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Author name code: ossendrijver
ADS astronomy entries on 2022-09-14
author:"Ossendrijver, Mathieu" year:1990-2010
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Title: Magnetoconvection and dynamo coefficients. III. α-effect
and magnetic pumping in the rapid rotation regime
Authors: Käpylä, P. J.; Korpi, M. J.; Ossendrijver, M.; Stix, M.
2006A&A...455..401K Altcode: 2006astro.ph..2111K
Aims.The α- and γ-effects, which are responsible for the generation
and turbulent pumping of large scale magnetic fields, respectively,
due to passive advection by convection are determined in the rapid
rotation regime corresponding to the deep layers of the solar convection
zone.<BR />Methods.A 3D rectangular local model is used for solving the
full set of MHD equations in order to compute the electromotive force
(emf), E = overline {u × b}, generated by the interaction of imposed
weak gradient-free magnetic fields and turbulent convection with varying
rotational influence and latitude. By expanding the emf in terms of the
mean magnetic field, E<SUB>i</SUB> = a<SUB>ij</SUB> overline B_j, all
nine components of a<SUB>ij</SUB> are computed. The diagonal elements
of a<SUB>ij</SUB> describe the α-effect, whereas the off-diagonals
represent magnetic pumping. The latter is essentially the advection
of magnetic fields by means other than the underlying large-scale
velocity field. Comparisons are made to analytical expressions of the
coefficients derived under the first-order smoothing approximation
(FOSA).<BR />Results.In the rapid rotation regime the latitudinal
dependence of the α-components responsible for the generation of the
azimuthal and radial fields does not exhibit a peak at the poles, as
is the case for slow rotation, but at a latitude of about 30°. The
magnetic pumping is predominantly radially down- and latitudinally
equatorward as in earlier studies. The numerical results compare
surprisingly well with analytical expressions derived under first-order
smoothing, although the present calculations are expected to lie near
the limits of the validity range of FOSA.<BR />
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Title: Alpha-Effect and Turbulent Pumping In The Rapid Rotation
Regime - Implications For Solar Dynamo Models
Authors: Käpylä, P. J.; Korpi, M. J.; Ossendrijver, M.; Stix, M.;
Tuominen, I.
2006IAUJD...8E..46K Altcode:
We use local 3D convection calculations to compute the alpha-effect and
turbulent pumping of mean magnetic fields in the rapid rotation regime
corresponding to the deep layers of the solar convection zone. We find
that in this regime the alpha-effect responsible for generating the
poloidal field out of the toroidal one peaks at around latitude 30
degrees, in contrast to the slow rotation case and the often adopted
prescription in mean-field models of the solar dynamo, where the
maximum values are found at the poles. Furthermore, the turbulent
pumping of mean fields is predominantly down- and equatorward. We
find that the downward pumping is decreased near the equator for rapid
rotation and can be upward for the toroidal field component. In order
to investigate the implications of the obtained local results for the
problems in mean-field dynamo theory arising from the helioseismically
determined solar rotation profile, namely the poleward migration of
activity belts at low latitudes and the activity being concentrated at
too high latitudes, we introduce the alpha-effect and turbulent pumping
as they were found in the local calculations into a kinematic mean-field
model of the solar dynamo. We also investigate the effect of a one-cell
counter-clockwise meridional flow pattern on the dynamo solutions. We
find that using the alpha-effect and turbulent pumping adapted from
the results of the local calculations, the migration of the activity
belts is equatorward also at low latitudes. When the meridional flow is
added, the activity belts are shifted further closer to the equator,
and a poleward migration belt appears at high latitudes. With all the
effects included, the activity still appears at too high latitudes
(5...60 degrees). Other remaining problems include the somewhat too
short cycle periods for the solar-like dipole solutions.
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Title: Local models of stellar convection. III. The Strouhal number
Authors: Käpylä, P. J.; Korpi, M. J.; Ossendrijver, M.; Tuominen, I.
2006A&A...448..433K Altcode: 2004astro.ph.10586K
Aims.The Strouhal number (St), which is a nondimensional measure of the
correlation time, is determined from numerical models of convection. The
Strouhal number arises in the mean-field theories of angular momentum
transport and magnetic field generation, where its value determines
the validity of certain widely used approximations, such as the first
order smoothing (hereafter FOSA). More specifically, the relevant
transport coefficients can be calculated by means of a cumulative
series expansion if St < St<SUB>crit</SUB> ≈ 1.Methods.We define
the Strouhal number as the ratio of the correlation and turnover times,
which we determine separately, the former from the autocorrelation of
velocity, and the latter by following test particles embedded in the
flow.Results.We find that the Strouhal numbers are, generally, of the
order of 0.1 to 0.4 which is close to the critical value above which
deviations from FOSA become significant. Increasing the rotational
influence tends to shorten both timescales in such a manner that St
decreases. However, we do not find a clear trend as a function of the
Rayleigh number for the parameter range explored in the present study.
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Title: Transport coefficients for solar and stellar dynamos
Authors: Ossendrijver, M.; Käpylä, P. J.
2006IAUS..233....3O Altcode:
The large-scale magnetic field of the Sun and solar-type stars is
probably generated near the interface of the radiative core and the
convection zone. One of the well-known difficulties of interface-type
and other dynamo models is to explain the latitudinal distribution
of magnetic fields as observed on the Sun. In this contribution new
results of numerical MHD simulations of transport coefficients of
magnetic fields in the rapid rotation regime, relevant for the base
of the solar convection zone, are presented that may contribute to
explaining the latitudinal distribution of magnetic fields observed on
the Sun. A brief outlook on further numerical simulations of transport
coefficients and their relevance for mean-field dynamo theory is given.
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Title: Approaching the solar dynamo
Authors: Ossendrijver, Mathieu
2006AdSpR..38..832O Altcode:
The origin of the solar magnetic field remains a stubborn challenge of
astrophysics. At the solar surface the magnetic field assumes a complex,
hierarchical structure in space and time. Systematic features such as
the solar cycle and the butterfly diagram point to the existence of a
deep-rooted large-scale predominantly toroidal magnetic field. In this
review of solar dynamo theory new developments in our understanding of
processes relevant for the solar dynamo are discussed. In recent years
there has been significant progress with regard to tachocline physics,
magnetic helicity conservation, the α effect, magnetic pumping,
and the storage and amplification of the magnetic field. Remaining
uncertainties about the nature of the deep-seated magnetic field and
the α effect have thus far prevented the formulation of a coherent
model for the solar dynamo. It is proposed that further progress is best
achieved through a combination of approaches including high-resolution
numerical simulations and global mean-field modeling. Along these
lines some recent numerical simulations of magnetic pumping and flux
expulsion in a magnetic layer are presented. The computations were
performed with a new anelastic Cartesian finite-difference MHD code,
which is the appropriate tool for studying processes near the base
of stellar convection zones with a high spatial resolution. Possible
implications for the solar dynamo are discussed.
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Title: Estimates of the Strouhal number from numerical models of
convection
Authors: Käpylä, P. J.; Korpi, M. J.; Ossendrijver, M.; Tuominen, I.
2005AN....326..186K Altcode: 2004astro.ph.10587K
We determine the Strouhal number (hereafter St), which is essentially
a nondimensional measure of the correlation time, from numerical
calculations of convection. We use two independent methods to estimate
St. Firstly, we apply the minimal tau-approximation (MTA) on the
equation of the time derivative of the Reynolds stress. A relaxation
time is obtained from which St can be estimated by normalising with
a typical turnover time. Secondly, we calculate the correlation and
turnover times separately, the former from the autocorrelation of
velocity and the latter by following test particles embedded in the
flow. We find that the Strouhal number is in general of the order of
0.1 to 1, i.e. rather large in comparison to the typical assumption
in the mean-field theories that St ≪ 1. However, there is a clear
decreasing trend as function of the Rayleigh number and increasing
rotation. Furthermore, for the present range of parameters the decrease
of St does not show signs of saturation, indicating that in stellar
convection zones, where the Rayleigh numbers are much larger, the
Strouhal number may indeed be significantly smaller.
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Title: The dynamo layer in solar-type stars
Authors: Ossendrijver, M.
2005AN....326..166O Altcode:
The role of the dynamo layer for dynamo action in solar-type
stars is discussed. As a result of instabilities, magnetic flux is
expelled from the dynamo layer into the convection zone above it. The
resulting partial evacuation of the dynamo layer and amplification
of the deep-seated toroidal magnetic field is investigated in an
illustrative 3D numerical simulation. The simulation is performed with
a finite-difference code that solves the anelastic MHD equations for a
Cartesian geometry. It is found that the field strength in the dynamo
layer is amplified by a factor of up to about 7.
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Title: Understanding the solar dynamo
Authors: Ossendrijver, M.
2004cosp...35.3775O Altcode: 2004cosp.meet.3775O
The solar magnetic field continues to be an outstanding challenge
of astrophysics. At the solar surface the magnetic field assumes
a complex, hierarchical structure consisting of widely different
spatial and temporal scales. Systematic features such as the solar
cycle and the butterfly diagram point to the existence of a deep-rooted
large-scale magnetic field, subject to variability on widely different
timescales. At the other end of the spatial scale are magnetic elements
and mixed-polarity magnetic fields. In order to explain these phenomena,
dynamo theory provides all the necessary ingredients including the
α effect, field amplification by differential rotation and other
mechanisms, magnetic pumping, turbulent diffusion, magnetic buoyancy,
flux storage, stochastic variations and nonlinear dynamics. Due to
advances in helioseismology, observations of stellar magnetic fields
and computer capabilities, significant progress has been made in our
understanding of these and other aspects such as the role of the
tachocline and magnetic helicity conservation. However, remaining
uncertainties about the nature of the deep-seated magnetic field and
the α effect have thus far prevented the formulation of a coherent
model for the solar dynamo. A preliminary evaluation of current
dynamo models favors a buoyancy-driven or distributed scenario. It
is proposed that progress in understanding the solar dynamo is best
achieved through a combination of approaches including high-resolution
numerical simulations and global mean-field modeling.
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Title: Book Review: ADVANCES IN NONLINEAR DYNAMOS / Taylor and
Francis, 2003
Authors: Ossendrijver, Mathieu
2003SoPh..218..359O Altcode:
No abstract at ADS
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Title: What can we learn from Local Convection Simulations in the
Context of mean Field Models of Stellar Rotation and Magnetism?
Authors: Käpylä, Petri J.; Korpi, Maarit J.; Ossendrijver, Mathieu;
Stix, Michael
2003ANS...324...63K Altcode: 2003ANS...324..I02K
No abstract at ADS
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Title: Do spherical α<SUP>2</SUP>-dynamos oscillate?
Authors: Rüdiger, G.; Elstner, D.; Ossendrijver, M.
2003A&A...406...15R Altcode: 2002astro.ph.12203R
The question of whether kinematic alpha <SUP>2</SUP>-shell-dynamos are
able to produce a cyclic activity is addressed. The alpha -effect is
allowed to be latitudinally inhomogeneous and anisotropic but it is
assumed as radially uniform in the turbulent shell. For a symmetric
alpha -tensor we only find oscillatory solutions if i) the alpha
-tensor component alpha <SUB>zz</SUB> vanishes or is of the opposite
sign as alpha <SUB>phi phi </SUB>, ii) the alpha -effect is strongly
concentrated in the equatorial region and iii) the alpha -effect is
concentrated in a rather thin outer shell. In the other cases almost
always the nonaxisymmetric field mode S1 (which slowly drifts along the
azimuthal direction) possesses the lowest critical dynamo number. The
uniform but anisotropic alpha -effect (e.g. alpha <SUB>zz</SUB> = 0)
leads to nonaxisymmetric solutions as is experimentally confirmed by
the Karlsruhe dynamo installation. One of the antisymmetric parts of
the alpha -tensor, however, plays the role of differential rotation in
the induction equation. Using the results of a numerical simulation
for the alpha -tensor of the solar convection zone for the radial
profile of this effect, one finds the possibility of oscillating alpha
<SUP>2</SUP>-dynamos even without the existence of a real nonuniform
rotation law. Compared with the results of numerical simulations of
the full alpha -tensor, the amplitude of the antisymmetric part of
the tensor must be artifically increased by only a factor of three in
order to find oscillating solutions.
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Title: Anelastic Simulations of Convection and Magnetic Fields in
Cartesian Geometry
Authors: Ossendrijver, Mathieu
2003ANS...324...64O Altcode: 2003ANS...324..I03O
No abstract at ADS
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Title: Crisis-induced intermittency due to attractor-widening in a
buoyancy-driven solar dynamo
Authors: Ossendrijver, M.; Covas, E.
2003IJBC....8.2327O Altcode:
In a recent paper [M. A. J. H. Ossendrijver, A&A, 359, 364 (2000)]
numerical simulations of a 2D mean-field model where shown to produce
grand minima, typical of the long-term behavior of solar magnetic
activity. The model consisted of dynamo that features an alpha effect
based on the buoyancy instability of magnetic fluxtubes, which gives
rise to the switching back and forth from grand minima to “regular”
solar behavior. In this Letter, we report evidence from a time-series
analysis of the model for the presence of crisis induced intermittency
due to attractor-widening. We support this finding by showing that the
average duration of the minima, , follows the theoretically predicted
scaling ~ (C-C*)^(gamma), where C is the bifurcation parameter of
interest, together with other statistical evidence. As far as we are
aware, this is the first time concrete and detailed evidence has been
produced for the occurrence of this type of crisisinduced intermittency
-- due to attractor widening - for such dynamo models.
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Title: The Solar Dynamo: A Challenge for Theory and Observations
(Invited review)
Authors: Ossendrijver, M.
2003ASPC..286...97O Altcode: 2003ctmf.conf...97O
No abstract at ADS
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Title: The solar dynamo
Authors: Ossendrijver, Mathieu
2003A&ARv..11..287O Altcode:
The solar dynamo continues to pose a challenge to observers and
theoreticians. Observations of the solar surface reveal a magnetic
field with a complex, hierarchical structure consisting of widely
different scales. Systematic features such as the solar cycle, the
butterfly diagram, and Hale's polarity laws point to the existence
of a deep-rooted large-scale magnetic field. At the other end of the
scale are magnetic elements and small-scale mixed-polarity magnetic
fields. In order to explain these phenomena, dynamo theory provides
all the necessary ingredients including the α effect, magnetic field
amplification by differential rotation, magnetic pumping, turbulent
diffusion, magnetic buoyancy, flux storage, stochastic variations and
nonlinear dynamics. Due to advances in helioseismology, observations
of stellar magnetic fields and computer capabilities, significant
progress has been made in our understanding of these and other aspects
such as the role of the tachocline, convective plumes and magnetic
helicity conservation. However, remaining uncertainties about the
nature of the deep-seated toroidal magnetic field and the α effect,
and the forbidding range of length scales of the magnetic field and
the flow have thus far prevented the formulation of a coherent model
for the solar dynamo. A preliminary evaluation of the various dynamo
models that have been proposed seems to favor a buoyancy-driven or
distributed scenario. The viewpoint proposed here is that progress
in understanding the solar dynamo and explaining the observations can
be achieved only through a combination of approaches including local
numerical experiments and global mean-field modeling.
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Title: The solar dynamo: a challenge for theory and observation
Authors: Ossendrijver, Mathieu
2002ESASP.506..797O Altcode: 2002svco.conf..797O; 2002ESPM...10..797O
The theory of the solar dynamo is reviewed with an emphasis on recent
advances of numerical simulations, and the possible consequences
for various dynamo scenarios. The solar dynamo poses a challenge
to both theoreticians and observers. Theoreticians are plagued by
the forbidding range of length scales in the solar convection zone,
which has thus far prevented succesful global MHD dynamo calculations
in spherical geometry. Observers have access only to the surface
magnetic field, which has been studied in great detail. While some
observations (solar cycle, butterfly diagram, Hale's polarity laws)
provide clues about the deep-rooted large-scale magnetic field, the
diagnostic value of other surface observations for the solar dynamo is
less easily established. With the increased of computer capabilities,
significant advances have been made through local simulations in
our understanding of various aspects of the solar dynamo. From box
simulations of magnetoconvection, transport coefficients for the
large-scale mean field have been estimated, and these can serve as input
information for global models. Several alternative dynamo scenarios
for the generation of the large-scale magnetic field coexist. They
differ mainly with regard to the nature of the α-effect. The viewpoint
proposed here is that progress in understanding the solar dynamo and
explaining the observations can be achieved only through a combination
of approaches which include local numerical experiments as well as
global mean-field modeling.
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Title: Magnetoconvection and dynamo coefficients. II. Field-direction
dependent pumping of magnetic field
Authors: Ossendrijver, M.; Stix, M.; Brandenburg, A.; Rüdiger, G.
2002A&A...394..735O Altcode: 2002astro.ph..2299O
We study the pumping of magnetic flux in three-dimensional compressible
magnetoconvection in the context of stellar dynamos. The simulation
domain represents a rectangular section from the lower part of
a stellar convection zone plus the underlying stably stratified
layer, with a total depth of up to five pressure scale heights. Once
convection has attained a statistically stationary state, a magnetic
field is introduced. The magnetic field is subsequently modified
by the convective motions, and the resulting pumping effects are
isolated by calculating various coefficients of the expansion of the
electromotive force, /line{u}x{b}, in terms of components of the mean
magnetic field. The dependence of the pumping effects on rotation,
latitude and other parameters is studied. First numerical evidence
is found for the existence of pumping effects in the horizontal
directions. Evidence is found that the pumping effects act differently
on different components of the mean magnetic field. Latitudinal pumping
is mainly equatorward for a toroidal field, and can be poleward for a
poloidal field. Longitudinal pumping is mainly retrograde for the radial
field but prograde for the latitudinal field. The pumping effect in the
vertical direction is found to be dominated by the diamagnetic effect,
equivalent to a predominating downward advection with a maximum speed in
the turbulent case of about 10% of the rms convective velocity. Where
possible, an attempt is made to identify the physical origin of the
effect. Finally, some consequences of the results for stellar dynamos
are discussed.
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Title: Solar Cycle
Authors: Ossendrijver, M.; Hoyng, P.
2002eaa..bookE1980O Altcode:
The term solar cycle refers to a quasi-periodic variation with a period
of about 11 years, visible in many of the Sun's observables. The
solar cycle is most easily observed in indices directly related to
the Sun's magnetic field, such as SUNSPOTS. During the last decades,
solar-cycle variations have also been found in many other aspects of
the Sun (irradiance, surface flows, coronal shape, oscil...
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Title: A theoretical analysis of the observed variability of the
geomagnetic dipole field
Authors: Hoyng, P.; Schmitt, D.; Ossendrijver, M. A. J. H.
2002PEPI..130..143H Altcode:
We present a detailed analysis of the Sint-800 virtual axial dipole
moment (VADM) data in terms of an αΩ mean field model of the geodynamo
that features a non-steady generation of poloidal from toroidal magnetic
field. The result is a variable excitation of the dipole mode and the
overtones, and there are occasional dipole reversals. The model permits
a theoretical evaluation of the statistical properties of the dipole
mode. We show that the model correctly predicts the distribution of
the VADM and the autocorrelation function inferred from the Sint-800
data. The autocorrelation technique allows us to determine the turbulent
diffusion time τ<SUB>d</SUB>= R<SUP>2</SUP>/ β of the geodynamo. We
find that τ<SUB>d</SUB> is about 10-15 kyr. The model is able to
reproduce the observed secular variation of the dipole mode, and the
mean time between successive dipole reversals. On the other hand, the
duration of a reversal is a factor ∼2 too long. This could be due to
imperfections in the model or to unknown systematics in the Sint-800
data. The use of mean field theory is shown to be selfconsistent.
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Title: Magnetic field reversals and secular variation in a bistable
geodynamo model
Authors: Schmitt, D.; Ossendrijver, M. A. J. H.; Hoyng, P.
2001PEPI..125..119S Altcode:
A method is described which enables the calculation of statistical
properties of geodynamo models. The result is a Fokker-Planck equation
for the probability distribution of the dipole moment. The method
only requires a non-oscillatory, predominantly dipolar magnetic field
and helicity fluctuations. The fluctuations perturb the fundamental
dynamo mode and lead to the excitation of higher modes. This results in
stochastic oscillations of the dipole field amplitude in a bistable
potential with minima representing normal and reversed polarity,
and occasional jumps between them. The shape of the potential is
determined by supercritical dynamo excitation and nonlinear limitation
of field growth. Application of the method to a mean-field dynamo model
with random fluctuations of the helicity parameter α reproduces the
observed relation between the secular variation and the reversal rate
of the geomagnetic field, as well as the amplitude distribution of
the dipole field inferred from the Sint-800 record.
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Title: Magnetoconvection and dynamo coefficients:. Dependence of
the alpha effect on rotation and magnetic field
Authors: Ossendrijver, M.; Stix, M.; Brandenburg, A.
2001A&A...376..713O Altcode: 2001astro.ph..8274O
We present numerical simulations of three-dimensional compressible
magnetoconvection in a rotating rectangular box that represents
a section of the solar convection zone. The box contains a
convectively unstable layer, surrounded by stably stratified layers
with overshooting convection. The magnetic Reynolds number, Rm, is
chosen subcritical, thus excluding spontaneous growth of the magnetic
field through dynamo action, and the magnetic energy is maintained by
introducing a constant magnetic field into the box, once convection
has attained a statistically stationary state. Under the influence
of the Coriolis force, the advection of the magnetic field results
in a non-vanishing contribution to the mean electric field, given by
<vec{u}xvec{b}>. From this electric field, we calculate the alpha
-effect, separately for the stably and the unstably stratified layers,
by averaging over time and over suitably defined volumes. From the
variation of alpha we derive an error estimate, and the dependence of
alpha on rotation and magnetic field strength is studied. Evidence is
found for rotational quenching of the vertical alpha effect, and for
a monotonic increase of the horizontal alpha effect with increasing
rotation. For Rm~ 30, our results for both vertical and horizontal
alpha effect are consistent with magnetic quenching by a factor [1+Rm
(B<SUB>0</SUB>/B<SUB>eq</SUB>)<SUP>2</SUP>]<SUP>-1</SUP>. The signs
of the small-scale current helicity and of the vertical component of
alpha are found to be opposite to those for isotropic turbulence.
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Title: Crisis-induced intermittency due to attractor-widening in a
buoyancy-driven solar dynamo
Authors: Covas, Eurico; Ossendrijver, Mathieu
2001nlin......7007C Altcode:
In a recent paper [M. A. J. H. Ossendrijver, A&A {\bf 359},
364 (2000)] numerical simulations of a 2D mean--field model where
shown to produce grand minima, typical of the long-term behavior of
solar magnetic activity. The model consisted of dynamo that features
an $\alpha$ effect based on the buoyancy instability of magnetic
fluxtubes, which gives rise to the switching back and forth from
grand minima to “regular” solar behavior. In this Letter, we report
evidence from a time-series analysis of the model for the presence
of crisis--induced intermittency due to attractor--widening. We
support this finding by showing that the average duration of the
minima, $<\tau>$, follows the theoretically predicted scaling
$<\tau> \sim (C_{\delta\alpha}-C_{\delta\alpha}^*)^{-\gamma}$,
where $C_{\delta\alpha}$ is the bifurcation parameter of interest,
together with other statistical evidence. As far as we are aware, this
is the first time concrete and detailed evidence has been produced
for the occurrence of this type of crisis--induced intermittency --
due to attractor widening -- for such dynamo models.
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Title: The geodynamo as a bistable oscillator
Authors: Hoyng, P.; Ossendrijver, M. A. J. H.; Schmitt, D.
2001GApFD..94..263H Altcode:
Our intent is to provide a simple and quantitative understanding of the
variability of the axial dipole component of the geomagnetic field on
both short and long time scales. To this end we study the statistical
properties of a prototype nonlinear mean field model. An azimuthal
average is employed, so that (1) we address only the axisymmetric
component of the field, and (2) the dynamo parameters have a
random component that fluctuates on the (fast) eddy turnover time
scale. Numerical solutions with a rapidly fluctuating alpha reproduce
several features of the geomagnetic field: (1) a variable, dominantly
dipolar field with additional fine structure due to excited overtones,
and sudden reversals during which the field becomes almost quadrupolar,
(2) aborted reversals and excursions, (3) intervals between reversals
having a Poisson distribution. These properties are robust, and appear
regardless of the type of nonlinearity and the model parameters. A
technique is presented for analysing the statistical properties of
dynamo models of this type. The Fokker-Planck equation for the amplitude
a of the fundamental dipole mode shows that a behaves as the position
of a heavily damped particle in a bistable potential ~(1-a^2)^2,
subject to random forcing. The dipole amplitude oscillates near the
bottom of one well and makes occasional jumps to the other. These
reversals are induced solely by the overtones. Theoretical expressions
are derived for the statistical distribution of the dipole amplitude,
the variance of the dipole amplitude between reversals, and the mean
reversal rate. The model explains why the reversal rate increases
with increasing secular variation, as observed. Moreover, the present
reversal rate of the geodynamo, once per (2-3)x10^5years, is shown
to imply a secular variation of the dipole moment of ~15% (about the
current value). The theoretical dipole amplitude distribution agrees
well with the Sint-800 data.
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Title: Grand minima in a buoyancy-driven solar dynamo
Authors: Ossendrijver, M. A. J. H.
2000A&A...359..364O Altcode:
Numerical simulations of a 2D mean-field model are presented which
show that grand minima, typical for the long-term behaviour of solar
magnetic activity, can be produced by a dynamo that features an alpha
effect based on the buoyancy instability of magnetic fluxtubes. The
buoyancy-driven alpha effect functions only if the magnetic field
strength exceeds a minimal value necessary for instability. It opens
the possibility of dynamo action within the solar overshoot layer,
where a strong magnetic field, B~ 10<SUP>5</SUP> G, is thought to be
stored. The existence of a magnetic threshold for dynamo action can lead
to interruptions of the magnetic cycle, similar to the grand minima
of solar activity. Transitions across the instability threshold are
triggered by magnetic-flux injections from the convection zone. This
is modelled by allowing for a small-scale kinematic alpha effect in
the convection zone, and convective downdrafts that penetrate the
overshoot layer.
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Title: The dynamo effect of magnetic flux tubes
Authors: Ossendrijver, M. A. J. H.
2000A&A...359.1205O Altcode:
It is shown that toroidal magnetic flux tubes in a rotating star do
not provide a net dynamo effect, even if they are subject to random
forcing, unless the tubes are unstable to small displacements. The
dynamo effect is produced by transverse helical waves that propagate
along the flux tube, resulting in an electric current (anti-) parallel
to the unperturbed magnetic field, equivalent to an alpha effect. For
unstable flux tubes, the alpha effect is enabled by a systematic phase
difference between the velocity and magnetic field perturbations. For
stable flux tubes subject to random forcing, phase differences do occur,
but they vanish in the mean. The requirement for instability is met
if the magnetic field strength exceeds a threshold value. Therefore
a stellar dynamo based on flux tubes is not self-excited, but needs
triggering until the magnetic threshold is surpassed.
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Title: The α-Effect of Toroidal Fluxtubes and its Application in
a Mean-Field Solar Dynamo
Authors: Ossendrijver, M.
1999ASPC..178..127O Altcode: 1999sdnc.conf..127O
No abstract at ADS
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Title: The geodynamo as a bistable oscillator
Authors: Ossendrijver, M. A. J. H.; Hoyng, P.; Schmitt, D.
1999AGAb...15...73O Altcode: 1999AGM....15..J13O
Paleomagnetism has convincingly shown that the Earth has reversed
the polarity of its almost dipolar magnetic field many times in the
past. The origin of the field is generally ascribed to dynamo action
in the liquid outer core. Numerical simulation of a mean-field model
demonstrates that random fluctuations of the dynamo alpha-effect
cause occasional rapid magnetic reversals, like those of the
Earth's field. The fluctuations perturb the fundamental stationary
dipolar mode and lead to the excitation of higher oscillatory dynamo
modes. This results in stochastic oscillations of the dipole field
amplitude in a bistable potential with minima representing normal and
reversed polarity, and occasional jumps between them. The shape of
the potential is determined by supercritical dynamo excitation and
nonlinear limitation of field growth. A statistical analysis shows
that the model reproduces the essential features of the geomagnetic
secular variation and reversal record.
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Title: Book Review: The cosmic dynamo / Kluwer, 1993
Authors: Ossendrijver, M.; Ossendrijver, M.
1994SoPh..155..207O Altcode: 1994SoPh..155..207K
No abstract at ADS
