Sunspot velocity fields

• Main Author: Holmes, J.
• Published: University of Oxford 1962
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.734629

Research in sunspot velocity fields was initiated by Evershed in 1909. He observed that, in general, Fraunhofer lines crossing a sunspot appeared to be equally displaced in wavelength in opposite directions on either aide of the spot, and subsequent measures of sunspot spectra enabled him to deduce a radial outflow from the spot centre over the surface of the sun, the maximum value of which approached 2 Km/sec, This sunspot feature has since been referred to as the Evershed effect. In addition, Evershed noticed that the velocity increased outwards from the umbra to the penumbra, where it reached the maximum value, and appeared to cease abruptly at the outer edge or the penumbra. Further work by St. John in 1913 confirmed Evershed's observations, but St. John maintained that the motion continued beyond the penumbra into the photosphere. Intending the investigation to chromospheric regions, St. John was also able to show that lines of calcium and hydrogen exhibited an instreaming motion. The recent investigations of sunspot velocity fields, notably by Kinman, Servajean and Bumba, have attempted more detailed analyses of the sunspot motions. Based on a scheme suggested by H.H. Plaskett, both Kinman and Servajean, after a series of detailed visual measures of sunspot spectra, attempted solutions for three sunspot velocity components viz. a radial component u along the surface of the sun, a vertical component w at right angles to the sunspot plane along a solar radius, and a tangential component v at right angles to u and w and in the plane of the spot. Their solutions confirmed Evershed's initial hypothesis, that the material flow in sunspot penumbrae is predominantly radial. In addition, both Kinman and Servajean deduced a variation in the maximum radial velocity u_max. Kinman suggested u_max increased linearly with umbral radius, while Servajean concluded that in the same spot, u_max decreased as the spot proceeded from the disk centre to the limb. In all such Evershed effect investigations, care must be taken to distinguish between line-shifts due to velocities in the sunspot along the line-of-sight, and shifts which have their origin in line-splitting produced by the magnetic field associated with the spot. The relation between magnetic lines of force and notarial motion is a matter of considerable importance in sunspot theory, and is the basis of a recent sunspot investigation by Bumba, who concludes that motion in sunspot penumbrae is along the lines of force of a fan-shaped magnetic field located outside the umbra. Verification of these results is desirable. The present work was designed to investigate velocity fields in sunspots from detailed measures of an Fe I line at λ5576, a Fraunhofer line whose Landé splitting factor g is zero, i.e. it is unaffected by the sunspot magnetic field. Using high dispersion sunspot spectra obtained by Professor Plaskett we again followed his suggested scheme to determine penumbral velocity components in one large and one small sunspot. A further investigation was concerned tilth the analysis of sunspot spectra taken during the passage of one and the same spot across the solar disk. This latter work was especially designed to determine the umbral velocity components and to investigate the variation of the penumbral velocity components with disk position. Two main series of results have emerged. First, the sunspot penumbral motion is confirmed to be predominantly radial, while the umbral motion is characterized by a small, descending vertical component. Also from the spectra of the same spot, we deduce that u_max decreases from to limb and find that simultaneously, the radial velocity pattern is considerably broadened. Light scattering suggests an explanation for this feature and a preliminary investigation of the effect of scattered light on the sunspot velocity field was attempted. The radial velocity corrections due to scattered light were greater than had been previously estimated and it is suggested that the centre-to-limb change in u_max may be due to this cause. An alternative explanation is provided by the possible existence of a radial velocity variation with depth in the sunspot. The second observational result concerns a phenomenon revealed by the high-dispersion spectra, and referred to as line-flare. This is a diffuse widening of the spectral line at the edge of the penumbra in the direction of the measured velocity. (It was simultaneously observed by Servajean and Bumba.) In this thesis, a detailed photometric study of the line-flare is undertaken to investigate intensity variations and to determine its influence on the visual measures of velocity fields in sunspots. The final survey of both our velocity field and photometric results leads us to the conclusion that no satisfactory interpretation of sunspot velocity fields can be suggested until two major issues have been decided viz. (a) the problem of a sunspot model to describe the variations of pressure and temperature with geometric and optical depths. (The determination of line equivalent widths and central intensities is a step in this direction.) If any depth variations in the sunspot are to be confirmed, the question of a sunspot model is essential. (b) The effect of scattered light on sunspot velocity and magnetic fields needs to be more carefully considered. Only when these problem have been decided can we attempt to relate our sunspot velocity field observations with our knowledge of umbral granulation, penumbral filaments and sunspot magnetic fields, in order to visualize the true pattern of the motions of material in a sunspot.