The superrotation of solar supergranules

Laboratoire d'Astrophysique de l'Observatoire Midi-Pyrénées, Université Paul Sabatier, CNRS, 57 Avenue d'Azereix, BP 826, 65008 Tarbes Cedex, France e-mail: [meunier;roudier]@ast.obs-mip.fr

Received: 22 November 2006
Accepted: 19 January 2007

Abstract

Context.Supergranules have long been believed to rotate faster than plasma and than magnetic structures. However, it has recently been shown that this could be due to a bias appearing when using Dopplergrams, which are strongly affected by projection effets.

Aims.Our purpose is to perform new measurements of supergranule angular velocities using a technique that would not be significantly influenced by projection effets.

Methods.We are therefore tracking horizontal divergence maps smoothed on the supergranular scale, either globally or on the scale of cells. Tracking the magnetic field using the same technique allows them to be used as a reference. Doppler tracking is performed for a direct comparison.

Results.We confirm that the tracking of Doppler features is not reliable when computing the dynamics on the supergranular scale. However, we find that divergence features are still rotating faster than the magnetic field, when using two independent time series.

Conclusions.We conclude that the long-standing puzzle of supergranular superrotation persists, and that interpreting it in terms of anchorage depth inside the convective zone may not be sufficient to explain it.