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Session 2 - Everything Else.
Display session, Friday, June 27
Ballroom C, Chair: Richard Canfield

[2.36] Velocity and Magnetic Field Fluctuations in the Photosphere of a Sunspot

J. H. Thomas (U. Rochester), B. W. Lites, T. J. Bogdan (HAO/NCAR)

We use a data set of exceptionally high quality, obtained with the HAO/NSO Advanced Stokes Polarimeter, to measure oscillations of Doppler velocity, intensity, and the vector magnetic field at photospheric heights in a sunspot. We find an upper limit of 4 G (rms) for 5-min oscillations in magnetic field strength in the umbra, based on the full Stokes inversion of the line profiles of Fe I 6301.5 and 6302.5. This magnitude of the oscillation in field strength is considerably lower than that found recently by Horn, Staude, and Landgraf (1997). We show, on the basis of the eigenmodes of oscillation in a simple theoretical model of the sunspot umbra, that magnetic field variations of order 1 G are all that is to be expected. Using measurements of velocity oscillations above the acoustic cutoff frequency, we determine the relative heights of formation in the umbra of four different photospheric spectral lines from the phase differences between velocities in these lines, assuming the oscillations propagate vertically at the local sound speed. In space-time maps of fluctuations in continuum intensity, Doppler velocity, magnetic field strength, and field inclination we see distinct features that migrate radially inward from the inner penumbra all the way to the center of the umbra. These moving features are probably a signature of the convective interchange of magnetic flux tubes in the sunspot, although we failed to find any strong correlation among the features in the different quantities, indicating that these features have not been fully resolved.

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