Motions in the Sun at the Photographic Level III. THE EVERSHED EFFECT IN SUNSPOTS OF DIFFERENT SIZES

Velocity fields are derived from the sight-line velocities at 315 points in the spectra of one large and three small sunspots, using Evershed's positiveon-negative method of measurement. Corrections for scattered light derived graphically from observations at the solar limb are applied to these velocities and also (since the scattering is mainly instrumental and therefore constant) to the previously determined velocities for Mt. Wilson No. 9987. The velocities in the small spots are also corrected for obliteration. The motion in the new large spot confirms the previous result of a purely radial horizontal flow reaching a maximum velocity in the penumbra and extending into the photosphere. A comparison of the velocity fields of all the spots shows (1) that the maximum radial velocity increases linearly with the umbral radius, and (2) that the time to reach maximum velocity is independent of the size of the umbra and equal to about $$13\times{10}^{8}$$ seconds. On a simple model of a sunspot this leads to the conclusion that the heat absorbed per gram of material in flowing from the umbra to the photosphere is the same for all spots.