ON THE VELOCITY FIELD IN SUNSPOTS.
OSLO UNIV (NORWAY) INST OF THEORETICAL ASTROPHYSICS
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Using the tower telescope of the Oslo Solar Observatory approximately 4600 spectra from 62 different sunspots have been obtained. From the study of weak Fraunhofer lines the velocity fields in isolated spots and sunspot groups have been derived. For the isolated spots the radial velocity component directed horizontally out of the spot is found to be the largest in most cases, but appreciable tangential velocities are present. An upper limit equal to 0.15 kmsec is found for the vertical velocity component. Sunspot groups show a higher frequency of tangential velocities as well as a faster change in the velocity field with time than is the case for isolated spots. An abrupt change in velocity at the outer prenumbral rim is detected. The observed line profiles when Evershed velocities are present have been discussed. A comparison between observed and calculated line profiles shows that the velocity changes much more quickly with optical depth than previously assumed. A discussion of the continuity of the motion shows that the vertical velocity component may be small since the density increases rapidly with depth as compared with the increase in horizontal velocity with increasing distance from the spot centre. Author