Bibcode
Beck, C.; Khomenko, E.; Rezaei, R.; Collados, M.
Bibliographical reference
Astronomy and Astrophysics, Volume 507, Issue 1, 2009, pp.453-467
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11
2009
Journal
Citations
36
Refereed citations
34
Description
Context: Acoustic waves are one of the primary suspects besides magnetic
fields for the chromospheric heating process to temperatures above
radiative equilibrium (RE). Aims: We derived the mechanical wave
energy as seen in line-core velocities on disc centre to obtain a
measure of mechanical energy flux with height for a comparison with the
energy requirements in a semi-empirical atmosphere model, the
Harvard-Smithsonian reference atmosphere (HSRA). Methods: We
analyzed a 1-hour time series and a large-area map of Ca II H spectra on
the traces of propagating waves. We analyzed the velocity statistics of
several spectral lines in the wing of Ca II H, and the line-core
velocity of Ca II H. We converted the velocity amplitudes into volume
(∝ ρ v^2) and mass energy densities (∝ v^2). For
comparison, we used the increase of internal energy (∝ R ρ
Δ T) necessary to lift a RE atmosphere to the HSRA temperature
stratification. Results: We find that the velocity amplitude
grows in agreement with linear wave theory and thus slower with height
than predicted from energy conservation. The mechanical energy of the
waves above around z ~ 500 km is insufficient to maintain on a long-term
average the chromospheric temperature rise in the semi-empirical HSRA
model. The intensity variations of the Ca line core (z ~ 1000 km) can,
however, be traced back to the velocity variations of the lowermost
forming spectral line considered (z ~ 250 km). Conclusions: The
chromospheric intensity, and hence, (radiation) temperature variations
are seen to be induced by passing waves originating in the photosphere.
The wave energy is found to be insufficient to maintain the temperature
stratification of the semi-empirical HSRA model above 500 km. We will in
a following paper of this series investigate the energy contained in the
intensity variations to see if the semi-empirical model is appropriate
for the spectra.
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