High-resolution observations of the Evershed effect in sunspots

High spatial resolution movies of sunspots taken at the Swedish Solar Observatory on La Palma reveal that the Evershed effect is time dependent. Outward proper motions are visible in both the continuum and Dopplergrams. These can be tracked over most of the width of the penumbra and overlap regions that show inward moving penubral grains. The radial spacing between the moving structures is about 2000 km, and they exhibit irregular repetitive behavior with a typical interval of 10 minutes. These are probably the cause of 10 minutes oscillations sometimes seen in a penumbral power spectra. Higher velocities are spatially correlated with the relatively darker regions between bright filaments. Regions with a strong variation in the Doppler signal show peak-to-peak modulations of 1 km/s on an average velocity of about 3-4 km/s. The proper motion velocity is approximately constant from the iner penumbra and generally larger than the Doppler velocity when both are interpreted as projections of horizontal motion. Regions where thay are consistent suggest a typical horizontal velocity of 3.5 km/s. Some proper motion velocites as high as 7 km/s are seen, but these are less certain. The temporal behavoir shows a correlation between increased Doppler signal and increased continuum intensity, the opposite of the spatial correlation. When spatially averaged across filaments and over time, the averaged Evershed effect has a peak horizontal component near the outer edge of the penumbra of 2.0 km/s with evidence for a 200-400 m/s upward component. The latter depends on an uncertain absolute velocity calibration. If real it could be an actual upward component or a penumbral analogue of the convective blueshift seen in the quiet Sun.