Issue |
A&A
Volume 389, Number 3, July III 2002
|
|
---|---|---|
Page(s) | 1020 - 1038 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361:20020638 | |
Published online | 01 July 2002 |
Penumbral structure and kinematics from
high-spatial-resolution observations of
K*
The Institute for Solar Physics of the Royal Swedish Academy of Sciences, SCFAB, 10691 Stockholm, Sweden
Corresponding author: rouppe@astro.su.se
Received:
8
March
2002
Accepted:
23
April
2002
High-spatial-resolution spectra of the 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
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° with the
horizontal, pointing downward towards the outer boundary.
Line blends in the extended
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-1.
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.
Key words: sunspots / Sun: photosphere / techniques: spectroscopic / techniques: high angular resolution
© ESO, 2002
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.