Title : La Palma observations of umbral flashes

Authors : L.H.M. Rouppe van der Voort, R.J. Rutten, P. Suetterlin,
P.J. Sloover, J.M. Krijger

accepted by Astronomy & Astrophysics (13 Feb 2003)

abstract:

   We present high-quality Ca II H & K data showing chromospheric
   flashes in sunspot umbrae collected with the Swedish Vacuum Solar
   Telescope, the Dutch Open Telescope, and the Swedish 1-m Solar
   Telescope at the Roque de los Muchachos Observatory on La Palma.
   Differential movies, time slices, spectrograms, and Fourier power
   maps demonstrate that umbral flashes and running penumbral waves
   are closely related oscillatory phenomena, combining upward shock
   propagation with coherent wave spreading over the entire spot.  We
   attribute the flash brightening to large redshift by post-shock
   material higher up.  We find no obvious relation between umbral
   dots and umbral flashes.