scholarly journals On rising magnetic flux tube and formation of sunspots in a deep domain

2020 ◽  
Vol 494 (2) ◽  
pp. 2523-2537 ◽  
Author(s):  
H Hotta ◽  
H Iijima
Keyword(s):  
Flux Tube ◽  
Temperature Region ◽  
Convection Zone ◽  
Solar Surface ◽  
Turbulent Transport ◽  
Numerical Code ◽  
Magnetic Flux Tube ◽  
Computational Domain ◽  
Divergent Flow ◽  
Horizontal Flux

ABSTRACT We investigate the rising flux tube and the formation of sunspots in an unprecedentedly deep computational domain that covers the whole convection zone with a radiative magnetohydrodynamics simulation. Previous calculations had shallow computational boxes (<30 Mm) and convection zones at a depth of 200 Mm. By using our new numerical code Radition and RSST for Deep Dynamics(r2d2), we succeed in covering the whole convection zone and reproduce the formation of the sunspot from a simple horizontal flux tube because of the turbulent thermal convection. The main findings are as follows. (1) The rising speed of the flux tube is larger than the upward convection velocity because of the low density caused by the magnetic pressure and the suppression of the mixing. (2) The rising speed of the flux tube exceeds 250 m s−1 at a depth of 18 Mm, while we do not see any clear evidence of the divergent flow 3 h before the emergence at the solar surface. (3) Initially, the root of the flux tube is filled with the downflows, and then the upflow fills the centre of the flux tube during the formation of the sunspot. (4) The essential mechanisms for the formation of the sunspot are the coherent inflow and the turbulent transport. (5) The low-temperature region is extended to a depth of at least 40 Mm in the matured sunspot, with the high-temperature region in the centre of the flux tube. Some of the findings indicate the importance of the deep computational domain for the flux emergence simulations.

Generation of nonlinear magnetic flux tube wave energies in Procyon A

2019 ◽  
Author(s):  
D E Fawzy ◽  
A T Saygac ◽  
K Stȩpień
Keyword(s):  
Magnetic Flux ◽  
Acoustic Wave ◽  
Flux Tube ◽  
Convection Zone ◽  
Frequency Factor ◽  
Turbulent Energy ◽  
Current Approach ◽  
Magnetic Flux Tube ◽  
Tube Wave ◽  
Wave Modes

Abstract The aim of the current study is the computation of the magnetic flux tube wave energies and fluxes generated in the convection zone of Procyon A. This is a subgiant of spectral type F5 IV-V showing chromospheric and coronal activities. The mechanisms responsible for the generation of different wave modes include the interaction of the thin and vertically oriented magnetic flux tube embedded in magnetic-free regions with turbulence in the convection zone of Procyon A. We are considering longitudinal, transverse and acoustic wave modes. Turbulence in the convection zone is modeled by the extended Kolmogorov turbulent energy spectrum and the modified Gaussian frequency factor. Different magnetic flux tube models with different degrees of magnetic activities were considered. The current approach takes the nonlinear effects into consideration. The results show that there is enough wave energy in the three different forms to heat the outer layers of the star. The obtained acoustic wave energies are larger than those of the longitudinal tube wave energies compared to the solar case. This can be explained by the relatively low magnetic field strength. On the other side, our computations show the importance of the transverse wave energies compared to the energies carried by the longitudinal waves. The former waves carry energy several (between 2 and 14) times higher than the latter. The obtained wave energies are essential for constructing time-dependent model chromospheres and for the predictions of atmospheric oscillations to be compared e.g. with the data collected by the CoRoT and Kepler missions.

Alpha-Effect, Current and Kinetic Helicities for Magnetically Driven Turbulence, and Solar Dynamo

2000 ◽  
Vol 179 ◽  
pp. 387-388
Author(s):  
Gaetano Belvedere ◽  
V. V. Pipin ◽  
G. Rüdiger
Keyword(s):  
Southern Hemisphere ◽  
Northern Hemisphere ◽  
Convection Zone ◽  
Solar Surface ◽  
Momentum Transport ◽  
Angular Momentum Transport ◽  
Magnetic Buoyancy ◽  
Alpha Effect ◽  
Current Helicity

Extended AbstractRecent numerical simulations lead to the result that turbulence is much more magnetically driven than believed. In particular the role ofmagnetic buoyancyappears quite important for the generation ofα-effect and angular momentum transport (Brandenburg & Schmitt 1998). We present results obtained for a turbulence field driven by a (given) Lorentz force in a non-stratified but rotating convection zone. The main result confirms the numerical findings of Brandenburg & Schmitt that in the northern hemisphere theα-effect and the kinetic helicityℋkin= 〈u′ · rotu′〉 are positive (and negative in the northern hemisphere), this being just opposite to what occurs for the current helicityℋcurr= 〈j′ ·B′〉, which is negative in the northern hemisphere (and positive in the southern hemisphere). There has been an increasing number of papers presenting observations of current helicity at the solar surface, all showing that it isnegativein the northern hemisphere and positive in the southern hemisphere (see Rüdigeret al. 2000, also for a review).

Probabilistic Tephra Hazard Assessment of Campi Flegrei, Italy

2021 ◽  
Author(s):  
Beatriz Martínez Montesinos ◽  
Manuel Titos ◽  
Laura Sandri ◽  
Sara Barsotti ◽  
Giovanni Macedonio ◽  
...  
Keyword(s):  
Hazard Assessment ◽  
Wide Spectrum ◽  
Numerical Code ◽  
Campi Flegrei ◽  
Computational Domain ◽  
Event Tree ◽  
Hazard Maps ◽  
Event Tree Analysis ◽  
Short And Long Term

<p>Campi Flegrei is an active volcano located in one of the most densely inhabited areas in Europe and under high-traffic air routes. There, the Vesuvius Observatory’s surveillance system, which continuously monitors volcanic seismicity, soil deformations and gas emissions, highlights some variations in the state of the volcanic activity. It is well known that fragmented magma injected into the atmosphere during an explosive volcanic eruption poses a threat to human lives and air-traffic. For this reason, powerful tools and computational resources to generate extensive and high-resolution hazard maps taking into account a wide spectrum of events, including those of low probability but high impact, are important to provide decision makers with quality information to develop short- and long- term emergency plans. To this end, in the framework of the Center of Excellence for Exascale in Solid Earth (ChEESE), we show the potential of HPC in Probabilistic Volcanic Hazard Assessment. On the one hand, using the ChEESE's flagship Fall3D numerical code and taking advance of the PRACE-awarded resources at CEA/TGCC-HPC facility in France, we perform thousands of simulations of tephra deposition and airborne ash concentration at different flight levels exploring the natural variability and uncertainty on the eruptive conditions on a 3D-grid covering a 2 km-resolution 2000 km x 2000 km computational domain. On the other hand, we create short- and long-term workflows, by updating current Bayesian-Event-Tree-Analysis-based prototype tools, to make them capable of analyze the large amount of information generated by the Fall3D simulations that finally gives rise to the hazard maps for Campi Flegrei.</p>

scholarly journals Convection and Dynamo in Newly Born Neutron Stars

2022 ◽  
Vol 924 (2) ◽  
pp. 75
Author(s):  
Youhei Masada ◽  
Tomoya Takiwaki ◽  
Kei Kotake
Keyword(s):  
Spherical Shell ◽  
Neutron Stars ◽  
Large Scale ◽  
Convection Zone ◽  
Turbulent Transport ◽  
Mean Field ◽  
Magnetic Component ◽  
Large Scale Structures ◽  
Scale Structures ◽  
Convective Model

Abstract To study properties of magnetohydrodynamic (MHD) convection and resultant dynamo activities in proto-neutron stars (PNSs), we construct a “PNS in a box” simulation model and solve the compressible MHD equation coupled with a nuclear equation of state (EOS) and simplified leptonic transport. As a demonstration, we apply it to two types of PNS model with different internal structures: a fully convective model and a spherical-shell convection model. By varying the spin rate of the models, the rotational dependence of convection and the dynamo that operate inside the PNS is investigated. We find that, as a consequence of turbulent transport by rotating stratified convection, large-scale structures of flow and thermodynamic fields are developed in all models. Depending on the spin rate and the depth of the convection zone, various profiles of the large-scale structures are obtained, which can be physically understood as steady-state solutions to the “mean-field” equation of motion. Additionally to those hydrodynamic structures, a large-scale magnetic component of  ( 10 15 ) G is also spontaneously organized in disordered tangled magnetic fields in all models. The higher the spin rate, the stronger the large-scale magnetic component grows. Intriguingly, as an overall trend, the fully convective models have a stronger large-scale magnetic component than that in the spherical-shell convection models. The deeper the convection zone extends, the larger the size of the convective eddies becomes. As a result, rotationally constrained convection seems to be more easily achieved in the fully convective model, resulting in a higher efficiency of the large-scale dynamo there. To gain a better understanding of the origin of the diversity of a neutron star’s magnetic field, we need to study the PNS dynamo in a wider parameter range.

scholarly journals Modeling the 3-d flow around a cylinder using the SAS hybrid model

2014 ◽  
Vol 16 (5) ◽  
pp. 901-918 ◽  
Keyword(s):  
Turbulence Modeling ◽  
Stokes Equations ◽  
Three Dimensional ◽  
Cross Flow ◽  
Numerical Code ◽  
Navier Stokes ◽  
Computational Domain ◽  
Mean Flow ◽  
Unstable Flow ◽  
Vortex Size

<div> <p>Three-dimensional calculations were performed to simulate the flow around a cylindrical vegetation element using the Scale Adaptive Simulation (SAS) model; commonly, this is the first step of the modeling of the flow through multiple vegetation elements. SAS solves the Reynolds Averaged Navier-Stokes equations in stable flow regions, while in regions with unstable flow it goes unsteady producing a resolved turbulent spectrum after reducing eddy viscosity according to the locally resolved vortex size represented by the von Karman length scale. A finite volume numerical code was used for the spatial discretisation of the rectangular computational domain with stream-wise, cross-flow and vertical dimensions equal to 30D, 11D and 1D, respectively, which was resolved with unstructured grids. Calculations were compared with experiments and Large Eddy Simulations (LES). Predicted overall flow parameters and mean flow velocities exhibited a very satisfactory agreement with experiments and LES, while the agreement of predicted turbulent stresses was satisfactory. Calculations showed that SAS is an efficient and relatively fast turbulence modeling approach, especially in relevant practical problems, in which the very high accuracy that can be achieved by LES at the expense of large computational times is not required.</p> </div> <p>&nbsp;</p>

scholarly journals Solar Opacities Constrained by Solar Neutrinos and Solar Oscillations

1990 ◽  
Vol 121 ◽  
pp. 61-80
Author(s):  
Arthur N. Cox
Keyword(s):  
Electric Fields ◽  
Oscillation Frequency ◽  
Convection Zone ◽  
Solar Surface ◽  
Solar Neutrinos ◽  
Photon Absorption ◽  
Solar Oscillations ◽  
Percent Reduction ◽  
Neutrino Production ◽  
Oscillation Frequencies

AbstractThis review discusses the current situation for opacities at the solar center, the solar surface, and for the few million kelvin temperatures that occur below the convection zone. The solar center conditions are important because they are crucial for the neutrino production, which continues to be predicted about 4 times that observed. The main extinction effects there are free-free photon absorption in the electric fields of the hydrogen, helium and the CNO atoms, free electron scattering of photons, and the bound-free and bound-bound absorption of photons by iron atoms with two electrons in the 1s bound level. An assumption that the iron is condensed-out below the convection zone, and the opacity in the central regions is thereby reduced, results in about a 25 percent reduction in the central opacity but only a 5 percent reduction at the base of the convection zone. Furthermore, the p-mode solar oscillations are changed with this assumption, and do not fit the observed ones as well as for standard models. A discussion of the large effective opacity reduction by weakly interacting massive particles (WIMPs or Cosmions) also results in poor agreement with observed p-mode oscillation frequencies. The much larger opacities for the solar surface layers from the Los Alamos Astrophysical Opacity Library instead of the widely used Cox and Tabor values show small improvements in oscillation frequency predictions, but the largest effect is in the discussion of p-mode stability. Solar oscillation frequencies can serve as an opacity experiment for the temperatures and densities, respectively, of a few million kelvin and between 0.1 and 10 g/cm3. Current oscillation frequency calculations indicate that possibly the Opacity Library values need an increase of typically 15 percent just at the bottom of the convection zone at 3×106K. Opacities have uncertainties at the photosphere and deeper than the convection zone ranging from 10 to 25 percent. The equation of state that supplies data for the opacity calculations fortunately has pressure uncertainties of only about 1 percent, but opacity uncertainties will always be much larger. A discussion is given about opacity experiments that the stars provide. Opacities in the envelopes of the Hyades G stars, the Cepheids, δ Scuti variables, and the β Cephei variables indicate that significantly larger opacities, possibly caused by iron lines, seem to be required.

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