Takaaki Yokoyama

We performed numerical simulations to investigate how coronal heating and magnetic field strength regulate the dynamics and morphology of coronal rain in the solar corona. Coronal rain is widely interpreted as a manifestation of thermal nonequilibrium and thermal instability. Many previous studies have investigated condensation under (quasi-)steady heating scenarios, but such heating models do not fully explain the observed properties, including multiple clumps and symmetric drainage toward both loop footpoints. Condensation driven by low-frequency impulsive heating has been much less explored and may produce distinct coronal rain morphology and drainage dynamics. The role of the coronal magnetic field also remains poorly understood. We conducted 1.5-dimensional magnetohydrodynamic simulations along a single strand with self-consistent coronal heating, upon which a single impulsive heating event was imposed. Two key parameters were systematically explored: the duration of localized heating and the strength of the coronal background magnetic field. We find that condensation can be triggered even by a single impulsive heating event, provided that the injected energy is about an order of magnitude larger than that in the steady heating case. Condensation becomes more likely for longer heating durations and weaker magnetic fields. The dynamics of coronal rain depend strongly on the heating duration. When the heating duration is shorter than the radiative cooling timescale, condensations drain toward both loop footpoints. We also find that weaker magnetic fields enhance the nonlinearity of Alfvén waves, promoting fragmented condensations. These results demonstrate that the heating duration and coronal magnetic field strength play key roles in regulating coronal rain formation and dynamics.

Rotation and magnetic fields in the cores of evolved massive stars in their final phase are thought to play an important role in the subsequent supernova explosion and the formation of a compact object, especially in hyperenergetic explosions. However, the interplay between rotation, magnetic fields, and convection up to the final collapse is a nonlinear, multidimensional effect that is difficult to capture with standard one-dimensional (1D) stellar evolution models. We quantify the magnetic angular momentum (AM) transport in the convective oxygen-burning shell in a three-dimensional (3D) rotating core-collapse progenitor model. We find that the radial direction of magnetic AM transport is directly related to the Rossby number of the convective oxygen shell. We also analyze the magnetic energy, which sets the amplitude of the magnetic AM flux. The magnetic energy is determined both by rotation and the nuclear energy generation rate, in a manner analogous to that for low-mass stars like the Sun. Based on these results, we construct a 1D model of magnetic AM transport in the convection zone for the first time in terms of properties of a given stellar evolution model. This model successfully reproduces the AM transport in the 3D model when the convective dynamo is in a quasi-steady state. Notably, our model for radial AM transport is the first to account for inward AM flux. This may result in interesting differences compared to the conventional treatment of magnetic AM transport in stellar evolution models, which assume AM is transported outward by a purely diffusive process.

Magnetic activity in solar-like stars shows a systematic dependence on stellar rotation rate, known as the “activity–rotation relationship.” Recent observations further reveal that large-scale surface magnetic fields in solar-like stars are anticorrelated with the Rossby number (Ro) in the moderate regime (0.1 < Ro < 1), reaching saturation at extremely low Rossby numbers (Ro < 0.1). The cause of this saturation remains one of the open issues in stellar dynamo theory. We performed nonlinear mean-field dynamo simulations with the stellar rotation rate and turbulent diffusivity ( ν ₀ ) as free parameters. The dependence of the magnetic field strength ( B ) in the deep convection zone is quantified as ( B ∝ Ro − 0.45 ν 0 0.54 ). Our analysis of the differential rotation energy shows that this dependence is maintained by a balance between the angular momentum transport by the turbulence and the large-scale magnetic field. Together with the reduction of the convective velocity revealed at extremely low Rossby numbers by recent local three-dimensional simulations, our results suggest that inefficient turbulent angular momentum transport might drive the saturation of the large-scale magnetic field.

Magnetic flux ropes are fundamental magnetic structures in solar eruptions, whose formation is generally attributed to: (1) the emergence of subsurface flux tubes; or (2) flux cancellation driven by photospheric horizontal flows and magnetic reconnection. Both mechanisms can operate simultaneously during active region (AR) evolution, making their relative contributions challenging to quantify. Here, we analyze the formation of a flux rope in a MURaM radiative magnetohydrodynamic simulation, which formed and evolved for approximately 3 hr before an M-class flare. The formation process is quantified by the magnetic helicity flux, which drives the nonpotential evolution of the magnetic field, with its advection and shear terms on the photosphere corresponding to the emergence and photospheric horizontal flows, respectively. Examining the helicity injected into the flux rope through the photosphere, we find both terms increase significantly as the eruption approaches, with the shear term prevailing overall. Height-dependent analysis of the helicity flux, together with the magnetic field and velocity distributions, further reveals a gradual transition from the shear to the advection term with an increasing altitude, which is driven by magnetic reconnection above the photosphere. Our results provide quantitative evidence that flux cancellation governs flux rope formation, arising naturally from magnetic field reorganization during AR evolution: as the flux emergence transports the magnetic flux upward, photospheric shearing motions adjust the magnetic field and inject helicity into the solar atmosphere, and magnetic reconnection ultimately assembles the main body of flux ropes.

Magnetic flux ropes (MFRs) are fundamental magnetic structures in solar eruptions, whose formation is generally attributed to (1) the emergence of subsurface flux tubes or (2) flux cancellation driven by photospheric horizontal flows and magnetic reconnection. Both mechanisms can operate simultaneously during active region evolution, making their relative contributions challenging to quantify. Here, we analyze the formation of a flux rope in a MURaM radiative magnetohydrodynamic (RMHD) simulation, which formed and evolved for approximately three hours before an M-class flare. The formation process is quantified by magnetic helicity flux, which drives the non-potential evolution of magnetic field, with its advection and shear terms on the photosphere corresponding to the emergence and photospheric horizontal flows, respectively. Examining the helicity injected into the flux rope through the photosphere, we find both terms increase significantly as the eruption approaches, with the shear term prevailing overall. Height-dependent analysis of helicity flux, together with magnetic field and velocity distributions, further reveals a gradual transition from the shear to the advection term with an increasing altitude, which is driven by magnetic reconnection above the photosphere. Our results provide quantitative evidence that flux cancellation governs flux rope formation, arising naturally from magnetic field reorganization during active region evolution: as flux emergence transports magnetic flux upward, photospheric shearing motions adjust magnetic field and inject helicity into solar atmosphere, and magnetic reconnection ultimately assembles the main body of flux ropes.

Magnetic reconnection is an important driving mechanism of many chromospheric phenomena, e.g., UV bursts and chromospheric jets. Information about magnetic fields is indispensable for analyzing chromospheric magnetic reconnection, which is mainly encoded in polarization signals. The purpose of this work is to predict possible Stokes features related to chromospheric reconnection events, from realistic two-dimensional magnetohydrodynamic simulation and Stokes profile synthesis. An emerging magnetic flux sheet is imposed at the bottom boundary of a well-relaxed unipolar atmosphere that spans from the upper convection zone to the corona. The reconnection region is heated to ∼7 kK and the outflow velocity reaches up to ∼35 km s ⁻¹ . Through Stokes profile synthesis, several Stokes features related to reconnections and plasmoids are reproduced. We found sign reversal features on circular polarization and amplitude reduction features on linear polarization at reconnection sites. Also, we report strong linear and circular polarization signals corresponding to huge (∼300 km) and tiny (∼40 km) plasmoids, respectively. We conclude that both linear and circular polarization signals may reveal the distinctive physical mechanisms in reconnections and enhance our understanding of magnetic reconnection in observations.

The coronal heating problem is one of the most critical challenges in solar physics. Recent observations have revealed that small-scale swirls are ubiquitous in the photosphere and chromosphere, suggesting that they may play a significant role in transferring magnetic energy into the corona. However, the overall contribution of swirls to the total magnetic energy supply and subsequent coronal heating remains uncertain. To address this, we perform statistical analyses of simulated swirls using a three-dimensional radiative magnetohydrodynamic simulation extending from the convection zone to the corona in the quiet Sun. Our results reveal that swirls account for approximately half of the total magnetic energy. Furthermore, they strongly suggest that swirls can trigger coronal heating events through magnetic reconnection. The occurrence frequency of these events follows a power-law-like distribution, consistent with observations of coronal heating signatures known as “nanoflares,” indicating that swirls are promising candidates as their drivers.

The coronal heating problem is one of the most critical challenges in solar physics. Recent observations have revealed that small-scale swirls are ubiquitous in the photosphere and chromosphere, suggesting that they may play a significant role in transferring magnetic energy into the corona. However, the overall contribution of swirls to the total magnetic energy supply and subsequent coronal heating remains uncertain. To address this, we perform statistical analyses of simulated swirls using a three-dimensional radiative magnetohydrodynamic simulation extending from the convection zone to the corona in the quiet Sun. Our results reveal that swirls account for approximately half of the total magnetic energy. Furthermore, they strongly suggest that swirls can trigger coronal heating events through magnetic reconnection. The occurrence frequency of these events follows a power-law-like distribution, consistent with observations of coronal heating signatures known as "nanoflares", indicating that swirls are promising candidates as their drivers.

This paper presents a comparison of plasma dynamics in coronal holes (CHs) and quiet Sun (QS) through 2.5D MHD flux emergence simulations. The magnetic reconnection between the emerging and the preexisting flux leads to the formation of cool, dense plasmoids with hot boundaries, and hot and cool jets with velocities ≈50 km s⁻¹. We perform a spectral synthesis in spectral lines probing transition region and coronal temperatures. CHs show reduced intensities, excess upflows (downflows), and widths during the jetting (downflow) period when compared to QS. During the jetting and downflow periods, the velocity and line width of the hot spectral lines in CHs show a strong positive correlation with the vertical magnetic field at z = 0, while the intensity of the cooler lines shows a weak correlation, which is not seen in QS. During the jetting period in CH, we find upflows in Si iv to be correlated (anticorrelated) with upflows (downflows) in other lines, and downflows in CH in Si iv to be correlated (anticorrelated) with upflows (downflows) in other lines when compared to QS. During the downflow, we find no strong correlation between Si iv and other line velocities. The correlation during the jetting period occurs due to coincident, cospatial origins of the hot and cool jet, while the lack of correlation during the downflow phase suggests a decoupling of hot and cool plasma. These results demonstrate that flux emergence and reconnection with preexisting flux in the atmosphere support a unified scenario for solar wind formation and coronal heating.

We performed numerical simulations to study mechanisms of solar prominence formation triggered by a single heating event. In the widely accepted “chromospheric-evaporation condensation” model, localized heating at footpoints of a coronal loop drives plasma evaporation and eventually triggers condensation. The occurrence of condensation is strongly influenced by the characteristics of the heating. Various theoretical studies have been conducted along one-dimensional field lines with quasi-steady localized heating. The quasi-steady heating is regarded as the collection of multiple heating events among multiple strands constituting a coronal loop. However, it is reasonable to consider a single heating event along a single field line as an elemental unit. We investigated the condensation phenomenon triggered by a single heating event using 1.5-dimensional magnetohydrodynamic simulations. By varying the magnitude of the localized heating rate, we explored the conditions necessary for condensation. We found that, when a heating rate approximately ~10⁴ times greater than that of steady heating was applied, condensation occurred. Condensation was observed when the thermal conduction efficiency in the loop became lower than the cooling efficiency, with the cooling rate significantly exceeding the heating rate. Using the loop length L and the Field length λ _F, the condition for condensation is expressed as λ _F ≲ L/2 under conditions where cooling exceeds heating. We extended the analytically derived condition for thermal nonequilibrium to a formulation based on heating amount.