Hideyuki Hotta

Evidence for the late formation of hydrous asteroids from young meteoritic carbonates

The accretion of small bodies in the Solar System is a fundamental process that was followed by planet formation. Chronological information of meteorites can constrain when asteroids formed. Secondary carbonates show extremely old (53)Mn-(53)Cr radiometric ages, indicating that some hydrous asteroids accreted rapidly. However, previous studies have failed to define accurate Mn/Cr ratios; hence, these old ages could be artefacts. Here we develop a new method for accurate Mn/Cr determination, and report a reliable age of 4,563.4+0.4/-0.5 million years ago for carbonates in carbonaceous chondrites. We find that these carbonates have identical ages, which are younger than those previously estimated. This result suggests the late onset of aqueous activities in the Solar System. The young carbonate age cannot be explained if the parent asteroid accreted within 3 million years after the birth of the Solar System. Thus, we conclude that hydrous asteroids accreted later than differentiated and metamorphosed asteroids.

Solar Parity Issue with Flux-transport Dynamo

We investigated the dependence of the solar magnetic parity between the hemispheres on two important parameters, the turbulent diffusivity and the meridional flow, by means of axisymmetric kinematic dynamo simulations based on the flux-transport dynamo model. It is known that the coupling of the magnetic field between hemispheres due to turbulent diffusivity is an important factor for the solar parity issue, but the detailed criterion for the generation of the dipole field has not been investigated. Our conclusions are as follows. (1) The stronger diffusivity near the surface is more likely to cause the magnetic field to be a dipole. (2) The thinner layer of the strong diffusivity near the surface is also more apt to generate a dipolar magnetic field. (3) The faster meridional flow is more prone to cause the magnetic field to be a quadrupole, i.e., symmetric about the equator. These results show that turbulent diffusivity and meridional flow are crucial for the configuration of the solar global magnetic field.

Large-scale magnetic fields at high Reynolds numbers in magnetohydrodynamic simulations

The 11-year solar magnetic cycle shows a high degree of coherence in spite of the turbulent nature of the solar convection zone. It has been found in recent high-resolution magnetohydrodynamics simulations that the maintenance of a large-scale coherent magnetic field is difficult with small viscosity and magnetic diffusivity (≲1012square centimenters per second). We reproduced previous findings that indicate a reduction of the energy in the large-scale magnetic field for lower diffusivities and demonstrate the recovery of the global-scale magnetic field using unprecedentedly high resolution. We found an efficient small-scale dynamo that suppresses small-scale flows, which mimics the properties of large diffusivity. As a result, the global-scale magnetic field is maintained even in the regime of small diffusivities—that is, large Reynolds numbers.

High-resolution Calculations of the Solar Global Convection with the Reduced Speed of Sound Technique. I. The Structure of the Convection and the Magnetic Field without the Rotation

We carry out non-rotating high-resolution calculations of the solar global convection, which resolve convective scales of less than 10?Mm. To cope with the low Mach number conditions in the lower convection zone, we use the reduced speed of sound technique (RSST), which is simple to implement and requires only local communication in the parallel computation. In addition, the RSST allows us to expand the computational domain upward to about 0.99 R ?, as it can also handle compressible flows. Using this approach, we study the solar convection zone on the global scale, including small-scale near-surface convection. In particular, we investigate the influence of the top boundary condition on the convective structure throughout the convection zone as well as on small-scale dynamo action. Our main conclusions are as follows. (1) The small-scale downflows generated in the near-surface layer penetrate into deeper layers to some extent and excite small-scale turbulence in the region >0.9 R ?, where R ? is the solar radius. (2) In the deeper convection zone (<0.9 R ?), the convection is not influenced by the location of the upper boundary. (3) Using a large eddy simulation approach, we can achieve small-scale dynamo action and maintain a field of about 0.15B eq-0.25B eq throughout the convection zone, where B eq is the equipartition magnetic field to the kinetic energy. (4) The overall dynamo efficiency varies significantly in the convection zone as a consequence of the downward directed Poynting flux and the depth variation of the intrinsic convective scales.

IMPORTANCE OF SURFACE TURBULENT DIFFUSIVITY IN THE SOLAR FLUX-TRANSPORT DYNAMO

A revision to the flux-transport dynamo model for the solar sunspot cycle is proposed and is demonstrated by using the axisymmetric kinematic simulations. The flux-transport dynamo has succeeded to explain the general cyclic behaviors of the sunspots. It has been known, however, that previous models failed to avoid the strong polar surface field and the strong toroidal field at the base in the high latitude, both of which are not consistent with observations. We propose a new regime of the flux-transport dynamo model by assuming an additional intense diffusivity profile near the surface. The surface poloidal field generated by the α effect is transported down to the base of the convection zone not by the meridional flow but by the surface diffusion mainly in the mid-latitude. With a moderate α quenching, this prevents the concentration of the polar surface field and the amplification of the toroidal field at the high latitude. The condition to obtain the proper magnetic field strength near the pole is ηsurf/u 0>2 × 109 cm, where ηsurf and u 0 are the surface diffusivity and the meridional flow speed, respectively. We also do some parameter studies to ensure the importance of the surface strong diffusivity. In addition, the dependence of the cycle period on free parameters, the speed of meridional flow and the surface diffusivity, is investigated.

MODELING OF DIFFERENTIAL ROTATION IN RAPIDLY ROTATING SOLAR-TYPE STARS

We investigate differential rotation in rapidly rotating solar-type stars by means of an axisymmetric mean field model that was previously applied to the Sun. This allows us to calculate the latitudinal entropy gradient with a reasonable physical basis. Our conclusions are as follows. (1) Differential rotation approaches the Taylor-Proudman state when stellar rotation is faster than solar rotation. (2) Entropy gradient generated by the attached subadiabatic layer beneath the convection zone becomes relatively small with a large stellar angular velocity. (3) Turbulent viscosity and turbulent angular momentum transport determine the spatial difference of angular velocity ΔΩ. (4) The results of our mean field model can explain observations of stellar differential rotation.

Numerical calculation of convection with reduced speed of sound technique

Context. The anelastic approximation is often adopted in numerical calculation with low Mach number, such as stellar internal convection. This approximation requires frequent global communication, because of an elliptic partial differential equation. Frequent global communication is negative factor for the parallel computing with a large number of CPUs. Aims. The main purpose of this paper is to test the validity of a method that artificially reduces the speed of sound for the compressible fluid equations in the context of stellar internal convection. The reduction of speed of sound allows for larger time steps in spite of low Mach number, while the numerical scheme remains fully explicit and the mathematical system is hyperbolic and thus does not require frequent global communication. Methods. Two and three dimensional compressible hydrodynamic equations are solved numerically. Some statistical quantities of solutions computed with different effective Mach numbers (due to reduction of speed of sound) are compared to test the validity of our approach. Results. Numerical simulations with artificially reduced speed of sound are a valid approach as long as the effective Mach number (based on the reduced speed of sound) remains less than 0.7.

Efficient small-scale dynamo in the solar convection zone

We investigate small-scale dynamo action in the solar convection zone through a series of high resolution MHD simulations in a local Cartesian domain with 1

Estimation of Turbulent Diffusivity with Direct Numerical Simulation of Stellar Convection

R_\odot

Generation of twist on magnetic flux tubes at the base of the solar convection zone

(solar radius) of horizontal extent and a radial extent from 0.715 to 0.96