Issue |
A&A
Volume 471, Number 2, August IV 2007
|
|
---|---|---|
Page(s) | 695 - 703 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20066492 | |
Published online | 18 June 2007 |
Tracking granules on the Sun's surface and reconstructing velocity fields
II. Error analysis
1
Laboratoire d'Astrophysique de Toulouse et Tarbes, UMR 5572, CNRS et Université Paul Sabatier Toulouse 3, 14 avenue E. Belin, 31400 Toulouse, France e-mail: [tkaczuk;rieutord]@ast.obs-mip.fr
2
Laboratoire d'Astrophysique de Toulouse et Tarbes, UMR 5572, CNRS et Université Paul Sabatier Toulouse 3, 57 avenue d'Azereix, BP 826, 65008 Tarbes Cedex, France e-mail: [meunier;roudier]@ast.obs-mip.fr
Received:
2
October
2006
Accepted:
30
April
2007
Context. The determination of horizontal velocity fields at the solar surface is crucial to understanding the dynamics and magnetism of the convection zone of the sun. These measurements can be done by tracking granules.
Aims. Tracking granules from ground-based observations, however, suffers from the Earth's atmospheric turbulence, which induces image distortion. The focus of this paper is to evaluate the influence of this noise on the maps of velocity fields.
Methods. We use the coherent structure tracking algorithm developed recently and apply it to two independent series of images that contain the same solar signal.
Results.
We first show that a filtering of the times series of images
is highly recommended as a pre-processing to decrease the noise, while,
in contrast, using destretching should be avoided. We also
demonstrate that the lifetime of granules has a strong influence on the error
bars of velocities and that a threshold on the lifetime should be imposed to
minimize errors. Finally, although solar flow patterns are easily
recognizable and image quality is very good, it turns out that a time
sampling of two images every 21 s is not frequent enough, since image
distortion still pollutes velocity fields at a 30% level on the
2500 km scale, i.e. the scale on which granules start to behave like
passive scalars.
Conclusions. The coherent structure tracking algorithm is a useful tool for noise control on the measurement of surface horizontal solar velocity fields when at least two independent series are available.
Key words: convection / turbulence / Sun: granulation / Sun: photosphere
© ESO, 2007
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