Analytical description of nonlinear acoustic waves in the solar chromosphere
1 Department of Mathematics, University of Waikato, PO Box 3105, 3240 Hamilton, New Zealand
2 Astronomy Program, Department of Physics and Astronomy, Seoul National University, 151-742 Seoul, Korea
Received: 23 August 2016
Accepted: 22 December 2016
Aims. Vertical propagation of acoustic waves of finite amplitude in an isothermal, gravitationally stratified atmosphere is considered.
Methods. Methods of nonlinear acoustics are used to derive a dispersive solution, which is valid in a long-wavelength limit, and a non-dispersive solution, which is valid in a short-wavelength limit. The influence of the gravitational field on wave-front breaking and shock formation is described. The generation of a second harmonic at twice the driving wave frequency, previously detected in numerical simulations, is demonstrated analytically.
Results. Application of the results to three-minute chromospheric oscillations, driven by velocity perturbations at the base of the solar atmosphere, is discussed. Numerical estimates suggest that the second harmonic signal should be detectable in an upper chromosphere by an instrument such as the Fast Imaging Solar Spectrograph installed at the 1.6-m New Solar Telescope of the Big Bear Observatory.
Key words: hydrodynamics / Sun: atmosphere / Sun: chromosphere / Sun: oscillations / waves
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