red/Orange colors indicate the hot granular regions while the dark colors show the cooler intergranular lanes. Right: line-of-sight velocity for the reconstructed volume also in a bottom-up-view. Bright color represent upflows and dark colors the downflows in the intergranular regions. One nicely can identify the onset of strong downflows in the mid photosphere.

Line-of-sight magnetic field
Figure 4: 3-dimensional reconstruction of line-of-sight magnetic field for the FOV (again bottom up). Black and white represent the different magnetic polarities. This figure also provides a nice slice view of at least three magnetic structures in the front plane (X-Z plane). The circle marks the magnetic structure which is already identified in the conventional magnetic flux map, but now in full 3-D.

Line-of-sight magnetic field Line-of-sight magnetic field
Figure 5. & 6: close up views of the same magnetic structure encircled in Fig.1. To allow a better "insight" into the 3-dimensional structure, the side walls (and the bottom plane in Figure 6) of the reconstructed data volume are made transparent. Iso-surface of 175 Gauss field strength are shown, which reveal the tube-like structuring of the magnetic field in the solar photosphere.


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