On the time variability of the Evershed effect

The Institute for Solar Physics of the Royal Swedish Academy of Sciences, AlbaNova University Center, 10691 Stockholm, Sweden

Corresponding author: rouppe@astro.su.se

Received: 9 September 2002
Accepted: 14 October 2002

Abstract

The time variability of the Evershed effect in the penumbra of a sunspot is studied from a time series of high-spatial resolution spectrograms covering the wings of the  K line accompanied by  K slit-jaw images obtained with the Swedish Vacuum Solar Telescope. Simultaneous G-band observations from the Dutch Open Telescope are used to study the penumbral scene around the spectrograph slit on the photospheric level. Four spectral lines are selected to probe the Evershed effect at different heights in the penumbral atmosphere. The Evershed flow is found to show variations on a time scale of 8–14 min which is a confirmation of earlier observations from tunable filter instruments. The largest amplitudes in the variation are found where the Evershed flow is strongest. No systematic height dependence is found for the propagation of the velocity variations. On several occasions, an increase in the Evershed signal is associated with an intensity increase. In the weak Evershed signal detected in bright filaments, no significant variability is found. As an alternative to the interpretation of temporal enhancements in the flow speed, it is speculated that temporal height variations of a flow channel hosting a steady Evershed flow can explain the observed variations in the Evershed effect.