THE SOURCE OF 5 MINUTE PERIOD PHOTOSPHERIC UMBRAL OSCILLATIONS

We observed the oscillations in the umbrae of two sunspots using the M CCD imaging spectrograph at the Mees Solar Observatory (MSO) on Haleak ala, Maui. We study velocity oscillations with 5 minute periods using the Doppler shifts of 18 molecular lines observed in both umbrae for o ver 50 hours each during the interval from 1991 May 11 to 16. In the u mbrae we find only a weak correlation between the velocity measured wi th molecular lines and the velocity measured using two iron lines. We remove solar rotation, image drift, and interpolate all the data onto an even time grid. We perform four spatial analyses of the umbral velo city and find (1) there is more power traveling toward the center of t he umbrae than leaving the center of the umbrae (this provides a direc t measure of the absorption of p-modes by the sunspot umbrae) (2) the umbral oscillations display power in the same spatial and temporal fre quency band as the quiet-Sun oscillations, within the resolution provi ded by the data, (3) Fourier-Bessel analysis of one umbra shows no obv ious resonant frequencies which might represent natural oscillation mo des, and (4) the centers of the umbrae have less rms velocity at high spatial wavenumber than the edges of the umbrae. From these analyses w e conclude: (1) the photospheric umbral oscillations are driven by a s ource external to the sunspot, the global p-mode oscillations, (2) the re are no resonant frequencies in the oscillations, which suggest ther e is no acoustic cavity formed by the umbrae, and (3) the absorption o f acoustic waves occurs inside the umbrae. We compare the observed abs orption function for one umbra with simple wave absorption models and find that a point absorber at the umbral center does not fit the obser vations; rather the data equally support either a linear acoustic abso rption across the umbra or total acoustic absorption on a ring surface at a radius of 5 Mm from the umbral center.