Alfvenic Motions Arising from Asymmetric Acoustic Wave Drivers in Solar Magnetic Structures

Alfvenic 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 magnetohydrodynamic 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 degrees with respect to the vertical axis of the magnetic structure and is equivalent 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 Alfvenic perturbations and present transverse motions with velocity amplitudes in the range 0.2-0.75 km s(-1) which exhibit two distinct oscillation regimes corresponding to 42 and 364 s, where the latter value is close to the period of the p-mode driver in the simulation.

Automated summary

In this study, we used a 3D magnetohydrodynamic numerical simulation to investigate the properties of Alfvenic motions in the solar atmosphere. We found that acoustic-gravity waves can generate transverse motions in a magnetic loop and obtained the contribution from different cylindrical modes through azimuthal Fourier analysis. Furthermore, we computed the perturbed vorticity to demonstrate the manifestation of transverse motions in the whole non-uniform space. Finally, we discovered two distinct oscillation regimes with velocity amplitudes in the range 0.2-0.75 km/s, corresponding to periods of 42 and 364 seconds, the latter being close to the period of the p-mode driver in the simulation.