Evolution and Advection of Solar Mesogranulation

摘要

Granular structure on the sun's surface, with a typical scale of 1-2 Mm, has beenknown since 1800, and one hundred years ago, with the first observations by spectroheliograph, a mesh-like bright network was found with a characteristic scale of 30 Mm (40 in.). This pattern was found, thirty years ago, to be coincident with close-packed convective cells ('supergranulation') revealed by Doppler observations to be nestling inside the bright network. More recently an intermediate 'mesogranular' structure was found, with a characteristic scale of 3-10 Mm. A three-hour sequence of observations was obtained at the Pic du Midi Observatory which shows the evolution of mesogranules from appearance to disappearance with unprecedented clarity. It is seen 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.