Summary: Floweffect on resonant absorption of pmodes in sunspots
Armagh Observatory, Armagh, College Hill, Armagh, BT61 9DG N.
Abstract. The effect of an equilibrium flow on resonant absorption of
pmode oscillations in sunspots is studied in compressible viscous MHD.
We show that an equilibrium shear flow can significantly influence the
absorption suffered by the acoustic driving waves.
1. Interaction of pmodes with sunspots
Braun et al. (1987) carried out a series of observations designed to probe the
nature of sunspots by detecting their influence on highdegree pmode oscilla
tions in the photosphere. Braun el al. decomposed the pmode oscillations in
the photospheric surroundings of several sunspots into waves travelling radially
towards and outwards from each spot. By comparing the power difference be
tween the incoming and outgoing waves they found that sunspots can absorb up
to 50 \Gamma 60% of the acoustic wave energy. The effect is present in pmodes with
wavelength on a large scale (with respect to the radius of the spots). For more
details see e.g. Braun et al. 1987, 1988, 1990, 1992; Braun & Duwall 1990.
The loss of power of pmode oscillations, as a physical mechanism, might
be modelled by the classical picture of resonant absorption: a periodic external