Recently, the Space Science and Aerospace Engineering team at the Institute of Frontier and Interdisciplinary Science, Shandong University, has made new progress in understanding the energy dissipation of propagating magnetohydrodynamic (MHD) waves in the solar atmosphere. The team has revealed a specific physical mechanism through which propagating kink waves in inhomogeneous open magnetic structures transfer energy toward smaller scales. The results have been published in The Astrophysical Journal Letters, a leading international journal in astronomy and astrophysics, under the title “Alfvénic Motions in a Stratified Open Flux Tube: Transition from Propagating to Locally Standing Motions and Implications for the Kelvin-Helmholtz Instability.” Dr. Guo Mingzhe from the Institute of Frontier and Interdisciplinary Science, Shandong University is the first and corresponding author of the paper, with collaborators including Prof. Li Bo, Prof. Chen Yao from Shandong University, and Dr. Gao Yuhang from Peking University.

Figure 1. (Left) Full-disk EUV image of the Sun, showing complex magnetic structures outlined by hot coronal plasma. Image credit: NASA/SDO. (Right) Schematic illustration of wave-driven coronal heating. MHD waves excited by motions at the solar surface propagate upward along magnetic field lines, transporting energy into the upper atmosphere, where the wave energy may ultimately be dissipated and converted into heat. Image credit: ISAS/JAXA.

The long-standing problem of coronal heating is one of the major frontier topics in solar physics, and wave-based heating is considered one of the principal candidate mechanisms (Figure 1). Abundant propagating kink waves have been observed in coronal plumes and other open-field regions. These waves are thought to be important carriers of energy from the lower solar atmosphere into the corona and even solar wind. However, how the energy carried by these waves is ultimately dissipated remains an important open question.

In closed magnetic structures, standing kink waves can transfer wave energy into localized Alfvénic motions near the boundaries of magnetic structures. The resulting enhancement of velocity shear can further trigger the Kelvin-Helmholtz instability (KHI). The vortical and turbulent structures generated by the KHI can transfer energy toward smaller spatial scales and are therefore regarded as a potentially important pathway for wave-energy dissipation. In contrast, conventional theoretical understanding suggests that propagating Alfvénic waves in open magnetic structures are less favorable for the development of the KHI and the subsequent formation of turbulent small structures. Consequently, the dissipation of energy carried by propagating kink waves in open magnetic structures has remained poorly understood.

Figure 2. Temporal evolution of physical quantities near the boundary of the magnetic flux tube. (b1) Velocity perturbation (black) and magnetic-field perturbation (blue); (b2) Elsässer variables z+(black) and z-(blue); (b3) temporal evolution of the axial component of the Poynting flux.

Figure 3.(a) Density distribution of the three-dimensional equilibrium magnetic flux tube in a gravitationally stratified atmosphere; (b1)–(b4) temporal evolution of the flux-tube cross section at z = 20Mm; (c1)–(c4) temporal evolution of the flux-tube cross section at z = 10Mm.

In this study, the researchers performed 3D MHD simulations of a gravitationally stratified open magnetic flux tube extending from the chromosphere into the corona, in which propagating kink waves were continuously driven from the lower boundary. The simulations show that, although the collective kink motion near the flux-tube axis largely retains the characteristics of an outward propagating wave, the dynamics near the flux tube boundary gradually become markedly different.

Because of the longitudinal inhomogeneity of the Alfvén speed, partial wave reflection develops near the boundary, causing the local Alfvénic motions to progressively acquire standing wave-like characteristics (Figure 2). This state, characterized by aglobally propagating but locally standing motions, allows velocity shear near the boundary to persist locally, thereby creating more favorable conditions for the growth of the KHI.

As the system evolves, clear vortical structures form at the flux-tube boundary and subsequently develop into complex turbulent small scales (Figure 3). These results demonstrate that, in an inhomogeneous open magnetic structure, propagating waves can also transfer energy toward smaller spatial scales through the KHI. Importantly, both longitudinal and transverse inhomogeneities of the medium play essential roles in this process.

The proposed mechanism may operate in open magnetic structures in coronal holes, such as coronal plumes, and provides a new theoretical perspective for understanding the nonlinear evolution of propagating kink waves, the cascade of wave energy toward small spatial scales, and the formation of fine structures in the open solar atmosphere.

This work was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Shandong Province, the Qilu Young Scholars Program of Shandong University, and the Tianshan Scholars Program of Shandong Province.