Issue |
A&A
Volume 430, Number 2, February I 2005
|
|
---|---|---|
Page(s) | 691 - 700 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20041286 | |
Published online | 20 January 2005 |
Radiative properties of magnetic elements
II. Center to limb variation of the appearance of photospheric faculae
Kiepenheuer-Institut für Sonnenphysik, Schöneckstrasse 6, 79104 Freiburg e-mail: steiner@kis.uni-freiburg.de
Received:
13
May
2004
Accepted:
15
September
2004
For the understanding of more complex numerical simulation results,
a basic facular model is created consisting of a magnetic flux sheet
embedded in a plane parallel atmosphere. The atmosphere within the
flux sheet is similar to the external one but shifted in the downward
direction to result in a Wilson depression of 150 km.
It is shown that the horizontal spatial extension of the contrast
enhancement produced by this model increases from center to limb
from a few tenths to up to as a consequence of enhanced
radiation from the limbward surface outside (behind) the magnetic
flux concentration. For a conceivable explanation of this radiative
effect it is noted that a plasma parcel on the solar surface sideways
of the flux sheet “sees” a more transparent sky in the direction
towards the flux sheet compared to a direction away from it because of
the rarefied atmosphere within the flux sheet.
This facular model also produces a dark lane at the disk-center side
(in front) of the flux sheet even though no flow is present.
It it is due to the deep layers of the flux sheet that have a
lower temperature gradient and are cooler than
the surrounding atmosphere at equal geometrical depth.
This implies that limb observations offer a glimpse of the
“cool bottom” of magnetic elements.
The center-to-limb variation of the size and the dark-lane effect
derived from this basic model is recovered in a self-consistent,
two-dimensional non-stationary simulation of a magnetic flux
concentration. These findings
are in excellent agreement with and provide an interpretation of
recent observations of faculae at very high resolution by
Lites et al. (2004, Sol. Phys., 221, 65).
Key words: Sun: magnetic fields / Sun: photosphere / Sun: faculae, plages
© ESO, 2005
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