Alfvénic motions arising from asymmetric acoustic wave drivers in solar magnetic structures

Abstract

Alfvénic motions are ubiquitous in the solar atmosphere and their observed properties are closely linked to those of photospheric p-modes. However, it is still unclear how a predominantly acoustic wave driver can produce these transverse oscillations in the magnetically dominated solar corona. In this study we conduct a 3D ideal MHD numerical simulation to model a straight, expanding coronal loop in a gravitationally stratified solar atmosphere which includes a transition region and chromosphere. We implement a driver locally at one foot-point corresponding to an acoustic-gravity wave which is inclined by $\theta =15^{\circ }$ with respect to the vertical axis of the magnetic structure and is similar to a vertical driver incident on an inclined loop. We show that transverse motions are produced in the magnetic loop, which displace the axis of the waveguide due to the breaking of azimuthal symmetry, and study the resulting modes in the theoretical framework of a magnetic cylinder model. By conducting an azimuthal Fourier analysis of the perturbed velocity signals, the contribution from different cylindrical modes is obtained. Furthermore, the perturbed vorticity is computed to demonstrate how the transverse motions manifest themselves throughout the whole non-uniform space. Finally we present some physical properties of the Alfvénic perturbations and present transverse motions with velocity amplitudes in the range of $0.2-0.75$ km s${}^{-1}$ which exhibit two distinct oscillation regimes corresponding to $42$ s and $364$ s, where the latter value is close to the period of the p-mode driver in the simulation.

arXiv comment15 pages, 8 figures, accepted in ApJ

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• Spectra of standing kink waves in loops and the effects of the lower solar atmosphere
• Polarization of decayless kink oscillations in a 3D MHD coronal loop model