Takehiro Miyagoshi

Filamentary structure on the Sun from the magnetic Rayleigh-Taylor instability.

Magnetic flux emerges from the solar surface as dark filaments connecting small sunspots with opposite polarities. The regions around the dark filaments are often bright in X-rays and are associated with jets. This implies plasma heating and acceleration, which are important for coronal heating. Previous two-dimensional simulations of such regions showed that magnetic reconnection between the coronal magnetic field and the emerging flux produced X-ray jets and flares, but left unresolved the origin of filamentary structure and the intermittent nature of the heating. Here we report three-dimensional simulations of emerging flux showing that the filamentary structure arises spontaneously from the magnetic Rayleigh–Taylor instability, contrary to the previous view that the dark filaments are isolated bundles of magnetic field that rise from the photosphere carrying the dense gas. As a result of the magnetic Rayleigh–Taylor instability, thin current sheets are formed in the emerging flux, and magnetic reconnection occurs between emerging flux and the pre-existing coronal field in a spatially intermittent way. This explains naturally the intermittent nature of coronal heating and the patchy brightenings in solar flares.

Formation of current coils in geodynamo simulations.

Computer simulations have been playing an important role in the development of our understanding of the geodynamo, but direct numerical simulation of the geodynamo with a realistic parameter regime is still beyond the power of today’s supercomputers. Difficulties in simulating the geodynamo arise from the extreme conditions of the core, which are characterized by very large or very small values of the non-dimensional parameters of the system. Among them, the Ekman number, E, has been adopted as a barometer of the distance of simulations from real core conditions, in which E is of the order of 10-15. Following the initial computer simulations of the geodynamo, the Ekman number achieved has been steadily decreasing, with recent geodynamo simulations performed with E of the order of 10-6. Here we present a geodynamo simulation with an Ekman number of the order of 10-7—the highest-resolution simulation yet achieved, making use of 4,096 processors of the Earth Simulator. We have found that both the convection flow and magnetic field structures are qualitatively different from those found in larger-Ekman-number dynamos. The convection takes the form of sheet plumes or radial sheet jets, rather than the columnar cell structures that are usually found. We have found that this sheet plume convection is an effective dynamo and the generated current is organized as a set of coils in the shape of helical springs or at times as a torus.

Zonal flow formation in the Earth's core.

Zonal jets are very common in nature. Well-known examples are those in the atmospheres of giant planets and the alternating jet streams found in the Earth’s world ocean. Zonal flow formation in nuclear fusion devices is also well studied. A common feature of these zonal flows is that they are spontaneously generated in turbulent systems. Because the Earth’s outer core is believed to be in a turbulent state, it is possible that there is zonal flow in the liquid iron of the outer core. Here we report an investigation at the current low-viscosity limit of numerical simulations of the geodynamo. We find a previously unknown convection regime of the outer core that has a dual structure comprising inner, sheet-like radial plumes and an outer, westward cylindrical zonal flow. We numerically confirm that the dual-convection structure with such a zonal flow is stable under a strong, self-generated dipole magnetic field.

Magnetohydrodynamic Simulation of Solar Coronal Chromospheric Evaporation Jets Caused by Magnetic Reconnection Associated with Magnetic Flux Emergence

We studied solar coronal X-ray jets by MHD numerical simulations with heat conduction effects based on a magnetic reconnection model. Key physical processes are included, such as the emergence of magnetic flux from the convection zone, magnetic reconnection with the coronal magnetic fields, heat conduction to the chromosphere, and chromospheric evaporation. Radiation, however, has been neglected. High-density evaporation jets were successfully reproduced in the simulations. The mass of the evaporation jets M is described as M = 6.8 × 1012g(B/10 G)15/7(Tcor/106 K)5/14(L/5000 km)12/7(t/400 s), where B is the strength of magnetic fields, Tcor is the coronal temperature, L is the loop height, and t is the duration of ejection, respectively. We also derived a theoretical model of the Mach number of the reconnection jets as a function of ambient coronal variables. Numerical simulations also show that two different types of jets (evaporation jets and low-density jets) exist simultaneously around the emerging flux region, and the energy of evaporation jets is somewhat larger than that of the low-density jets.

Magnetohydrodynamic Numerical Simulations of Solar X-Ray Jets Based on the Magnetic Reconnection Model That Includes Chromospheric Evaporation

We studied solar coronal X-ray jets by MHD numerical simulations with thermal conduction effects based on the magnetic reconnection model. Key physical processes are included, such as the emergence of magnetic fluxes from the convection zone, magnetic reconnection with the coronal magnetic fields, heat conduction to the chromosphere, and chromospheric evaporation. High-density evaporation jets were successfully reproduced in the simulations. The mass of the evaporation jets M is described as M = 6.8 × 1012 g(B/10 G)15/7(Tcor/106 K)5/14 × (sflare/5000 km)12/7(t/400 s), where B is the magnetic field strength, Tcor is the coronal temperature, sflare is the loop height, and t is the duration of the ejection.

ON THE VIGOR OF MANTLE CONVECTION IN SUPER-EARTHS

Numerical models are presented to clarify how adiabatic compression affects thermal convection in the mantle of super-Earths ten times the Earths mass. The viscosity strongly depends on temperature, and the Rayleigh number is much higher than that of the Earths mantle. The strong effect of adiabatic compression reduces the activity of mantle convection; hot plumes ascending from the bottom of the mantle lose their thermal buoyancy in the middle of the mantle owing to adiabatic decompression, and do not reach the surface. A thick lithosphere, as thick as 0.1 times the depth of the mantle, develops along the surface boundary, and the efficiency of convective heat transport measured by the Nusselt number is reduced by a factor of about four compared with the Nusselt number for thermal convection of incompressible fluid. The strong effect of adiabatic decompression is likely to inhibit hot spot volcanism on the surface and is also likely to affect the thermal history of the mantle, and hence, the generation of magnetic field in super-Earths.

Dependence of the Magnetic Energy of Solar Active Regions on the Twist Intensity of the Initial Flux Tubes

We present a series of numerical experiments that model the evolution of magnetic flux tubes with a different amount of initial twist. As a result of calculations, tightly twisted tubes reveal a rapid two-step emergence to the atmosphere with a slight slowdown at the surface, while weakly twisted tubes show a slow two-step emergence waiting longer the secondary instability to be triggered. This picture of the two-step emergence is highly consistent with recent observations. These tubes show multiple magnetic domes above the surface, indicating that the secondary emergence is caused by interchange mode of magnetic buoyancy instability. As for the weakest twist case, the tube exhibits an elongated photospheric structure and never rises into the corona. The formation of the photospheric structure is due to inward magnetic tension force of the azimuthal field component of the rising flux tube (i.e., tubes twist). When the twist is weak, azimuthal field cannot hold the tubes coherency, and the tube extends laterally at the subadiabatic surface. In addition, we newly find that the total magnetic energy measured above the surface depends on the initial twist. Strong twist tubes follow the initial relation between the twist and the magnetic energy, while weak twist tubes deviates from this relation, because these tubes store their magnetic energy in the photospheric structures.

Formation of sheet plumes, current coils, and helical magnetic fields in a spherical magnetohydrodynamic dynamo

Aiming at understanding of magnetic field generation process in rapidly rotating stars and planets represented by the Earth, computer simulations of magnetohydrodynamic (MHD) dynamo were performed in a rotating spherical shell geometry. Thermal convection and dynamo process with Ekman number of the order of 10−7 were studied. New structures of convection motion, dynamo-generated electrical current, and magnetic field are found. The flow is organized as a set of thin, sheet-like plumes. The current is made of small-scale coil structure with magnetic flux tubes within each of the coil. These flux tubes are connected each other to form a large scale helical magnetic field structure.

Thermal convection and the convective regime diagram in super-Earths

Numerical models of bottom-heated thermal convection of highly compressible fluid with strongly temperature-dependent viscosity are presented to understand how the Rayleigh number Ra and the temperature dependence of viscosity exert control over the regimes of thermal convection in massive super-Earths. Thermodynamic properties of mantle materials are pressure dependent, but other material properties including the viscosity are not. A stagnant lid develops along the surface of the planet, when the viscosity contrast across the mantle due to temperature dependence r exceeds 106 at high Rayleigh number relevant to super-Earths. The threshold in r, which increases with increasing Ra, is higher than that expected for the Earth from earlier Boussinesq models. The efficiency of convective heat transport measured by the Nusselt number Nu is considerably lower than that expected from Boussinesq models; Nu depends on Ra and r as Nu = 59 ⋅ r− 0.23 ⋅ (Ra/109)0.27, when r ≤ 105. Strong adiabatic compression significantly reduces the activity of hot ascending plumes especially at high r. At r relevant for super-Earths, hot ascending plumes lose their buoyancy on their way and hardly reach the surface boundary: hot spot volcanism due to ascending plumes is probably suppressed on super-Earths. The lithosphere is considerably thicker than that suggested by earlier Boussinesq models and is unlikely to show a plate-like behavior.

Extremely long transition phase of thermal convection in the mantle of massive super-Earths

Adiabatic compression is a key factor that exerts control over thermal convection in the compressible solid mantle of super-Earths. To discuss the effects of adiabatic compression, we present a numerical model of transient convection in the cooling mantle of a super-Earth that is ten times larger in size than the Earth. The calculations started with the shallow mantle that was hotter than expected by the extrapolation from the deep mantle conditions. This type of initial thermal state of the mantle is expected to naturally occur in real super-Earths due to heating by giant impacts at the time of their formation. With our initial setup conditions, the convection temporarily occurs as a layered convection for the first several to ten billion years of the calculation and then changes its style into a whole layer convection. The long duration of the transient stage suggests that mantle convection currently occurs as a temporal layered convection in many of the super-Earths. A temporal layered convection, if it occurs, can exert control over the tectonic activities of super-Earths. Future studies should clarify how internal heating and complicated rheological properties of mantle materials including their pressure dependence affect the duration of the temporal layered convection.Graphical abstract.