Temporal Behavior of the Evershed Effect

Abstract

We study the Evershed flow in the photosphere and the reverse Evershed flow in the chromosphere from simultaneous observations, giving emphasis to the temporal evolution of the phenomena. We compute the components of the velocity vector as a function of distance from the center of the sunspot, assuming an axial symmetry of both the spot and the flow. A five-minute oscillatory pattern is obvious in the penumbra at photospheric level. Our results verify that the velocity of the Evershed flow has a maximum above the penumbra in the photosphere and well outside the penumbra in the chromosphere. We find evidence of temporal variations prominent in the radial component of the average photospheric velocity with a characteristic timescale of 25 minutes. We consider a transient siphon flow or a wave superimposed on a steady flow as possible explanations for the temporal behavior of the photospheric Evershed flow. The radial component of the chromospheric reverse Evershed flow shows a repetitive temporal variation with a typical timescale of 15 minutes. The variation consists of enhanced velocity amplitudes that propagate to an opposite direction from the flow with a velocity of about 5-6 km s-1. This behavior cannot be easily explained in the frame of a transient flow and strongly suggests that it is related to the propagation of a wave. We examine the possibility of its being associated with the propagation of running penumbral waves in the superpenumbra. The temporal evolution of the line-of-sight velocity across superpenumbral fibrils presents alterations that can be associated with a time-dependent flow. However, we also observe propagating velocity packets that can be associated with a wave.