Bibcode
Beck, C. ; Rezaei, R.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 531, id.A173
Advertised on:
7
2011
Revista
Número de citas
10
Número de citas referidas
9
Descripción
Aims: We present constraints on the thermodynamical structure of
the chromosphere from ground-based observations of the Ca ii H line
profile near and off the solar limb. Methods: We obtained a
slit-spectrograph data set of the Ca ii H line with a high
signal-to-noise ratio in a field of view extending 20'' across the limb.
We analyzed the spectra for the characteristic properties of average and
individual off-limb spectra. We used various tracers of the Doppler
shifts, such as the location of the absorption core, the ratio of the
two emission peaks H2V and H2R, and intensity
images at a fixed wavelength. Results: The average off-limb
profiles show a smooth variation with increasing limb distance. The line
width increases up to a height of about 2 Mm above the limb. The profile
shape is fairly symmetric with nearly identical H2V and
H2R intensities; at a height of 5 Mm, it changes into a
single Gaussian without emission peaks. We find that all off-limb
spectra show large Doppler shifts that fluctuate on the smallest
resolved spatial scales. The variation is more prominent in cuts
parallel to the solar limb than on those perpendicular to it. As far as
individual structures can be unequivocally identified at our spatial
resolution, we find a specific relation between intensity enhancements
and Doppler shifts: elongated brightenings are often flanked all along
their extension by velocities in opposite directions.
Conclusions: The average off-limb spectra of Ca ii H present a good
opportunity to test static chromospheric atmosphere models because they
lack the photospheric contribution that is present in disk-center
spectra. We suggest that the observed relation between intensity
enhancements and Doppler shifts could be caused by waves propagating
along the surfaces of flux tubes: an intrinsic twist of the flux tubes
or a wave propagation inclined to the tube axis would cause a helical
shape of the Doppler excursions, visible as opposite velocity at the
sides of the flux tube. Spectroscopic data allow one to distinguish this
from a sausage-mode oscillation where the maximum Doppler shift and the
tube axis would coincide.
Appendices are available in electronic form at http://www.andaa.orgThe Data set is
available in electronic form at the CDS via anonymous ftp to
cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/531/A173
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