The Evershed Flow: Flow Geometry and Its Temporal Evolution

A diffraction-limited 120 minute time sequence of Evershed flows along penumbral filaments was obtained using high-order adaptive optics in conjunction with postprocessing. We observe individual Evershed flow channels and study their evolution in time. The vast majority of flow channels originate in bright, inner footpoints of size 02-04 with an upflow. The upflow turns into a horizontal outflow along a dark penumbral filament within fractions of 1'' (300-500 km). The time sequence clearly shows that both (bright) upflow and (dark) horizontal flow move around and evolve as a unit, indicating that they are part of the same feature. The inner footpoints are brighter than the average quiet photosphere and move inward at 0.5-1 km s-1. Our observations provide strong evidence that penumbral grains are the inner footpoints of Evershed flows where a hot upflow occurs. We observe an Evershed flow channel as it appears to emerge near the outer penumbra and track the flow over a period of about 100 minutes as it moves toward the penumbra-umbra edge, where it disappeared. We observe a steep decline (≤02) of the velocity at outer end of individual flow channels, even for flow channels that end well within the penumbra. This sharp outer edge of the flow channels is also observed to move inward toward the penumbra-umbra boundary. Flows in dark-cored penumbral filaments appear to be produced by the Evershed effect. We discuss our observational results in the context of models of the Evershed effect. Some aspects of our observations provide strong support for the moving tube model of the Evershed flow.

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