Evershed Flow, Oscillations, and Sunspot Structure

Abstract : Using high resolution spectroscopy at high cadence, we probe oscillatory properties of the Evershed effect flows. We employ Doppler measurements in several spectral lines to show that the Evershed flow is modulated at periods lasting a few tens of minutes, at the photosphere and chromosphere. The phase of this modulation is always outward irrespective of whether the spectral line originates in the photosphere or chromospheres. From a power-spectrum analysis, we show that periods of peak power shift to longer periods as the magnetic field increases (going from the umbra to the outer penumbra), at photospheric levels. At the chromospheres the periods shift to longer periods as the magnetic field shifts from stronger to weaker fields. An analysis of these phenomena and their influence on the sunspot structure is presented.