A large-scale meridional circulation in the convective zone

Abstract

Many attempts have been made to detect, at the solar photospheric level, large-scale motions related to the Sun's global convection and dynamo (see ref. 1). Because these motions are likely to be influenced by solar rotation, one might expect that they would take the form of ‘bananas’ aligned with the solar rotation axis², as a result of the Taylor–Proudman constraint. The Paris observatory has one of the longest series of spectroheliograms, dating back to 1919. In view of the current interest in the history of solar activity, we have started to digitize the collection³. This allows us to measure rotational rate and meridional drift for individual sunspots with an accuracy of a few ni s⁻¹ for the best seeing conditions at Meudon. A new phenomenon, which may lead us drastically to revise our ideas on the large-scale convection of the Sun, can immediately be seen. Newly-born sunspots trace a roughly axisymmetric meridional circulation, in the form of four zonal bands, with relatively large velocity amplitude. If these drifts are assumed to trace fluid motions at some level in the solar envelope, the resulting circulation pattern cannot be associated with the ‘banana’ cells mentioned above. We report here results based on two sets of data; one involves a detailed analysis of two periods, each covering five solar rotations while the other is essentially qualitative and results from eye-estimates of the spot meridional drifts over much longer time sequences.