Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002mnras.335..628w&link_type=abstract
Monthly Notice of the Royal Astronomical Society, Volume 335, Issue 3, pp. 628-640.
Astronomy and Astrophysics
Astronomy
6
Sun: Atmosphere, Sun: Oscillations
Scientific paper
Oscillations in velocity and intensity are detectable over a range of height in the solar atmosphere. Properties of the atmosphere can be deduced from the stratification revealed by the corresponding power and phase-difference spectra, potentially in a manner independent of optical spectroscopy. In this paper, published acoustic power spectra and phase-difference spectra, mainly derived from observations in the NaD2 line, are interpreted through a simple model of the oscillations in an exploratory investigation along these lines. The difference between the `heights of formation' at two positions in the line profile is deduced from the ratio of oscillatory power. The corresponding phase difference between oscillations in velocity at the two heights is compared with the phase spectrum derived from observations. Deduced acoustically, the `height of formation' of the core of the D2 line is about 700 km above that of the pressure-broadened wing. The indicated acoustic cut-off frequency is about 4.5 mHz, a value significantly lower than that usually quoted. A variety of observational evidence is consistent with this value. The low value is probably a consequence of radiative damping, with a relaxation time of about 55 s. The phase relations between the vertical and horizontal components of motion, the temperature and the pressure are considered. At low frequencies the velocity/intensity phase difference, for oscillations of moderate to high degree, is significantly affected by the horizontal motion, even if observations are restricted to a relatively small window at disc centre. There is evidence that at the edge of the Doppler cores of strong absorption lines, fluctuations in intensity are in anti-phase with fluctuations in temperature.
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