The Generation of Long Period Acoustic Waves by Solar Global Oscillations
We study the response of a stratified isothermal atmosphere to long period disturbances with frequencies below the acoustic cut-off frequency ω co . Simulating the 5-min and other long period oscillations by a sinusoidally moving piston, we find that a few scale heights above the piston the atmosphere does not oscillate anymore with the excitation period but with the cut-off period (≈ 180s). We therefore suggest that the observed chromospheric 3-min oscillations are not due to a cavity but to a very simple — and in principle well known — basic effect of the dynamical behaviour of atmospheres, namely the excitation of the “cut-off frequency mode”.
KeywordsEvanescent Wave Scale Height Excitation Period Period Disturbance Partial Reflection
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