explanation blue bibcodes open ADS page with paths to full text
Author name code: yokoi
ADS astronomy entries on 2022-09-14
author:"Yokoi, Nobumitsu"
---------------------------------------------------------
Title: Modeling turbulent transport associated with plumes in
stellar convection
Authors: Yokoi, Nobumitsu; Masada, Youhei; Takiwaki, Tomoya
2022cosp...44.2554Y Altcode:
Direct numerical simulations (DNSs) of astrophysical turbulent flows
with real physical parameters, resolving all the scales of motion
ranging from the largest to smallest scales without resorting to
any artificial viscosity, are just impossible. In this situation,
modeling the effective turbulent transport provides a powerful tool
for analysing turbulent flows of astrophysical interests. The solar
and stellar convection is one of such astrophysical problems. There,
small-scale plumes and jets are considered to play an important
role in determining the effective transport such as the turbulent
mass, momentum and heat fluxes. Plumes, local and temporary fluid
motions, are generated by surface cooling and/or bottom heating in the
stellar convection zone. They are coherent structures in turbulence,
but will be smeared out under a simple space or ensemble averaging
procedure. Local dynamical and statistical properties of turbulence
are expected to change along the plume motion. This can be regarded
as a nonequilibrium effect, which alters the time and length scales of
turbulence. In order to incorporate such local structure effects in the
turbulence modeling of stellar convection, we introduce a time--space
double averaging procedure. In this formulation, a field quantity $f$
is divided into $f = \langle {\overline{f}} \rangle + \widetilde{f} +
f”$. Then, the plume motions are treated as the coherent fluctuation
$\widetilde{\bf{u}}$ while the random fluctuations are treated as the
incoherent ones ${\bf{u}}”$ ($\overline{f}$: time average, $\langle
{f} \rangle$: space average). The nonequilibrium effect is taken into
account through the Lagrangian derivative based on the coherent plume
motion $\widetilde{\bf{u}}$. The transport coefficients such as the
turbulent mass flux, the Reynolds stress, and the turbulent energy
flux are expressed in the combination of the usual eddy diffusivity
and viscosity (expressed in terms of the turbulent energy $K$ and its
dissipation rate $\epsilon$ as $K^2/\epsilon$) and the nonequilibrium
effect (expressed by the Lagrangian or advective derivative). Depending
on the sign of $ (\partial / \partial t + \widetilde{\bf{u}} \cdot
\nabla) (K^2/\epsilon)$, turbulent fluxes are enhanced or suppressed. A
turbulence model with the nonequilibrium effect incorporated through the
advective derivative along the plume flow is applied to the transport
problem caused by the surface cooling in stellar convection zone. The
prominent characteristics of the surface cooling driven convection
are \def\theenumi{\roman{enumi}} \def\labelenumi{(\theenumi)}
\begin{enumerate} \item Much enhanced turbulent fluxes in the
whole region of the convection zone; \item Strongly localized peak
turbulent fluxes near surface region. \end{enumerate} Neither of these
characteristics can be reproduced at all by the standard gradient-flux
model with the mixing-length expression of the turbulent transport
coefficients. The present nonequilibrium turbulence model successfully
reproduces these prominent features. These results suggest that
turbulence modeling with the nonequilibrium effect paves the way
for developing realistic treatments of the astrophysical convective
flow phenomena with plumes. References Yokoi, N., Masada, Y. , and
Takiwaki, T. (2022) “Modelling stellar convective transport with
plumes: I. Non-equilibrium turbulence effect in double-averaging
formulation,” submitted to Man. Not. Roy. Astron. Soc. arXiv:2111.08921
---------------------------------------------------------
Title: Magnetoclinicity Instability
Authors: Yokoi, Nobumitsu; Tobias, Steven M.
2022arXiv220514453Y Altcode:
In strongly compressible magnetohydrodynamic turbulence, obliqueness
between the large-scale density gradient and magnetic field gives
an electromotive force mediated by density variance (intensity of
density fluctuation). This effect is named “magnetoclinicity”, and
is expected to play an important role in large-scale magnetic-field
generation in astrophysical compressible turbulent flows. Analysis of
large-scale instability due to the magnetoclinicity effect shows that
the mean magnetic-field perturbation is destabilised at large scales
in the vicinity of strong mean density gradient in the presence of
density variance.
---------------------------------------------------------
Title: Helical fluid and (Hall)-MHD turbulence: a brief review
Authors: Pouquet, Annick; Yokoi, Nobumitsu
2022RSPTA.38010087P Altcode: 2021arXiv210412855P
Helicity, a measure of the breakage of reflectional symmetry
representing the topology of turbulent flows, contributes in a
crucial way to their dynamics and to their fundamental statistical
properties. We review several of their main features, both new and
old, such as the discovery of bi-directional cascades or the role of
helical vortices in the enhancement of large-scale magnetic fields in
the dynamo problem. The dynamical contribution in magnetohydrodynamic
of the cross-correlation between velocity and induction is discussed as
well. We consider next how turbulent transport is affected by helical
constraints, in particular in the context of magnetic reconnection
and fusion plasmas under one- and two-fluid approximations. Central
issues on how to construct turbulence models for non-reflectionally
symmetric helical flows are reviewed, including in the presence of
shear, and we finally briefly mention the possible role of helicity
in the development of strongly localized quasi-singular structures
at small scale. <P />This article is part of the theme issue `Scaling
the turbulence edifice (part 2)'.
---------------------------------------------------------
Title: Modeling Convective Turbulent Transport with Plumes Using
Double-Averaging Formulation
Authors: Yokoi, Nobumitsu; Masada, Youhei; Takiwaki, Tomoya
2021AGUFMNG33A..02Y Altcode:
Plumes in a convective flow, whose flow structure is localised in
space and time, are considered to be relevant to the turbulent
transport in convection. The effective mass, momentum, and heat
transports in the convective turbulence are investigated in the
framework of time--space double averaging procedure, where a field
quantity is decomposed into three parts: the spatiotemporal mean
(spatial average of the time-averaged) field, the dispersion or coherent
fluctuation (deviation from the spatiotemporal mean), and the random or
incoherent fluctuation. With this double-averaging framework, turbulent
correlations such as the Reynolds stress, turbulent mass flux, turbulent
internal-energy flux, etc., in the mean-field equations are divided into
the dispersion/coherent correlation part and the random/incoherent
correlation part. The evolution equations of these two parts of
the correlation show what are responsible for the conversion of the
fluctuation energy between the coherent and incoherent components. By
reckoning the plume as the coherent fluctuation, a transport model
for the convective turbulence is constructed with the aid of the
non-equilibrium effect along plume motions, and applied to a stellar
convective flow. One of the prominent characteristics of a surface
cooling-driven convection, the enhanced and localised turbulent mass
flux below the surface layer, which cannot be reproduced at all by the
usual eddy-diffusivity model with mixing length theory (MLT), is well
reproduced by the present model with the non-equilibrium effect. Our
results show that the incorporation of plume motion into turbulent
transport model through the non-equilibrium effect is an important
and very relevant extension of mean-field theory beyond the heuristic
gradient transport model with MLT.
---------------------------------------------------------
Title: Modelling stellar convective transport with plumes:
I. Non-equilibrium turbulence effect in double-averaging formulation
Authors: Yokoi, Nobumitsu; Masada, Youhei; Takiwaki, Tomoya
2021arXiv211108921Y Altcode:
Plumes in a convective flow are considered to be relevant to the
turbulent transport in convection. The effective mass, momentum,
and heat transports in the convective turbulence are investigated
in the framework of time--space double averaging procedure, where
a field quantity is decomposed into three parts: the spatiotemporal
mean (spatial average of the time-averaged) field, the dispersion or
coherent fluctuation, and the random or incoherent fluctuation. With
this framework, turbulent correlations in the mean-field equations
are divided into the dispersion/coherent and random/incoherent
correlation part. By reckoning the plume as the coherent fluctuation,
a transport model for the convective turbulence is constructed with the
aid of the non-equilibrium effect, in which the change of turbulence
characteristics along the mean stream is taken into account for the
modelling of the turbulent transport coefficients. In this work, for
the first time, change of turbulence properties along plume motions
is incorporated into the expression of the turbulent transport
coefficients. This non-equilibrium model is applied to a stellar
convective flow. One of the prominent characteristics of a surface
cooling-driven convection, the enhanced and localised turbulent mass
flux below the surface layer, which cannot be reproduced at all by
the usual eddy-diffusivity model with mixing length theory (MLT),
is well reproduced by the present model. Our results show that the
incorporation of plume motion into turbulent transport model is an
important and very relevant extension of mean-field theory beyond the
heuristic gradient transport model with MLT.
---------------------------------------------------------
Title: High Oxygen Fugacity of Lunar Anorthosites as Revealed by
Iron Micro-XANES of Plagioclase
Authors: Mikouchi, T.; Yokoi, N.; Takenouchi, A.; Arai, T.
2019LPI....50.2341M Altcode:
Synchrotron Fe-XANES analysis of plagioclase in 12 lunar rocks suggests
formation of anorthosites (FAN and MAN) at high fO2 probably well
above the IW buffer.
---------------------------------------------------------
Title: Mass and internal-energy transports in strongly compressible
magnetohydrodynamic turbulence
Authors: Yokoi, N.
2018JPlPh..84f7703Y Altcode:
Turbulent mass and internal-energy transports in strongly compressible
magnetohydrodynamic (MHD) turbulence are investigated in the framework
of the multiple-scale direct-interaction approximation, an analytical
closure scheme for inhomogeneous turbulence at very high Reynolds
numbers. Utilising the analytical representations for the turbulent
mass and internal-energy fluxes and their transport coefficients, which
are expressed in terms of the correlation and response functions,
turbulence models for these fluxes are proposed. In addition to
the usual gradient-diffusion transports, cross-diffusion transports
mediated by the density variance and the transports along the mean
magnetic field mediated by the compressional or dilatational turbulent
cross-helicity (velocity-magnetic-field correlation coupled with
compressive motions) are shown to arise. These compressibility effects
are of fundamental importance since they provide deviations from the
usual gradient-diffusion transports. Analogies of the dilatational
cross-helicity effects to the magnetoacoustic waves are also argued.
---------------------------------------------------------
Title: Electromotive force in strongly compressible
magnetohydrodynamic turbulence
Authors: Yokoi, Nobumitsu
2018JPlPh..84e7301Y Altcode:
Fully compressible magnetohydrodynamic (MHD) turbulence is
investigated in the framework of the multiple-scale direct-interaction
approximation. With the aid of the propagators (correlation and Green's
functions), fluctuating fields are solved, and turbulent correlations
are estimated in highly compressible turbulence. We focus on the
expression of the turbulent electromotive force (EMF). Obliqueness
between the mean magnetic field and the mean-density gradient, the
mean internal density gradient and the non-equilibrium mean velocity
contributes to the EMF in the presence of the density variance, which
is ubiquitous in turbulence in strongly variable density flows such
as the shock-front region. This density-variance effect is expected
to locally enhance the turbulence intensity across the shock front,
leading to a fast reconnection.
---------------------------------------------------------
Title: Path integrals for mean-field equations in nonlinear dynamos
Authors: Sokoloff, Dmitry; Yokoi, Nobumitsu
2018JPlPh..84c7307S Altcode: 2018arXiv180202842S
Mean-field dynamo equations are addressed with the aid of the path
integral method. The evolution of magnetic field is treated as a
three-dimensional Wiener random process, and the mean magnetic-field
equations are obtained with the Wiener integrals taken over all the
trajectories of the fluid particles. The form of the equations is
just the same as the conventional mean-field equations, but here the
equations are derived with the velocity field realisation affected
by the force exerted by the magnetic field. In this sense, we derive
nonlinear dynamo equations.
---------------------------------------------------------
Title: Generation of a Large-scale Magnetic Field in a Convective
Full-sphere Cross-helicity Dynamo
Authors: Pipin, V. V.; Yokoi, N.
2018ApJ...859...18P Altcode: 2017arXiv171201527P
We study the effects of the cross-helicity in the full-sphere
large-scale mean-field dynamo models of a 0.3 M <SUB>⊙</SUB>
star rotating with a period of 10 days. In exploring several
dynamo scenarios that stem from magnetic field generation by the
cross-helicity effect, we found that the cross-helicity provides the
natural generation mechanisms for the large-scale scale axisymmetric
and nonaxisymmetric magnetic field. Therefore, the rotating stars
with convective envelopes can produce a large-scale magnetic field
generated solely due to the turbulent cross-helicity effect (we call
it γ <SUP>2</SUP>-dynamo). Using mean-field models we compare the
properties of the large-scale magnetic field organization that stems
from dynamo mechanisms based on the kinetic helicity (associated
with the α <SUP>2</SUP> dynamos) and cross-helicity. For the fully
convective stars, both generation mechanisms can maintain large-scale
dynamos even for the solid body rotation law inside the star. The
nonaxisymmetric magnetic configurations become preferable when
the cross-helicity and the α-effect operate independently of each
other. This corresponds to situations with purely γ <SUP>2</SUP>
or α <SUP>2</SUP> dynamos. The combination of these scenarios, i.e.,
the γ <SUP>2</SUP> α <SUP>2</SUP> dynamo, can generate preferably
axisymmetric, dipole-like magnetic fields at strengths of several
kGs. Thus, we found a new dynamo scenario that is able to generate an
axisymmetric magnetic field even in the case of a solid body rotation
of the star. We discuss the possible applications of our findings to
stellar observations.
---------------------------------------------------------
Title: Iron Valence States of Plagioclase in Some Lunar Meteorites
Authors: Yokoi, N. Y.; Takenouchi, A. T.; Mikouchi, T. M.
2018LPI....49.2227Y Altcode:
By measuring Fe valences of plagioclase in lunar meteorites of different
rock types, we investigated redox states and relationships with their
water contents.
---------------------------------------------------------
Title: Inhomogeneous turbulence in magnetic reconnection
Authors: Yokoi, Nobumitsu
2016cosp...41E2119Y Altcode:
Turbulence is expected to play an essential role in enhancing magnetic
reconnection. Turbulence associated with magnetic reconnection is
highly inhomogeneous: it is generated by inhomogeneities of the field
configuration such as the velocity shear, temperature gradient,
density stratification, magnetic shear, etc. This self-generated
turbulence affects the reconnection through the turbulent transport. In
this reconnection--turbulence interaction, localization of turbulent
transport due to dynamic balance between several turbulence effects
plays an essential role. For investigating inhomogeneous turbulence
in a strongly nonlinear regime, closure or turbulence modeling
approaches provide a powerful tool. A turbulence modeling approach for
the magnetic reconnection is introduced. In the model, the mean-field
equations with turbulence effects incorporated are solved simultaneously
with the equations of turbulent statistical quantities that represent
spatiotemporal properties of turbulence under the effect of large-scale
field inhomogeneities. Numerical simulations of this Reynolds-averaged
turbulence model showed that self-generated turbulence enhances
magnetic reconnection. It was pointed out that reconnection states may
be divided into three category depending on the turbulence level: (i)
laminar reconnection; (ii) turbulent reconnection, and (iii) turbulent
diffusion. Recent developments in this direction are also briefly
introduced, which includes the magnetic Prandtl number dependence,
spectral evolution, and guide-field effects. Also relationship of this
fully nonlinear turbulence approach with other important approaches
such as plasmoid instability reconnection will be discussed.
---------------------------------------------------------
Title: A New Simple Dynamo Model for Stellar Activity Cycle
Authors: Yokoi, N.; Schmitt, D.; Pipin, V.; Hamba, F.
2016ApJ...824...67Y Altcode: 2016arXiv160106348Y
A new simple dynamo model for stellar activity cycle is proposed. By
considering an inhomogeneous flow effect on turbulence, it is shown
that turbulent cross helicity (velocity-magnetic-field correlation)
enters the expression of turbulent electromotive force as the coupling
coefficient for the mean absolute vorticity. This makes the present
model different from the current α-Ω-type models in two main
ways. First, in addition to the usual helicity (α) and turbulent
magnetic diffusivity (β) effects, we consider the cross-helicity effect
as a key ingredient of the dynamo process. Second, the spatiotemporal
evolution of cross helicity is solved simultaneously with the mean
magnetic fields. The basic scenario is as follows. In the presence of
turbulent cross helicity, the toroidal field is induced by the toroidal
rotation. Then, as in usual models, the α effect generates the poloidal
field from the toroidal one. This induced poloidal field produces a
turbulent cross helicity whose sign is opposite to the original one
(negative production). With this cross helicity of the reversed sign,
a reversal in field configuration starts. Eigenvalue analyses of the
simplest possible model give a butterfly diagram, which confirms the
above scenario and the equatorward migrations, the phase relationship
between the cross helicity and magnetic fields. These results suggest
that the oscillation of the turbulent cross helicity is a key for the
activity cycle. The reversal of the cross helicity is not the result
of the magnetic-field reversal, but the cause of the latter. This
new model is expected to open up the possibility of the mean-field or
turbulence closure dynamo approaches.
---------------------------------------------------------
Title: Large-scale flow generation by inhomogeneous helicity
Authors: Yokoi, N.; Brandenburg, A.
2016PhRvE..93c3125Y Altcode: 2015arXiv151108983Y
The effect of kinetic helicity (velocity-vorticity correlation) on
turbulent momentum transport is investigated. The turbulent kinetic
helicity (pseudoscalar) enters the Reynolds stress (mirror-symmetric
tensor) expression in the form of a helicity gradient as the coupling
coefficient for the mean vorticity and/or the angular velocity (axial
vector), which suggests the possibility of mean-flow generation in
the presence of inhomogeneous helicity. This inhomogeneous helicity
effect, which was previously confirmed at the level of a turbulence-
or closure-model simulation, is examined with the aid of direct
numerical simulations of rotating turbulence with nonuniform helicity
sustained by an external forcing. The numerical simulations show
that the spatial distribution of the Reynolds stress is in agreement
with the helicity-related term coupled with the angular velocity,
and that a large-scale flow is generated in the direction of angular
velocity. Such a large-scale flow is not induced in the case of
homogeneous turbulent helicity. This result confirms the validity
of the inhomogeneous helicity effect in large-scale flow generation
and suggests that a vortex dynamo is possible even in incompressible
turbulence where there is no baroclinicity effect.
---------------------------------------------------------
Title: Modeling helicity dissipation-rate equation
Authors: Yokoi, Nobumitsu
2016arXiv160208015Y Altcode:
Transport equation of the dissipation rate of turbulent helicity is
derived with the aid of a statistical analytical closure theory of
inhomogeneous turbulence. It is shown that an assumption on the helicity
scaling with an algebraic relationship between the helicity and its
dissipation rate leads to the transport equation of the turbulent
helicity dissipation rate without resorting to a heuristic modeling.
---------------------------------------------------------
Title: Shock--turbulence interaction in magnetic reconnection:
Density variance effects
Authors: Yokoi, N.
2014AGUFMSM13E4216Y Altcode:
Effects of density variance (, : density fluctuation, :
mean) in magnetic reconnection shocks are theoretically
investigated. Shock--turbulence interaction is one of the most
challenging problems in turbulence modeling. A strong variation of the
mean density () leads to a strong density variance. The expressions for
the turbulent correlations such as the Reynolds and turbulent Maxwell
stresses, the turbulent electromotive force, etc. are examined in a
compressible magnetohydrodynamic (MHD) turbulence. It is shown that in
the presence of the density variance, a mean density gradient () oblique
or perpendicular to the mean magnetic field gives rise to the turbulent
electromotive force. Since the electric current density induced by
this effect is in the direction of the reconnection electric current
density, the turbulent energy near the slow shock in the fast magnetic
reconnection is expected to be enhanced. The physical origin of this
effect is discussed. A turbulence model incorporating this effect is
proposed. This model is expected to reproduce the spatial distribution
of the turbulent energy around the fast magnetic reconnection.
---------------------------------------------------------
Title: Influence of Turbulence on the Reconnection Rate
Authors: Widmer, Fabien; Büchner, Jörg; Yokoi, Nobumitsu; Schmidt,
Wolfram
2014cosp...40E3627W Altcode:
Magnetic reconnection requires an, at least locally, non-ideal plasma
response. In collisionless space and astrophysical plasmas, turbulence
could provide this instead of the too rare binary collisions. We
investigated the possible influence of turbulence on the reconnection
rate in the framework of a single fluid compressible MHD simulation of
a tearing-unstable double current sheet model in order to test, whether
unresolved, sub-grid for an MHD simulations, turbulent transport can
enhance the reconnection process. For this sake we solve, simultaneously
with the grid-scale MHD equations, evolution equations for the sub-grid
turbulent energy and cross-helicity according to Yokoi's (2013) model
as well as their feedback into the MHD reconnection process. Preliminary
results are presented for a two-dimensional case
---------------------------------------------------------
Title: Turbulent Magnetic Reconnection and Particles Acceleration
Authors: Hoshino, Masahiro; Higashimori, Katsuaki; Yokoi, Nobumitsu
2014cosp...40E1227H Altcode:
Magnetic reconnection in the earth’s magnetotail involves a variety
of plasma processes across many scales from a several 10Re down to
ion/electron inertia scales, and those excited waves in many scales
show more or less turbulent behavior. The generation of such turbulent
waves is believed to be responsible not only to dynamics of magnetic
reconnection but also supra-thermal particle acceleration. In
this presentation, we review our recent progress on turbulent
reconnection and particle acceleration by using Particle-in-cell and MHD
simulations. Firstly, we discuss the interplay of magnetic reconnection
and turbulence based on a newly developed Reynolds-averaged MHD
simulation, and show that the turbulent diffusivity self-consistently
generated around the X-type region dramatically enhances the global
magnetic reconnection rate. Secondly, we argue that the scattering
process of particles with those turbulences plays an important role
on plasma heating and particle acceleration. In the earth’s plasma
sheet, it is expected that many magnetic reconnection sites with
many different scales can be generated. We discuss that the multiple
interaction of the energetic particle with those reconnection regions
leads to energization of supra-thermal particles.
---------------------------------------------------------
Title: A Reynolds-averaged turbulence modelling approach to the
maintenance of the Venus superrotation
Authors: Yoshizawa, A.; Kobayashi, H.; Sugimoto, N.; Yokoi, N.;
Shimomura, Y.
2013GApFD.107..614Y Altcode: 2013arXiv1308.1417Y
A maintenance mechanism of an approximately linear velocity profile of
the Venus zonal flow or superrotation is explored, with the aid of a
Reynolds-averaged turbulence modelling approach. The basic framework
is similar to that of Gierasch (Meridional circulation and maintenance
of the Venus atmospheric rotation. J. Atmos. Sci. 1975, 32, 1038-1044)
in the sense that the mechanism is examined under a given meridional
circulation. The profile mimicking the observations of the flow is
initially assumed, and its maintenance mechanism in the presence of
turbulence effects is investigated from a viewpoint of the suppression
of energy cascade. In the present work, the turbulent viscosity is
regarded as an indicator of the intensity of the cascade. A novelty of
this formalism is the use of the isotropic turbulent viscosity based
on a non-local time scale linked to a large-scale flow structure. The
mechanism is first discussed qualitatively. On the basis of these
discussions, the two-dimensional numerical simulation of the proposed
model is performed, with an initially assumed superrotation, and the
fast zonal flow is shown to be maintained, compared with the turbulent
viscosity lacking the non-local time scale. The relationship of the
present model with the current general circulation model simulation
is discussed in light of a crucial role of the vertical viscosity.
---------------------------------------------------------
Title: Transport enhancement and suppression in turbulent magnetic
reconnection: A self-consistent turbulence modela)
Authors: Yokoi, N.; Higashimori, K.; Hoshino, M.
2013PhPl...20l2310Y Altcode: 2014arXiv1401.1498Y
Through the enhancement of transport, turbulence is expected to
contribute to the fast reconnection. However, the effects of turbulence
are not so straightforward. In addition to the enhancement of transport,
turbulence under some environment shows effects that suppress the
transport. In the presence of turbulent cross helicity, such dynamic
balance between the transport enhancement and suppression occurs. As
this result of dynamic balance, the region of effective enhanced
magnetic diffusivity is confined to a narrow region, leading to the
fast reconnection. In order to confirm this idea, a self-consistent
turbulence model for the magnetic reconnection is proposed. With the aid
of numerical simulations where turbulence effects are incorporated in
a consistent manner through the turbulence model, the dynamic balance
in the turbulence magnetic reconnection is confirmed.
---------------------------------------------------------
Title: Explosive Turbulent Magnetic Reconnection
Authors: Higashimori, K.; Yokoi, N.; Hoshino, M.
2013PhRvL.110y5001H Altcode: 2013arXiv1305.6695H
We report simulation results for turbulent magnetic reconnection
obtained using a newly developed Reynolds-averaged magnetohydrodynamics
model. We find that the initial Harris current sheet develops in three
ways, depending on the strength of turbulence: laminar reconnection,
turbulent reconnection, and turbulent diffusion. The turbulent
reconnection explosively converts the magnetic field energy into
both kinetic and thermal energy of plasmas, and generates open fast
reconnection jets. This fast turbulent reconnection is achieved by the
localization of turbulent diffusion. Additionally, localized structure
forms through the interaction of the mean field and turbulence.
---------------------------------------------------------
Title: Explosive Turbulent Magnetic Reconnection: A New Approach of
MHD-Turbulent Simulation
Authors: Hoshino, Masahiro; Yokoi, Nobumitsu; Higashimori, Katsuaki
2013EGUGA..15.1775H Altcode:
Turbulent flows are often observed in association with magnetic
reconnection in space and astrophysical plasmas, and it is often
hypothesized that the turbulence can contribute to the fast magnetic
reconnection through the enhancement of magnetic dissipation. In this
presentation, we demonstrate that an explosive turbulent reconnection
can happen by using a new turbulent MHD simulation, in which the
evolution of the turbulent transport coefficients are self-consistently
solved together with the standard MHD equations. In our model, the
turbulent electromotive force defined by the correlation of turbulent
fluctuations between v and B is added to the Ohm's law. We discuss
that the level of turbulent can control the topology of reconnection,
namely the transition from the Sweet-Parker reconnection to the
Petscheck reconnection occurs when the level of fluctuations becomes
of order of the ambient physical quantities, and show that the growth
of the turbulent Petscheck reconnection becomes much faster than the
conventional one.
---------------------------------------------------------
Title: Cross helicity and related dynamo
Authors: Yokoi, N.
2013GApFD.107..114Y Altcode: 2013arXiv1306.6348Y
The turbulent cross helicity is directly related to the coupling
coefficients for the mean vorticity in the electromotive force and for
the mean magnetic-field strain in the Reynolds stress tensor. This
suggests that the cross-helicity effects are important in the
cases where global inhomogeneous flow and magnetic-field structures
are present. Since such large-scale structures are ubiquitous in
geo/astrophysical phenomena, the cross-helicity effect is expected
to play an important role in geo/astrophysical flows. In the presence
of turbulent cross helicity, the mean vortical motion contributes to
the turbulent electromotive force. Magnetic-field generation due to
this effect is called the cross-helicity dynamo. Several features
of the cross-helicity dynamo are introduced. Alignment of the mean
electric-current density J with the mean vorticity Ω , as well as
the alignment between the mean magnetic field B and velocity U , is
supposed to be one of the characteristic features of the dynamo. Unlike
the case in the helicity or α effect, where J is aligned with B in the
turbulent electromotive force, we in general have a finite mean-field
Lorentz force J × B in the cross-helicity dynamo. This gives a
distinguished feature of the cross-helicity effect. By considering the
effects of cross helicity in the momentum equation, we see several
interesting consequences of the effect. Turbulent cross helicity
coupled with the mean magnetic shear reduces the effect of turbulent
or eddy viscosity. Flow induction is an important consequence of this
effect. One key issue in the cross-helicity dynamo is to examine how
and how much cross helicity can be present in turbulence. On the basis
of the cross-helicity transport equation, its production mechanisms
are discussed. Some recent developments in numerical validation of
the basic notion of the cross-helicity dynamo are also presented.
---------------------------------------------------------
Title: Dynamic balance in turbulent reconnection
Authors: Yokoi, N.; Higashimori, K.; Hoshino, M.
2012AGUFMSM21B2270Y Altcode:
Dynamic balance between the enhancement and suppression of
transports due to turbulence in magnetic reconnection is discussed
analytically and numerically by considering the interaction of the
large-scale field structures with the small-scale turbulence in a
consistent manner. Turbulence is expected to play an important role in
bridging small and large scales related to magnetic reconnection. The
configurations of the mean-field structure are determined by turbulence
through the effective transport. At the same time, statistical
properties of turbulence are determined by the mean-field structure
through the production mechanisms of turbulence. This suggests that
turbulence and mean fields should be considered simultaneously in
a self-consistent manner. Following the theoretical prediction on
the interaction between the mean-fields and turbulence in magnetic
reconnection presented by Yokoi and Hoshino (2011), a self-consistent
model for the turbulent reconnection is constructed. In the model,
the mean-field equations for compressible magnetohydrodynamics are
treated with the turbulence effects incorporated through the turbulence
correlation such as the Reynolds stress and turbulent electromotive
force. Transport coefficients appearing in the expression for these
correlations are not adjustable parameters but are determined through
the transport equations of the turbulent statistical quantities such
as the turbulent MHD energy, the turbulent cross helicity. One of the
prominent features of this reconnection model lies in the point that
turbulence is not implemented as a prescribed one, but the generation
and sustainment of turbulence through the mean-field inhomogeneities are
treated. The theoretical predictions are confirmed by the numerical
simulation of the model equations. These predictions include the
quadrupole cross helicity distribution around the reconnection region,
enhancement of reconnection rate due to turbulence, localization of
the reconnection region through the cross-helicity effect, etc. Some
implications to the satellite observation of the magnetic reconnection
will be also given. Reference: Yokoi, N. and Hoshino, M. (2011)
Physics of Plasmas, 18, 111208.
---------------------------------------------------------
Title: Near Horizon Superconformal Symmetry of Rotating BPS Black
Holes in Five Dimensions
Authors: Nakamura, M.; Yokoi, N.
2012PThPh.128..251N Altcode: 2011arXiv1109.6481N
We investigate the asymptotic supersymmetry group of the near horizon
region of the BMPV black holes, which are the rotating BPS black
holes in five dimensions. When considering only bosonic fluctuations,
we find that there exist consistent boundary conditions and the
corresponding asymptotic symmetry group is generated by a chiral
Virasoro algebra with the vanishing central charge. After turning on
fermionic fluctuations with the boundary conditions, we construct the
conserved charges associated with the infinitesimal asymptotic Killing
spinors. The conserved charges satisfy a chiral super-Virasoro algebra
without central extension. The super-Virasoro algebra is originated
in the AdS_2 isometry supergroup of the near horizon solution.
---------------------------------------------------------
Title: Cross-helicity effects and turbulent transport in
magnetohydrodynamic flow
Authors: Yokoi, Nobumitsu; Balarac, Guillaume
2011JPhCS.318g2039Y Altcode: 2011arXiv1107.1154Y
In the presence of large-scale vortical motions and/or magnetic-field
strains, the turbulent cross helicity (velocity-magnetic-field
correlation in fluctuations) may contribute to the turbulent
electromotive force and the Reynolds stress. These effects of cross
helicity are considered to balance the primary effects of turbulence
such as the turbulent magnetic diffusivity in magnetic-field evolution
and the eddy viscosity in the momentum transport. The cross-helicity
effects may suppress the enhanced transports due to turbulence. Physical
interpretation of the effects is presented with special emphasis on
the difference between the cross-helicity effect and the usual a or
helicity effect in the dynamo action. The relative importance of the
cross-helicity effect in dynamo action is validated with the aid of
a direct numerical simulation (DNS) of the Kolmogorov flow with an
imposed magnetic field. Several mechanisms that provide turbulence
with the cross helicity are also discussed.
---------------------------------------------------------
Title: Turbulence and flow structures in magnetic reconnection
Authors: Yokoi, N.; Hoshino, M.
2011AGUFMSH43A1926Y Altcode:
Magnetic reconnection is viewed from the interaction between the
large-scale inhomogeneous structure and turbulence. On the one hand,
turbulence determines large-scale structures through the turbulent
transport coefficients. On the other hand, large-scale inhomogeneous
structure determines the statistical properties of turbulence through
the production rates of turbulent quantities. In the context of magnetic
reconnection, this mutual interaction between the large-scale structures
and turbulence is modeled with special emphasis on the pseudo-scalar
effects, which represents some symmetry breakage in turbulence. In
addition to the magnetic reconnection, magnetic-flux freezing in
turbulence media, turbulent dynamo, transport suppression are also
discussed.
---------------------------------------------------------
Title: Maintenance mechanism of Venus superrotation in light of
turbulent-viscosity suppression
Authors: Yoshizawa, A.; Yokoi, N.; Shimomura, Y.; Kobayashi, H.;
Sugimoto, N.
2011AGUFMNG43B1493Y Altcode:
Venus rotates with the speed whose magnitude is about 1/200 of the Earth
counterpart. Under this quite slow rotation, the atmosphere at height
65-70 km flows with velocity 100 ms-1 in the zonal or longitudinal
direction. The direction is from the east to west and is the same
as that of the Venus surface. There are two phases to be clarified
concerning this flow: (a) Evolution phase (the process in which the
zonal flow is generated and reaches a steady state); (b) Maintenance
phase (the process in which the fast zonal flow is maintained in a
turbulent atmospheric state). The present work focuses on the latter or
maintenance phase. It is founded on the recognition that the existence
of such a fast flow is due to the decrease in the turbulent viscosity in
the Reynolds-mean turbulence modeling. Special attention is paid to the
nonlocal time scale representing parts of global flow structures. The
maintenance of the Venus superrotation is discussed in light of the
turbulent-viscosity suppression given rise to by the time scale.
---------------------------------------------------------
Title: Flow-turbulence interaction in magnetic reconnection
Authors: Yokoi, N.; Hoshino, M.
2011PhPl...18k1208Y Altcode: 2011arXiv1105.6343Y
Roles of turbulence in the context of magnetic reconnection are
investigated with special emphasis on the mutual interaction between
flow (large-scale inhomogeneous structure) and turbulence. In order to
evaluate the effective transport due to turbulence, in addition to the
intensity information of turbulence represented by the turbulent energy,
the structure information represented by pseudoscalar statistical
quantities (helicities) is important. On the basis of the evolution
equation, mechanisms that provide turbulence with cross helicity
are presented. Magnetic-flux freezing in highly turbulent media
is considered with special emphasis on the spatial distribution
of the turbulent cross helicity. The cross-helicity effects in
the context of magnetic reconnection are also investigated. It is
shown that the large-scale flow and magnetic-field configurations
favorable for the cross-helicity generation is compatible with the
fast reconnection. Difference between the spatial distributions of
the turbulent MHD energy and cross helicity plays an essential role
for localizing the reconnection region. In this sense, turbulence and
large-scale structures promote magnetic reconnection mediated by the
turbulent cross helicity.
---------------------------------------------------------
Title: Modeling the turbulent cross-helicity evolution: production,
dissipation, and transport rates
Authors: Yokoi, N.
2011JTurb..12...27Y Altcode: 2010arXiv1005.2762Y; 2011JTurb..12N..27Y
It has been recognized that the turbulent cross helicity (correlation
between the velocity and magnetic-field fluctuations) can play an
important role in several magnetohydrodynamic (MHD) plasma phenomena
such as the global magnetic-field generation, turbulence suppression,
etc. Despite its relevance to the cross-helicity evolution, little
attention has been paid to the dissipation rate of the turbulent cross
helicity, $\epsilon_W$. In this paper, we consider the model expression
for the dissipation rate of the turbulent cross helicity. In addition to
the algebraic model, an evolution equation of $\epsilon_W$ is proposed
on the basis of the statistical analytical theory of inhomogeneous
turbulence. A turbulence model with the modeling of $\epsilon_W$
is applied to the solar-wind turbulence. Numerical results on the
large-scale evolution of the cross helicity is compared with the
satellite observations. It is shown that, as far as the solar-wind
application is concerned, the simplest possible algebraic model for
$\epsilon_W$ is sufficient for elucidating the large-scale spatial
evolution of the solar-wind turbulence. Dependence of the cross-helicity
evolution on the large-scale velocity structures such as velocity
shear and flow expansion is also discussed.
---------------------------------------------------------
Title: Cross-helicity turbulence model: Application to MHD phenomena
from solar convection zone to heliosphere
Authors: Yokoi, N.; Kitiashvili, I. N.; Kosovichev, A. G.
2010AGUFMSH31A1793Y Altcode:
Cross helicity (velocity-magnetic field correlation) is expected
to play a key role in several geo/astrophysical processes including
dynamo action, suppression of turbulent transport, etc. We discuss the
relevance of the cross-helicity effects with the aid of the turbulence
model. A turbulence model with the cross-helicity effects incorporated
may be called the “cross-helicity turbulence model”. This model is
applied to several MHD phenomena ranging from the formations of magnetic
fields and plasma motions in the solar convection zone to the solar-wind
evolution in the heliosphere. Generation of turbulence quantities
depends on the inhomogeneity of large-scale fields, and turbulence
in turn determines the configuration of the mean fields through the
turbulent transport. Such nonlinear interactions between the mean- and
fluctuation-fields are explored with the aid of numerical simulations
with cross-helicity turbulence model. Through the comparisons to the
observation, validity of the turbulence model is examined. Examinations
include (i) A large-eddy simulation of the sunspot flow reveals how and
how much cross helicity is generated there; (ii) A eddy-viscosity-type
turbulence model shows how the turbulence quantities evolves under
the influence of the large-scale velocity and magnetic-field shears.
---------------------------------------------------------
Title: Inhomogeneity and anisotropy effects in magnetohydrodynamic
turbulence
Authors: Yokoi, N.
2009AGUFMSM43B1762Y Altcode:
Most turbulence of interest encountered in the scientific and
engineering fields is inhomogeneous: accompanied by the inhomogeneity of
large-scale or mean fields such as flow shear, rotation, magnetic field,
etc. In addition, as in the geo/astrophysical phenomena, the presence of
rotation and/or magnetic field makes turbulence anisotropic. Statistical
property of turbulence coupled with the mean field (inhomogeneity)
determines the effective transports due to turbulence. In this work, the
effects of anisotropy and inhomogeneity on the turbulent transport are
investigated. In the current closure theory of inhomogeneous turbulence
such as the two-scale direct-interaction approximation (multiple-scale
analysis combined with a closure theory of turbulence), the basic or
non-perturbed field has been assumed to be homogeneous and isotropic;
the effects of inhomogeneity are incorporated in a perturbative
manner. In this work, we consider a homogeneous but anisotropic state
as the basic field. As compared with the previous formulation, where
the anisotropy effects appear in the higher-order contribution, in
the present formulation the anisotropy appears as a primary effect. As
this consequence, this analysis is expected to be appropriate in the
case where the rotation and/or large-scale magnetic field play an
essential role. The possibility of the turbulent transport suppression
(reduction of eddy transport) due to anisotropy is also discussed.
---------------------------------------------------------
Title: Modeling of the turbulent cross-helicity dissipation rate:
Comparison using the solar-wind observations
Authors: Yokoi, N.
2008AGUFMSH31A1664Y Altcode:
The turbulent cross helicity (velocity--magnetic-field correlation
of turbulence) W ≡ <u' · b'>, as well as the turbulent
magnetohydrodynamic (MHD) energy K ≡ < u'2 + b'2 > / 2,
is a quantity of primary importance which represents statistical
properties of turbulence. The presence of the cross helicity in
turbulence may alter the transport properties of turbulence, then
it affects the magnitude and configuration of large-scale fields
much. A typical example is the turbulent dynamo. If the cross helicity
exists in turbulence accompanied by the large-scale vortical motions,
electromotive force parallel to the vorticity is induced. This may
counterbalance a huge magnetic diffusivity due to turbulence, and work
for the magnetic-field generation. Although spacecraft observations
of solar-wind turbulence have provided precious information on the
turbulent cross helicity, their results have not been fully utilized in
the studies of the MHD turbulence modeling. As for the dissipation rate
of the turbulent cross helicity, ɛW, very little is known. This is in
marked contrast with the dissipation rate of the turbulent energy, ɛ,
whose model equation has long been discussed. We propose a few models
for the turbulent cross-helicity dissipation rate ɛW: an algebraic
model, a model equation for ɛW evolution, etc. Using comparison with
the large-scale behavior of the cross helicity obtained by several
solar-wind observations, we evaluate these models. The detailed
observations by Roberts et al. (1987) inferred that in the absence
of flow shear the turbulent cross helicity W remains to be relatively
large value as the heliocentric distance increases. We will show that
a turbulence model simulation with the algebraic model of ɛW can
reproduce this W behavior with a reasonable model constant. Further
discussions including the model equation for the ɛW evolution will
be also presented.
---------------------------------------------------------
Title: Statistical Analysis of the Nonlinear Mixing Correlations in
Magnetohydrodynamic Turbulence and its Application to the Solar Wind
Authors: Yokoi, N.
2007AGUFMSH23A1170Y Altcode:
The velocity strain is related to the non-Gaussian nature of turbulence
through the dynamics of vorticity. With the aid of a spectral closure
theory coupled with the multiple-scale method, the nonlinear mixing
correlations in the inhomogeneous magnetohydrodynamic turbulence is
analyzed. Using the analytical results, a turbulence model for MHD
turbulence is proposed. The model is expected to be useful in the MHD
turbulence with the mean velocity shears. The system of model equations
is applied to the solar wind, and shown to be appropriate to describe
the radial evolutions of the cross helicity (velocity-magnetic-field
correlation) and the residual energy (difference between the kinetic
and magnetic energies) in solar-wind turbulence. The Alfven ratio
(ratio of the kinetic to magnetic energies) of ~ 0.5 stationary in
space in the outer heliosphere is elucidated as a stationary solution
of the turbulence model.
---------------------------------------------------------
Title: An application of the turbulent magnetohydrodynamic
residual-energy equation model to the solar wind
Authors: Yokoi, Nobumitsu; Hamba, Fujihiro
2007PhPl...14k2904Y Altcode:
A magnetohydrodynamic (MHD) turbulence model incorporating the
turbulent MHD residual energy (difference between the kinetic and
magnetic energies) is applied to solar-wind turbulence. In the model,
the dynamics of the turbulent cross-helicity (cross-correlation between
the velocity and magnetic field) and the turbulent MHD residual energy,
which are considered to describe the degree of Alfvénicity of the MHD
turbulence, are solved simultaneously with the dynamics of the turbulent
MHD energy and its dissipation rate. The transition of solar-wind
turbulence from the Alfvén-wave-like fluctuations near the Sun in
the inner heliosphere to the fully developed MHD turbulence in the
outer heliosphere is discussed. Magnetic dominance in the solar-wind
fluctuations is addressed from the dynamics of the evolution equation
of the residual energy. An interpretation of the observed Alfvén ratio
(ratio of the kinetic to magnetic energies) of ~0.5 is proposed from
the viewpoint of a stationary solution of the turbulence model.
---------------------------------------------------------
Title: Periodic Change of Solar Differential Rotation
Authors: Itoh, S. -I.; Itoh, K.; Yoshizawa, A.; Yokoi, N.
2005ApJ...618.1044I Altcode:
The periodic oscillation of the inhomogeneous rotation of the Sun
is studied by use of the MHD dynamo theory. There exists a turbulent
electromotive force that is driven by the vorticity of the flow (i.e.,
the γ dynamo). In addition, its counterpart exists in the vorticity
equation, that is, the rotation is induced by an inhomogeneous magnetic
field in turbulent plasmas through the γ-dynamo process. Based on this
dynamo theory, a periodic change of solar differential rotation with a
period of 11 yr is theoretically explained under the prescribed solar
magnetic cycle. The predicted amplitude is compared with observations.
---------------------------------------------------------
Title: TOPICAL REVIEW: Dynamos and MHD theory of turbulence
suppression
Authors: Yoshizawa, Akira; Itoh, Sanae-I.; Itoh, Kimitaka; Yokoi,
Nobumitsu
2004PPCF...46R..25Y Altcode:
Characteristics of electrically conducting media are reviewed from
the macroscopic viewpoint based on mean-field magnetohydrodynamics,
while being compared using the methodology and knowledge in fluid
mechanics. The themes covered in this review range from the mechanism of
generating stellar magnetic fields (dynamo) to transport properties in
fusion. The primary concern here is to see the characteristics common
to these apparently different phenomena, within the framework of the
mean-field theory. Owing to the intrinsic limitation of the approach,
the present discussions are limited more or less to specific aspects
of phenomena. They are supplemented with reference to theoretical,
numerical, and observational approaches intrinsic to each theme. In
the description of dynamo phenomena, emphasis is laid on the cross
helicity dynamo. Features common to stellar magnetic-field generation
and the rotational-motion drive in toroidal plasmas are illustrated
on this basis.
---------------------------------------------------------
Title: Mean Field Theory Interpretation of Solar Polarity Reversal
Authors: Yoshizawa, Akira; Kato, Hirofumi; Yokoi, Nobumitsu
2000ApJ...537.1039Y Altcode:
A mechanism of the polarity reversal of the solar magnetic field
is explored on the basis of the mean field or turbulent dynamo
theory. In the low-latitude region of the convective zone, the toroidal
magnetic field, which is the origin of sunspots, is generated by the
rotational motion of fluids, with the turbulent cross helicity as the
intermediary. This field generates the poloidal field of dipole type
through the alpha or turbulent helicity effect. The latter, in turn,
contributes to the annihilation of the turbulent cross helicity,
resulting in the decay of the toroidal magnetic field. This process
indicates less room for the occurrence of the fully developed poloidal
field in the low-latitude region and paves the way for the polarity
reversal through the change of the sign of the turbulent cross
helicity. A simple model mimicking the periodic polarity reversal is
presented, and the relationship of the reversal period to the ratio of
the poloidal to toroidal fields is given. The meridional-flow velocity
at the solar surface is estimated, giving a result consistent with
observations.
---------------------------------------------------------
Title: Collimation mechanism of magnetohydrodynamic jets based on
helicity and cross-helicity dynamos, with reference to astronomical
jets
Authors: Yoshizawa, Akira; Yokoi, Nobumitsu; Kato, Hirofumi
2000PhPl....7.2646Y Altcode:
A collimation mechanism of magnetohydrodynamic jets is sought
on the basis of the turbulent-dynamo theory within the framework
of fluid incompressibility. The momentum diffusion giving rise to
the broadening of the jets is suppressed through the cross-helicity
effect, whereas the diffusion of the magnetic field is hampered owing
to the helicity effect. These two effects lead to the collimation of
electrically conducting jets in the presence of turbulent fluctuations
of the velocity and magnetic field. The relevance to collimated
astronomical jets is discussed with the reservation about effects of
fluid compressibility.
---------------------------------------------------------
Title: Erratum: Analysis of Toroidal Magnetic Fields in Accretion
Disks Using the Cross-Helicity Effect and Estimate of the Jet Velocity
Authors: Nishino, Satoru; Yokoi, Nobumitsu
1999PASJ...51..173N Altcode:
In the paper [PASJ 50, 653--665 (1998)], equations (27) and (29) should
be substituted with the following expressions; DK/Dt = P_K - \varepsilon
+ \nabla\cdot T_K P_K \equiv - E_M \cdot J + R :\nabla U, respectively.
---------------------------------------------------------
Title: Analysis of Toroidal Magnetic Fields in Accretion Disks Using
the Cross-Helicity Effect and Estimate of the Jet Velocity
Authors: Nishino, Satoru; Yokoi, Nobumitsu
1998PASJ...50..653N Altcode:
The magnetic fields in an accretion disk are examined using
a magnetohydrodynamic (MHD) turbulent dynamo model consisting of
transport equations for the mean fields, turbulent energy, dissipation
rate, and cross helicity. The velocity of accreting gases is assumed
to obey the Keplerian and rigid rotations in the outer and inner
regions of a disk, respectively, except for the central part. Under
the condition of axisymmetry around the rotation axis and uniformity
in the direction perpendicular to the disk, the turbulent model is
examined both numerically and analytically. As a result, it is pointed
out that the cross-helicity effect generates a toroidal magnetic field,
resulting in the occurrence of a current in a direction perpendicular to
the disk in the central part. This toroidal magnetic field enables gas
to escape from a central high-mass body as bipolar jets, because the
magnetic energy may become comparable to the gravitational one. The
velocity of the jets in a protoplanetary system was estimated by a
numerical simulation of the preset model.
---------------------------------------------------------
Title: Stationary large-scale magnetic fields generated by turbulent
motion in a spherical region
Authors: Yoshizawa, Akira; Yokoi, Nobumitsu
1996PhPl....3.3604Y Altcode:
Stationary large-scale magnetic fields generated by an electrically
conducting fluid in a spherical region are examined analytically,
using the concept of the turbulent dynamo based on helicity and
cross-helicity effects. Under this concept, the toroidal magnetic
field is induced through the combination of a rotational motion and
the turbulent cross-helicity effect. This field generates the poloidal
one through the turbulent residual-helicity (alpha) effect. A new
magnetic-field generation mechanism in the vicinity of the poles is
also described. These findings are discussed in the context of the
dimension of the convection part of a stellar object.
---------------------------------------------------------
Title: Large-scale magnetic fields in spiral galaxies viewed from
the cross-helicity dynamo.
Authors: Yokoi, N.
1996A&A...311..731Y Altcode:
Turbulent magnetohydrodynamic dynamo using the cross-helicity effect is
applied to the interstellar or galactic magnetic fields. The strength
of the interstellar magnetic fields estimated with the aid of the
cross-helicity (velocity/magnetic field correlation) dynamo solution
is in good agreement with the observed magnetic-field strength. Two
typical configurations of the magnetic fields in galactic disks,
which are bisymmetric and axisymmetric spiral fields, are elucidated
by using the cross-helicity dynamo, and the configurations of the
vertical magnetic fields are also discussed. Some implications about the
stability of the turbulent cross-helicity configuration are presented
with special emphasis on the role of the vertical magnetic field,
the frozen-in field, and the jet from the galactic center.
---------------------------------------------------------
Title: Turbulent Magnetohydrodynamic Dynamo for Accretion Disks
Using the Cross-Helicity Effect
Authors: Yoshizawa, Akira; Yokoi, Nobumitsu
1993ApJ...407..540Y Altcode:
Accretion disks are studied using the concept of the turbulent
magnetohydrodynamic (MHD) dynamo. Under this concept, the effect of
cross helicity (magnetic-field/velocity correlation function) plays
a key role as does the effect of turbulent viscosity and anomalous
resistivity. In the presence of the cross helicity, the rotational
motion of the disk can generate the toroidal magnetic field. The
magnetic field produces the thrust for launching the jet which, in
turn, induces the poloidal magnetic field under the cross-helicity
effect. The close relationship between the magnetic field and the
plasma velocity is a primary feature of the cross-helicity dynamo.
---------------------------------------------------------
Title: Vortex dynamo and large-scale turbulent structures in a
rotating system
Authors: Yoshizawa, Akira; Yokoi, Nobumitsu
1991JPSJ...60.2500Y Altcode: 1991PSJaJ..60.2500Y
Turbulent flows subject to the Coriolis force are examined theoretically
to study the vortex dynamo or generation of large-scale turbulent
structures. A major term in the turbulent vortex-motive force that plays
a key role in the mean vorticity equation is shown to be proportional to
the angular velocity vector of a rotating system with the proportional
coefficient expressed in terms of the helicity. This result shows
that the vortical structure with its axis in the zonal direction near
the equator is a promising candidate of Saturn's large atmospheric
structures called white spots. Formation of the vortex with its axis
normal to the spherical surface, as in typhoons, is also discussed.
