On collective nature of non-linear torsional Alfvén waves

С. А. Белов, D. S. Riashchikov,Dmitrii Y. Kolotkov,S. Vasheghani Farahani, Н. Е. Молевич, Vladislavs Bezrukovs

Monthly Notices of the Royal Astronomical Society(2023)

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Abstract
ABSTRACT Torsional Alfvén waves in coronal plasma loops are usually considered to be non-collective, i.e. consist of cylindrical surfaces evolving independently, which significantly complicates their detection in observations. This non-collective nature, however, can get modified in the non-linear regime. To address this question, the propagation of non-linear torsional Alfvén waves in straight magnetic flux tubes has been investigated numerically using the astrophysical MHD code Athena++ and analytically, to support numerical results, using the perturbation theory up to the second order. Numerical results have revealed that there is radially uniform-induced density perturbation whose uniformity does not depend on the radial structure of the mother Alfvén wave. Our analysis showed that the ponderomotive force leads to the induction of the radial and axial velocity perturbations, while the mechanism for the density perturbation is provided by a non-equal elasticity of a magnetic flux tube in the radial and axial directions. The latter can be qualitatively understood by the interplay between the Alfvén wave perturbations, external medium, and the flux tube boundary conditions. The amplitude of these non-linearly induced density perturbations is found to be determined by the amplitude of the Alfvén driver squared and the plasma parameter β. The existence of the collective and radially uniform density perturbation accompanying non-linear torsional Alfvén waves could be considered as an additional observational signature of Alfvén waves in the upper layers of the solar atmosphere.
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Key words
alfvén waves,collective nature,non-linear
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