VELOCITY AND MAGNETIC-FIELD FLUCTUATIONS IN THE PHOTOSPHERE OF A SUNSPOT

We use a data set of exceptionally high quality to measure oscillation s of Doppler velocity, intensity, and the vector magnetic held at phot ospheric heights in a sunspot. Based on the full Stokes inversion of t he line profiles of Fe I 630.15 and 630.25 MI, in the sunspot umbra we find upper limits of 4 G (root mean square [rms]) for the amplitude o f 5 minute oscillations in magnetic held strength and 0.degrees 09 (rm s) for the corresponding oscillations of the inclination of the magnet ic held to the line of sight. Our measured magnitude of the oscillatio n in magnetic field strength is considerably lower than that found in 1997 by Horn, Staude, & Landgraf. Moreover, we find it likely that our measured magnetic held oscillation is at least partly due to instrume ntal and inversion cross talk between the velocity and magnetic signal s, so that the actual magnetic field strength fluctuations are even we aker than 4 G. In support of this we show, on the basis of the eigenmo des of oscillation in a theoretical model of the sunspot umbra, that m agnetic field variations of at most 0.5 G are all that is to be expect ed. The theoretical model also provides an explanation of the shift of power peaks in Doppler velocity to the 3 minute band in chromospheric umbral oscillations, as a natural consequence of the drastic change i n character of the eigenmodes of oscillation between frequencies of ab out 4.5 and 5.0 mHz due to increased tunneling through the acoustic cu toff-frequency barrier. Using measurements of the phase of velocity os cillations above the acoustic cutoff frequency, we determine the relat ive velocity response height in the umbra of four different photospher ic spectral lines from the phase differences between velocities in the se lines, assuming that the oscillations propagate vertically at the l ocal sound speed. In spacetime maps of fluctuations in continuum inten sity, Doppler velocity, magnetic held strength, and held inclination, we see distinct features that migrate radially inward from the inner p enumbra all the way to the center of the umbra, at speeds of a few ten ths of a kilometer per second. These moving features are probably a si gnature of the convective interchange of magnetic flux tubes in the su nspot, although we failed to find any strong correlation among the fea tures in the different quantities, indicating that these features have not been fully resolved.