Title : Penumbral structure and kinematics from high-spatial-resolution
observations of Ca II K.

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

accepted by Astronomy & Astrophysics (23 April 2002)

High-spatial-resolution spectra of the Ca II K line obtained with the
Swedish Vacuum Solar Telescope (SVST) on La Palma are used to study
the penumbra of a sunspot.  The observed radiation temperatures in the
Ca II K wing are used to derive the temperature stratification of
fine-structure elements in the penumbra.  It is found that in general,
over the observed atmospheric depth range, penumbral structures keep
their relative brightness identity with respect to their local
surroundings, i.e., bright (dark) structures in the lower photosphere
remain bright (dark) in the upper photosphere.  Hot structures have a
larger temperature difference between the bottom and the top of the
photosphere than cool structures.  Three semi-empirical atmosphere
models, a cool, hot and mean model, are presented as being
representative for the temperature stratification of penumbral fine
structure.  The mean temperature distribution of the centre-side
penumbra is found to be up to 50 K hotter in the higher photosphere as
compared to the limb-side penumbra. Hot structures being more numerous
in the centre-side penumbra can account for this difference. These are
primarily found near the outer penumbral boundary. It is suggested
that the asymmetry can be explained by a differential line-of-sight
effect that is caused by isotherms in bright structures having in the
higher photosphere a tilt angle of approximately 7 degrees with the
horizontal, pointing downward towards the outer boundary.  Line blends
in the extended Ca II K wing are selected to study the Evershed effect
and its height dependence.  At a number of locations, the Evershed
effect is found to be concentrated in channels which have a tendency
to coincide with dark filaments.  A weak correlation between
brightness and velocity signal is found but also a number of bright
structures with a significant Evershed signal.  Simple numerical tests
of flow channels in the penumbral atmosphere are performed to confront
existing theoretical models with the observations.  From these
experiments it is found that the bulk of the flow must be concentrated
in the lower atmospheric layers, i.e., below 200 km, and must have a
velocity not higher than 6 km/s.  A channel width of 200~km is found
to give the best reproduction of the observed velocities, so that the
flow is either concentrated in a single channel or in a bundle of
narrower channels.  No direct indication is found of the Evershed
channels being elevated above the continuum, and it is estimated that
the flow channels reach down to at least 50~km above the continuum.