Bo Li

Context. Investigating the characteristics and formation mechanisms of plasma upflows at the edges of active regions (ARs) is essential for understanding the coupling between different layers of the solar atmosphere and the origin of the slow solar wind. Aims. The present study aims to statistically characterize AR upflows and investigate their properties and interrelation with the underlying photospheric magnetic field and chromospheric emission. Methods. We performed a statistical analysis of 30 ARs observed with the Extreme ultraviolet Imaging Spectrometer (EIS) on board the Hinode satellite using the Fe XII 195.12 Å line. We also analyzed the corresponding chromospheric 1600 Å intensity from the Atmospheric Imaging Assembly (AIA), the photospheric magnetic field measured by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO), and the associated coronal magnetic field derived from the Potential-field Source-surface (PFSS) model. Results. Regions with blueshifted Fe XII 195.12 Å emission are present in all selected ARs. The blueshift speeds are strongly correlated with the corresponding nonthermal velocities. Compared with the AR same-polarity magnetic field concentration areas, blueshifted regions exhibit significantly weaker coronal intensity, comparable chromospheric intensity, and a weaker photospheric magnetic field. In the upflow regions, the Fe XII line intensity, blueshift speed, and nonthermal velocity are all positively correlated with the corresponding chromospheric intensity and photospheric magnetic field strength. For the ARs with unbalanced magnetic flux, regions with PFSS open magnetic field lines account for 78.6% and 35.7% of the majority- and minority-polarity areas, respectively. The corresponding fraction is 37.5% for ARs with relatively balanced magnetic flux. Blueshift regions with speeds exceeding 7 km s ⁻¹ occur at nearly identical rates in areas with (71.4%) and without (71.9%) PFSS open magnetic field lines. However, their area coverage is approximately 24% larger in regions with open magnetic field lines. Conclusions. The occurrence of AR upflow shows no clear association with PFSS open magnetic field lines, although blueshifted regions associated with open fields are approximately one-quarter larger. This suggests that a substantial fraction of AR boundary upflows may be associated with large-scale closed magnetic structures. The coronal upflow properties are positively correlated with the photospheric magnetic field strength and chromospheric intensity. This result suggests that stronger magnetic fields and enhanced chromospheric activity at the loop footpoints may drive greater upward transport of mass and energy. These findings support a cross-layer coupling scenario in which AR upflows are jointly regulated by the footpoint magnetic field, lower-atmosphere activity, and coronal open magnetic field topology.

Standing transverse waves in closed coronal structures have been widely studied as a possible route to energy dissipation, with resonant absorption transferring kink wave energy to localized Alfvénic motions and the Kelvin–Helmholtz instability (KHI) accelerating the formation of small dissipative scales. However, it remains unclear whether the same mechanism applies to the open corona, given the long-standing consensus that the KHI tends to be prohibited for propagating Alfvénic waves. Within the framework of magnetohydrodynamics (MHD), we perform three-dimensional MHD simulations of boundary-driven kink waves in a gravitationally stratified open flux tube extending from the chromosphere into the corona. We find that propagating waves in open magnetic structures can also drive the system toward a turbulent state, with Kelvin–Helmholtz vortices clearly identifiable across the flux tube. This occurs because resonant absorption transfers energy from the propagating kink waves to azimuthal Alfvénic motions near the tube boundary, and wave reflection off the gradient of the Alfvén speed subsequently enables these boundary motions to acquire a locally standing character. Our results provide a possible answer to the long-standing question of whether and how propagating waves in open magnetic structures can generate nonlinear turbulent fine structures despite their globally propagating nature.

Coronal plumes and chromospheric jetlike structures are believed to be highly dynamic. We report the first direct observations of a propagating kink wave in a chromospheric jetlike structure and its associated plumelet structure in the upper corona of the solar polar region, using data from the High Resolution Imager of the Extreme Ultraviolet Imager on board Solar Orbiter. The dark jetlike structure exhibits transverse oscillation during upward propagation, with a period of approximately 95 s and a displacement of about 193 km. The corresponding plumelet also displays transverse motion, with an oscillation period of around 99 s and a displacement of about 315 km. Given that both the dark jetlike structure and the plumelet share the same magnetic skeleton and have similar oscillation period, we suggest that these oscillations are the same transverse propagating wave originating in the chromosphere. This scenario is further supported by a 3D magnetohydrodynamic (MHD) simulation, in which both vertical and transverse perturbations were introduced in a stratified magnetic flux tube. The simulation successfully reproduces the upward propagation of a kink wave through both the chromospheric jetlike structure and the coronal plumelet. These results highlight the potential role of transverse waves in transferring energy from the lower solar atmosphere to the corona.

Context. Little attention has been paid to group velocities of three-dimensional (3D) magnetohydrodynamic (MHD) waves in solar coronal seismology. Aims. This study presents a comprehensive examination of the group velocities of trapped 3D kink modes in coronal slabs, emphasizing the connection of mode analysis to both mode characterization and impulsively excited 3D kink waves. Methods. We worked in linear, ideal, pressureless MHD, and took the equilibrium slab to be symmetrically structured only in one transverse direction. The dispersion relation was numerically solved, and the results were understood by making in-depth analytical progress. We addressed both the transverse fundamental and its first overtone. Results. We developed a three-subgroup scheme for categorizing 3D kink modes on the plane spanned by the axial and out-of-plane wavenumbers. The group ( v _gr ) and phase velocities ( v _ph ) lie on the same side of the equilibrium magnetic field ( B ₀ ) for the B ₀ -same-side A and B ₀ -same-side F subgroups, which are further discriminated by the directional similarity of v _gr and B ₀ . The B ₀ -straddling subgroup is peculiar in that v _gr and v _ph lie astride B ₀ , a feature that cannot be found for waves in unbounded uniform media in pressureless MHD. This B ₀ -straddling subgroup pertains to both the fundamental and its overtones. We further place our results in the context of impulsive waves, employing the method of stationary phase to predict the large-time wavefront morphology in the plane of symmetry of the equilibrium slab. Wavefronts directed toward B ₀ derive exclusively from B ₀ -straddling modes, and are confined to narrow sectors. Conclusions. The directional information of impulsively excited 3D wavefronts carries rich seismic information, whose inversion requires a thorough understanding of the behavior of group velocities of 3D modes.

Context. Kink oscillation frequency is a key parameter for coronal seismology. It is still unclear how gravitational stratification affects the kink frequency in curved coronal loops. Aims. This work aims to investigate the effect of gravitational stratification on the frequency of kink oscillations in curved coronal loops and discuss their seismological potential. Methods. We conducted numerical computations within the ideal magnetohydrodynamic framework to study different kink polarizations and harmonics in a curved, gravitationally stratified coronal loop. The oscillation frequencies derived from the Lagrangian displacement were compared with the Wentzel-Kramers-Brillouin (WKB) approximation. Results. For the vertically polarized fundamental mode, the oscillation frequency deviates from the WKB approximation by about 18% in the current numerical setup. Nevertheless, the oscillation frequency closely matches the local Alfvén frequency near the loop apex. On the other hand, the frequency of the horizontally polarized fundamental mode exhibits only a 7% deviation in our current model from the WKB approximation and closely matches the local Alfvén frequency near one quarter of the loop. For the first overtones, the frequencies for both polarizations can be well described by the WKB approximation. Conclusions. The frequency of vertically polarized fundamental kink modes can be predicted by the local Alfvén frequency near the loop apex. In contrast, the WKB approximation remains highly reliable for estimating the frequency of horizontally polarized fundamental modes and first overtones, which is also well described by the local Alfvén frequency near one quarter of the loop. These results therefore pave the way for spatially dependent coronal seismology, enabling, for example, the probing of magnetic field strength at different locations along a coronal loop.

Quasiperiodic fast-propagating (QFP) waves excited by an impulsive, spatially localized velocity perturbation in a curved coronal loop are investigated with three-dimensional numerical simulations. The equilibrium consists of a density-enhanced loop embedded in a magnetic arcade. The initial perturbation is a velocity pulse with either sausage symmetry or kink symmetry (vertically or horizontally polarized), applied near one loop footpoint. Fast magnetoacoustic wave trains guided along the loop (i.e., trapped fast wave trains) arise in both cases due to waveguide dispersion. However, in the kink case, the trapped wave trains are less pronounced than in the sausage case because kink modes exhibit weaker dispersion and weaker compression. Leaky fast magnetoacoustic wave trains also form for all considered symmetries. In the sausage case, the leaky-wave density perturbation is symmetric about the loop plane; in the kink cases it is either symmetric or antisymmetric, depending on whether the excitation is vertically or horizontally polarized. Forward modelling of the observational manifestation of the waves in the optically thin regime shows that trapped wave trains are detectable for all examined viewing angles, whereas leaky-wave signals can be very faint for certain viewing directions. These results support the notion that coronal QFP waves can be excited by a velocity pulse and suggest that the infrequent detection of broad QFP waves may stem from line-of-sight integration effects.

Coronal cavities, often associated with prominences, are crucial structures in understanding coronal heating and the eruption mechanism of coronal mass ejections. Previous studies have identified their lower density, higher temperature, and flux-rope structures. However, spectroscopic observations are still relatively scarce. In this study, we utilize the newly developed Spectral Imaging Coronagraph (SICG), Chinese H α Solar Explorer, and Atmospheric Imaging Assembly (AIA) Solar Dynamics Observatory to analyze the morphology, temperature, Doppler shift, and nonthermal velocity of two coronal cavities observed on 2024 November 13. We find that coronal cavities are distinctly visible in SICG Fe xiv 5303 Å and AIA 193 Å, whereas they are nearly absent in SICG Fe x 6374 Å and AIA 171 Å. The spectroscopic measurements show that the two coronal cavities display asymmetric, ringlike structures in the Fe xiv 5303 Å Doppler shift maps. The nonthermal velocities inside coronal cavities are significantly higher than those of the surrounding streamer areas. In addition, the core regions of coronal cavities, located directly above the prominences, exhibit the highest nonthermal velocities and Doppler velocities. Our results suggest the presence of waves and turbulence in coronal cavities, which are likely more intense than those in the adjacent streamer regions. We suggest that the interaction and exchange between the cold, dense prominence materials and the hot, low-density coronal materials are the main drivers of the waves and turbulence inside coronal cavities.

Remote brightening (RB) refers to brightening at a footpoint of magnetic loops in the solar atmosphere, which is passively triggered by an eruption occurring at the other footpoint. Here, we present observations of an RB event driven by an extreme-ultraviolet (EUV) jet. The leading front of the jet exhibits flows exceeding the characteristic sound speed, and it triggers RBs at two remote sites that are connected by coronal loops. We focus on the RB that occurred at one of the footpoints, in which the magnetic structure is characterized by a dome-like feature with magnetic null points. When the jet propagates to this footpoint, the brightening displays a distinct inverted-Y-shaped structure. Bidirectional flows with a velocity of about 30 km s ⁻¹ are detected in any two branches of this structure, which are suggestive of the occurrence of magnetic reconnection at this footpoint. To investigate the physics of the interaction between the jet and the remote magnetic null point, we conducted a 2D MHD simulation with adaptive mesh refinement. We find that the collapse of the magnetic null point is induced by a fast-mode wave, and the flow associated with the jet elongates the current sheet, thereby triggering plasmoid instability. Subsequent magnetic reconnection not only forms the bright inverted-Y-shaped structure but also drives bidirectional flows along the structures, being consistent with the observational findings. This study provides new insights into the dynamic interaction between EUV jets and remote magnetic nulls in the solar atmosphere, and helps understand the diverse natures of RBs.