Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003padeu..13...61m&link_type=abstract
In: Contributions to NATO Advanced Research Workshop Turbulence, Waves, and Instabilities in the Solar Plasma; Hungary, 16-20 Se
Astronomy and Astrophysics
Astronomy
Scientific paper
The effect of space- and time-dependent random mass density, velocity, and pressure fields on frequencies and amplitudes of solar acoustic waves is considered by means of the analytical perturbative method. The analytical results, which are valid for weak fluctuations and long wavelength waves, reveal frequency and amplitude alteration, the effect which depends on the type of a random field. In particular, short-wavelength frequencies of the sound waves that propagate along the constant gravity in an isothermal atmosphere are lifted up (reduced) by a space-dependent random mass density (pressure) field. Higher values of the acoustic cut-off frequency result in an increase of wave frequencies. This effect is stronger for longer waves. As a consequence of that in the case of the random pressure field the frequency correction exhibits a cross-over at intermediate values of wavenumber; long (short) sound waves experience a positive (negative) frequency shift. In the limit of a gravity-free medium, the effect of a random mass density field is to increase wave frequencies. Space-dependent random velocity and pressure fields reduce wave frequencies. While space-dependent random fields attenuate wave amplitudes, their time-dependent counterparts lead to wave amplification. In the other example, the sound waves that are trapped in the vertical direction but are free to propagate horizontally are effected by a space-dependent random mass density field. This effect depends on a direction along which this field is varying. A random field, which varies along the horizontal direction, does not couple standing modes but increase their frequencies and attenuates amplitudes. These modes are coupled by a random field which depends on the vertical coordinate but the dispersion relation remains the same as in the case of the deterministic medium.
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