Recently, the Solar Activity and CME Theory Research Group of Yunnan Observatories of the Chinese Academy of Sciences, in collaboration with the Indian Institute of Astrophysics and the University of Sheffield, has made significant progress in the study of coronal heating and solar wind origin in coronal holes. The findings are published in The Astrophysical Journal Letters.
Coronal heating remains a central puzzle in modern astrophysics: why is the outermost layer of the Sun's atmosphere—the corona—hundreds of times hotter than the solar surface? Resolving this apparent violation of the second law of thermodynamics requires answering two key questions: how is energy transported from the solar surface to the corona, and how is that energy dissipated into heat to maintain the high temperatures? Magnetohydrodynamic waves, ubiquitous in the solar atmosphere, are considered one of the most important candidate mechanisms for coronal heating. It is generally believed that these different types of waves are excited near the solar surface and propagate upward through the lower atmosphere to the corona. Among them, slow-mode waves tend to develop into shocks and dissipate while propagating through the chromosphere, and have therefore been considered to contribute little to coronal heating.
Solar coronal holes are regions where magnetic field lines open into interplanetary space and serve as important source regions for the formation of fast solar wind. With lower density and temperature than the surrounding corona, they appear as dark "holes" in extreme-ultraviolet wavelengths. Spicules, on the other hand, are slender, transient brightening phenomena distributed throughout the solar chromosphere. They connect the solar surface to the corona and act as crucial channels for the supply of mass and energy. However, their detailed physical processes and the specific key contributions to coronal heating and the origin of the solar wind remain not yet fully understood.
The research team extended their self-developed radiative and partially ionized modules to cover the region from the upper convection zone to the low corona. Within the context of the open magnetic field configuration in coronal holes, they successfully performed magnetohydrodynamic simulations that capture the full physical process from self-excited convective turbulence to the spontaneous generation of spicules. The simulation results show that convective and turbulent motions frequently trigger small-scale magnetic reconnection and shock structures in the lower atmosphere. The combined action of these two mechanisms drives the quasiperiodic formation of spicule groups. Although only a small amount of plasma ultimately flows into the low corona during the rising phase of the spicules, the average mass flux injected into the low corona (approximately 10⁻⁹ kg m⁻² s⁻¹) is sufficient to compensate for the mass loss of the solar wind in coronal holes. These plasma flows continuously excite localized slow-mode waves and shocks in the low corona, carrying an average outward energy flux of about 10–100 W m⁻², which is dissipated through thermal conduction and compression to heat the low corona. The research team notes that these slow-mode waves and slow-mode shocks are locally re-excited by the plasma flows that enter the corona, rather than being generated in the lower atmosphere and propagating upward to the corona.
Using high-resolution data from joint observations by the IRIS satellite at 2796 Å and SDO/AIA at 171 Å and 193 Å, the research team identified quasiperiodic upward-propagating perturbations commonly present in coronal holes. These perturbations appear as alternating bright and dark slanted ridges in time–distance maps, with measured propagation velocities of approximately 100–150 km/s. The onset of these perturbations typically coincides with the rising phase of spicules in the lower atmosphere. By synthesizing extreme-ultraviolet images from simulation data and analyzing the evolutionary paths of spicules, the team found that these perturbations are jointly caused by the density increase resulting from spicule upflows entering the corona and the temperature increase due to heating by slow-mode waves and slow-mode shocks. The mutual validation between numerical simulations and observational results demonstrates for the first time that slow-mode waves and shocks can be locally excited and efficiently dissipated in the low corona of coronal holes, establishing them as a non-negligible mechanism for coronal heating in open-field regions.
This study moves beyond the conventional view that slow-mode waves struggle to propagate into the corona. It shows that slow-mode waves excited and dissipated within the coronal hole environment also serve as an important mechanism for low coronal heating, thereby enriching the theoretical framework of coronal heating. Additionally, the work provides valuable insights for understanding coronal heating and stellar wind origins in Sun-like stars and in lower-mass stars with stronger convection.
This research is supported by the Strategic Priority Research Program of the Chinese Academy of Sciences; the NSFC grants; the China’s Space Origins Exploration Program; the Yunling Scholar Project of the Yunnan Province and the Yunnan Province Scientist Workshop of Solar Physics; the Yunnan Key Laboratory of Solar Physics and Space Science.
Figure 1: Numerical simulation results of solar spicules. (A-C) The distributions of logarithmic temperature, logarithmic density, and velocity perpendicular to the solar surface for a group of spicules during their rising phase, respectively. The black arrow points to the top of one of the spicules. (D) shows the pressure distribution along the white dashed line in (A). (E) presents the absolute value of the energy density variation rate contributed by different physical terms. (F) and (G) show the height distributions of the average energy fluxes and average mass, respectively. Image by NI.
Figure 2: (A-B) The synthesized emission count rate in AIA 171 Å and 193 Å varying with time along a simulated spicule, the red solid curve and the green dashed curve correspond to the two locations where T = 80,000 and 8000 K, respectively. (C-D) The time-distance plot along a spicule observed in SDO/AIA 171 Å and 193 Å, corresponding to the upward propagating disturbances. (E-F) The time-distance plot along a spicule observed in IRIS 2796 Å, corresponding to the parabolic trajectory of the spicule. Image by NI.
Contact:
NI Lei
Yunnan Observatories, CAS
e-mail:leini@ynao.ac.cn
