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Astron. Astrophys. 321, 323-329 (1997)

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1. Introduction

Various tracers (particularly sunspots) have been used to characterize solar dynamics. Cycle 21 sunspots of Meudon [FORMULA] spectroheliograms have been studied previously by Nesme-Ribes et al. (1993a; 1993b, referred to hereafter as Papers I and II, respectively), where their angular rotation and meridional motion were investigated. These reveal new sunspot properties, namely variable rotation through the solar cycle and a very small latitudinal angular momentum transport via Reynolds stresses. However, these authors also found some net meridional circulation, which could contribute to maintaining the observed differential rotation.

The present work consists in extending the above previous studies to cycle 19 sunspots. Our aim was first to confirm or refute the properties detected in Papers I and II, and secondly to determine the time-variability of the azimuthal field: cycle 19 was the most active since Galileo's time. Our result will be compared extensively to cycle 21 sunspot properties.

Observational data are presented in Sect. 2, the angular velocity is discussed in Sect. 3, and the meridional circulation in Sect. 4. The correlation between these two motions (the "covariance") is calculated in Sect. 5, in order to estimate the Reynolds stress contribution to angular momentum transport, which is a question related to the origin of the differential rotation.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998