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 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 θ = 15° 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 Alfvénic 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.

Modeling of Transverse Oscillations Driven by p-modes in Short Coronal Loops

“…Note that a convergence study has been conducted to ensure that the current results are not influenced by the numerical resolution. As shown in previous studies (Riedl et al 2019;Skirvin et al 2023), an inclined p-mode driver can excite longitudinal waves, especially slow sausage modes, in the loop. Figure 2 presents the evolution of vertical/longitudinal velocity v z .…”

“…However, we need to be careful because sausage mode or fluting mode is also accompanied by the fluctuation of horizontal (radial) velocity. Therefore, in order to prove the existence of transverse kink mode with an azimuthal wavenumber m = 1, we need to further confirm whether the axisymmetry of the loop is (see Skirvin et al 2023). In Figure 4(A), we plot v x and v y along the loop axis at several different time steps.…”

“…The driver has a velocity amplitude (V 0 ) of 500 m s −1 , a period (P) of 300 s, and an inclination angle (θ) of 15°relative to the loop axis. It is expected that the inclination can excite transverse Alfvénic motions/oscillations in the loop by breaking the azimuthal symmetry of the system (see Riedl et al 2019;Skirvin et al 2023). To ensure that the driver is located inside the loop, we multiply it with a Gaussian function of the form…”

“…Meanwhile, wave excitation by p-modes is also widely studied in a number of analytical and modeling works. Some previous works show that transverse waves can be generated from p-modes through mode conversion (e.g., Khomenko & Cally 2012;Cally 2017), while Riedl et al (2019) and Skirvin et al (2023) have found another scenario in which an inclined p-mode driver can directly excite kink oscillations in a flux tube.…”

The Role of High-frequency Transverse Oscillations in Coronal Heating

“…Interestingly, all power spectra showed enhanced power in the frequency region between 3 and 5 mHz (Tomczyk et al 2007;Tomczyk & McIntosh 2009;Morton et al 2016Morton et al , 2019. This feature proffered the possibility of coronal transverse waves excited by photospheric p-modes (Jefferies et al 2006;Cally & Goossens 2008;Riedl et al 2021;Skirvin et al 2023).…”