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Astron. Astrophys. 349, 301-311 (1999)


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Determination of horizontal velocity fields at the sun's surface with high spatial and temporal resolution

Th. Roudier 1, M. Rieutord 1, J.M. Malherbe 2 and J. Vigneau 1

1 Laboratoire d'Astrophysique de Toulouse, Observatoire Midi-Pyrénées, 14 avenue E. Belin, F-31400 Toulouse, France
2 DASOP, Observatoire de Paris, Section de Meudon, F-92195 Meudon, France

Received 30 March 1999 / Accepted 14 June 1999

Abstract

We analyse the two algorithms which have been used in the past few years to determine the horizontal flow fields at the Sun's surface, namely the Local Correlation Tracking (LCT) of L. November and the Feature Tracking of L. Strous. Analysing the systematic errors introduced by LCT, we show that these errors come from the averaging processes. More precisely, they arise from the interpolating step of the algorithm: granules' motions determine the flow on an irregular grid which is then interpolated to derive quantities such as horizontal divergence or vertical vorticity. Interpolation is therefore a crucial step since mesoscale structures have mainly been studied through divergences and vorticities. We conclude that a reliable algorithm should be based on the tracking of coherent structures, like granules, since they are representative of the fluid motion, and should contain an interpolator which keeps track of the errors introduced either by location of the data (the shape of the irregular grid) or by the noise in the data.

Key words: Sun: atmosphere – Sun: general – Sun: granulation – Sun: photosphere

Send offprint requests to: Th. Roudier

© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999

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