Motions in the Sun at the Photospheric Level: II. The Evershed Effect in Sunspot Mt. Wilson No. 9987

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Abstract
Sight-line velocities are measured for 237 points in the penumbra and surrounding photosphere of a large regular spot (Mt. Wilson No. 9987), from short exposure spectra obtained under good conditions of seeing. These spectra are measured by Evershed's positive-on-negative method with an r.m.s. error of 0.1 km sec–1 per point. The sight-line velocity is a linear function of the cylindrical components of the velocity at any point in the penumbra and these cylindrical components may be found from the sight-line velocities of a group of points which have a motion in common. Least squares solutions for the 237 points show that the penumbral motion has cylindrical symmetry. This motion is found to consist of an outward horizontal radial velocity which increases from approximately 1 km sec–1 at the edge of the umbra to 2 km sec–1 in the middle zone of the penumbra and finally falls to zero in the photosphere well outside the spot. Tangential and vertical velocity components are less than the errors of measurement. In a brief investigation of the energy exchange in the penumbra, it is shown that the energy required both to warm up the penumbral material and to provide its kinetic energy is comparable with the radiation absorbed by the material.

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