Supergranule and mesogranule evolution

The MDI instrument on the SOHO satellite obtained a nearly continuous 45.5- hr run in high-resolution mode on 17-18 January 1997, collecting continuum, Dopplergram, and magnetogram images once per minute. This is one of the lo ngest data sets yet obtained in this mode and shows significant evolution o f the supergranulation pattern. After allowing for solar rotation within th e fixed field of view, an area spanning 17 degrees in latitude and 11 degre es in longitude was extracted that covers the same area of the solar surfac e for the entire run. From the de-rotated continuum images, we computed flo w maps of photospheric motions using local correlation techniques (LCT). Ho rizontal divergence maps constructed from the flow maps show local maxima o f the size of mesogranules (5-10''). We interpret these as mesogranules alt hough the LCT flow map resolution (4.8'' FWHM) may not completely resolve s maller mesogranules. Movies made from the divergence maps clearly show the outward convection (advection) of these mesogranules within each supergranu le, and narrow boundaries of negative divergence outlining the supergranule s. Several new supergranules are observed forming. These appear as areas of strong divergence that pop up between pre-existing supergranules and grow, pushing their neighbors apart. Others seem to perish between growing neigh bors. We also computed the vertical component of vorticity from the flow ma ps. Movies of this vorticity do not show any obvious patterns.