Issue |
A&A
Volume 450, Number 1, April IV 2006
|
|
---|---|---|
Page(s) | 365 - 374 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20054310 | |
Published online | 03 April 2006 |
Dynamics of the solar photosphere with IBIS
I. Reversed intensity structure in the mid-photosphere
INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy e-mail: kjanssen@arcetri.astro.it
Received:
5
October
2005
Accepted:
16
December
2005
Context.We investigate the quiet solar photosphere with the new Interferometric BI-dimensional
Spectrometer (IBIS), recently installed at the Dunn Solar Telescope of the National
Solar Observatory, Sacramento Peak, NM.Aims. We study the occurrence of reverse
granulation in the mid-photosphere, and its properties.Methods. IBIS, a double
Fabry-Perot system with a narrow passband of 20 mÅ at 6000,
was used to acquire a one hour sequence of 2D scans in the photospheric Fe I
7090.4
and Fe II 7224.5
lines, and in the chromospheric
Ca II 8542
line. Photospheric velocities are measured for statistical
study. To visualize the reversed intensity structure, the influence of velocity
on the steep photospheric profiles is taken into account. The correlation between
mid-photospheric intensities and continuum is then studied, considering the
effects of spatial smoothing and time delay. The temporal evolution of the cross-correlation
coefficients is analyzed. Results. The distribution of photospheric velocities
is consistent with values in the literature, thus confirming the reliability
of the instrument for this kind of study. “Reversed granulation” is clearly
visible in the Fe I 7090.4
line center, formed at about 200 km.
It provides a weak anti-correlation with the continuum, as expected from numerical
simulations. Images acquired in the far wings of the Ca II line display the
same pattern with a slightly higher correlation. In both cases a spatial smoothing
larger than
decreases the level of anti-correlation, while
a time delay of about 2 min increases it. Together, these facts suggest
that the phenomenon is mainly due to a reversal of temperature fluctuations
between granules and intergranules, occurring in the mid-photosphere. The temporal
evolution of the anti-correlation between the Fe I line center and the continuum
reveals a clear oscillatory signal that we consider as highly suggestive of
the presence of gravity waves developing from the mid-photosphere outwards.
© ESO, 2006
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