Near-surface flow fields deduced using correlation tracking and time-distance analyses

Near-photospheric flow fields on the Sun are deduced using two independent methods applied to the same time series of velocity images observed by SOI- MDI on SOHO. Differences in travel times between f modes entering and leavi ng each pixel measured using time-distance helioseismology are used to dete rmine sites of supergranular outflows. Alternatively, correlation tracking analysis of mesogranular scales of motion applied to the same time series i s used to deduce the near-surface flow field. These two approaches provide the means to assess the patterns and evolution of horizontal flows on super granular scales even near disk center, which is not feasible with direct li ne-of-sight Doppler measurements. We find that the locations of the supergr anular outflows seen in flow fields generated from correlation tracking coi ncide well with the locations of the outflows determined from the time-dist ance analysis, with a mean correlation coefficient after smoothing of (r) o ver bar(s)=0.890. Near-surface velocity field measurements can be used to s tudy the evolution of the supergranular network, as merging and splitting e vents are observed to occur in these images. The data consist of one 2048-m in time series of high-resolution (0.6'' pixels) line-of-sight velocity ima ges taken by MDI on 1997 January 16 -18 at a cadence of one minute.