Title : On the time variability of the Evershed effect

Author : L.H.M. Rouppe van der Voort

accepted by Astronomy & Astrophysics (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 \ion{Ca}{ii}~K line
  accompanied by \ion{Ca}{ii}~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 minutes 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.