EVOLUTION AND ADVECTION OF SOLAR MESOGRANULATION

GRANULAR structure on the Sun's surface, with a typical scale of 1-2 Mm, has been known since 1800, and one hundred years ago, with the first observations by spectroheliograph 1,2, a mesh-like bright network was found with a characteristic scale of 30 Mm (40"). This pattern was found, thirty years ago, to be coincident with dose-packed convective cells ('supergranulation') revealed by Doppler observations 3-5 to be nestling inside the bright network. More recently 6,7 an intermediate 'mesogranular' structure was found, with a characteristic scale of 3-10 Mm. We have obtained a three-hour sequence of observations at the Pic du Midi observatory which shows the evolution of mesogranules from appearance to disappearance with unprecedented clarity. We see that the supergranules, which are known to carry along (advect) the granules with their convective motion, also advect the mesogranules to their boundaries. This process controls the evolution and disappearance of mesogranules.