The response of an isothermal atmosphere to pressure fluctuations at its base and the five-minute oscillations in the solar photosphere
✍ Scribed by R. L. Moore
- Publisher
- Springer
- Year
- 1974
- Tongue
- English
- Weight
- 800 KB
- Volume
- 36
- Category
- Article
- ISSN
- 0038-0938
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✦ Synopsis
The steady-state vertical-velocity response of an isothermal atmosphere to pressure fluctuations of arbitrary period and horizontal wavelength at its base is derived in the approximation of dissipationless polytropic motion in the atmosphere. It is pointed out that, since only upward modes can be excited in an isothermal atmosphere perturbed from below, the infinite response found by Worrall (1972) at the critical frequency cog does not occur. The correct behavior of the response is presented in some detail.
Comparison of the response of the model, for the case of isothermal oscillations, with observed features of the photospheric oscillations indicates that, in addition to the evanescent photospheric oscillations which occur at the compression-wave propagation cut-off frequencies and which have horizontal wavelengths > 3000 kin, in the lower photosphere there are also smaller-scale evanescent oscillations which have horizontal wavelengths <1000 kin, periods ranging from 200 to 400 s, amplitudes comparable to that of the larger-scale oscillations, and in which the phase of the vertical velocity oscillation leads the phase of the pressure oscillation.