Differential speckle interferometry: in-depth analysis of the solar photosphere

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Aims.We present the results of an experiment performed at the solar telescope THEMIS in 2002 to measure the depth over which the solar granulation extends in the photosphere. Methods: Observations made in the 523.3 nm and 557.6 nm photospheric non-magnetic iron lines were correlated with images in the continuum using spectrograms. The difference in depth between the different levels in the photosphere is projected into a difference of position along the slit of the spectrograph, using a perspective effect similar to the well-known Wilson effect for sunspots. This requires measuring displacements, ones much smaller than the telescope resolution. This is made possible by using a differential speckle interferometric technique, cross-correlating images taken in the continuum and the line. The method is not adapted to following displacements of structures in the core of strong lines, due to their difference in shapes with the structures observed in the continuum. In this case, a sequential cross-spectrum method is developed to cross-correlate images taken at close wavelengths. Results: The raw results are surprising: displacements measured in the blue and the red wings of a line have opposite signs! North and South observations, however, clearly show the expected behavior attributed to a perspective effect. After a description of the observations, we give a first interpretation of the results. The main part of the observed displacement comes from the effect of unresolved Doppler shifts produced by horizontal velocities in the solar photosphere. The perspective effect we seek appears as a second-order term; we find that its amplitude is 2 or 3 times larger than predicted by theoretical 1D models. In the core of strong lines we detect a contrast inversion that also shows up in the cross-correlation function as an anti-correlation peak at line center. Conclusions: .This first use of the differential speckle interferometry technique on the Sun is quite promising for 3D studies at high spatial resolution. Further observations with very good image quality are needed to take advantage of this new technique. THEMIS is operated on the Island of Tenerife by CNRS-CNR in the Spanish Observatorio del Teide of the Instituto de Astrofísica de Canarias.

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