Bibcode
Beck, C.; Rezaei, R.; Puschmann, K. G.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 544, id.A46
Advertised on:
8
2012
Revista
Número de citas
15
Número de citas referidas
13
Descripción
Context. The energy source powering the solar chromosphere is still
undetermined, but leaves its traces in observed intensities.
Aims: We investigate the statistics of the intensity distributions as a
function of the wavelength for Ca ii H and the Ca ii IR line at 854.2 nm
to estimate the energy content in the observed intensity fluctuations.
Methods: We derived the intensity variations at different heights
of the solar atmosphere, as traced by the line wings and line cores of
the two spectral lines. We converted the observed intensities to
absolute energy units employing reference profiles calculated in
non-local thermal equilibrium (NLTE). We also converted the intensity
fluctuations to corresponding brightness temperatures assuming LTE. Results: The root-mean-square (rms) fluctuations of the emitted
intensity are about 0.6 (1.2) W m-2 ster-1
pm-1 near the core of the Ca ii IR line at 854.2 nm (Ca ii
H), corresponding to relative intensity fluctuations of about 20% (30%).
For the line wing, we find rms values of about 0.3 W m-2
ster-1 pm-1 for both lines, corresponding to
relative fluctuations below 5%. The relative rms values show a local
minimum for wavelengths forming at a height of about 130 km, but
otherwise increase smoothly from the wing to the core, i.e., from
photosphere to chromosphere. The corresponding rms brightness
temperature fluctuations are below 100 K for the photosphere and up to
500 K in the chromosphere. The skewness of the intensity distributions
is close to zero in the outer line wing and positive throughout the rest
of the line spectrum, owing to the frequent occurrence of high-intensity
events. The skewness shows a pronounced local maximum at locations with
photospheric magnetic fields for wavelengths in-between those of the
line wing and the line core (z ≈ 150-300 km), and a global maximum at
the very core (z ≈ 1000 km) for both magnetic and field-free
locations. Conclusions: The energy content of the intensity
fluctuations is insufficient to create a chromospheric temperature rise
that would be similar to the one in most reference models of the solar
atmosphere. The increase in the rms fluctuations with height indicates
the presence of upwardly propagating acoustic waves of increasing
oscillation amplitude. The intensity and temperature variations indicate
that there is a clear increase in dynamical activity from photosphere
towards the chromosphere, but the variations fall short of the magnitude
predicted by fully dynamical chromospheric models by a factor of about
five. The enhanced skewness between the photosphere and lower solar
chromosphere at magnetic locations is indicative of a mechanism that
acts solely on magnetized plasma.
Appendices are available in electronic form at http://www.aanda.org
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