Title : Observations of umbral flashes

Authors : L.H.M. Rouppe van der Voort & J.M. Krijger

To appear in: "The future of cool-star astrophysics", Procs. 12th
Cambridge workshop on cool stars, stellar systems and the Sun, 2001

Abstract:

 We present observations of oscillations in the chromosphere of the
umbra of sunspots. The observations were obtained with the Swedish
Vacuum Solar Telescope (SVST) and the Dutch Open Telescope (DOT) on La
Palma, comprising spectrograms and filtergrams in the Ca II H line.
The sawtooth pattern in the spectroscopic time evolution of the Ca II
H core is shown as well as evidence for a connection between umbral
flashes and running penumbral waves from image sequences.  Running
waves, coherent over a large fraction of the penumbra, seem to be
excited by flashes that occur close to the umbra-penumbral
boundary. Comparing the intensity oscillations in the Ca II H line
with TRACE observations in the 1600 A passband, we find a phase
difference of approximately 25 degrees with 1600 A leading the Ca II H
intensity oscillation which we attribute to complex dynamical
behaviour.