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Paper: Nonlinear Fast MHD Waves in Open Coronal Magnetic Structures
Volume: 184, Third Advances in Solar Physics Euroconference: Magnetic Fields and Oscillations
Page: 253
Authors: Oliver, R.; Ballester, J. L.; Murawski, K.
Abstract: An open coronal magnetic structure is modeled as a slab of cold plasma threaded by a vertical, uniform magnetic field. A periodic driver acting at the coronal base is assumed to drive the velocity component normal to the equilibrium magnetic field. Previous works indicate that, in the linear regime, only fast mode perturbations propagate, since Alfvén waves are excluded from the model and the slow wave is absent in the cold plasma limit. However, in this work it is shown that nonlinear terms in the magnetohydrodynamic (MHD) equations give rise to excitation of the velocity component parallel to the equilibrium B, with a lower amplitude than the normal component. Another consequence of nonlinearities is the generation of higher-frequency Fourier modes, which can be detected by Fourier analyzing the velocity variations above the photosphere. The nature of the nonlinear interactions in the MHD equations determines the frequency of those modes. These interactions are quadratic in the case of the parallel component, while they are cubic in the case of the normal component. Therefore, nonlinearly excited frequencies 2ω_d, 4ω_d, 6ω_d, ... are present in the parallel velocity, whereas frequencies 3ω_d, 5ω_d, 7ω_d, ... are present in the normal velocity, with ω_d the driving frequency.
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