188 research outputs found
Reply to comments of Dikpati et al
We present here our response to Dikpati et al.'s criticism of our recent
solar dynamo model.Comment: 8 pages, 2 figure
The Waldmeier Effect in Sunspot Cycles
We discuss two aspects of the Waldmeier Effect, namely (1) the rise times of
sunspot cycles are anti-correlated to their strengths (WE1) and (2) the rates
of rise of the cycles are correlated to their strengths (WE2). From analysis of
four different data sets we conclude that both WE1 and WE2 exist in all the
data sets. We study these effects theoretically by introducing suitable
stochastic fluctuations in our regular solar dynamo model.Comment: Magnetic Coupling between the Interior and Atmosphere of the Sun;
Astrophysics and Space Science Proceeding
On the compatibility of a flux transport dynamo with a fast tachocline scenario
The compatibility of the fast tachocline scenario with a flux transport
dynamo model is explored. We employ a flux transport dynamo model coupled with
simple feedback formulae relating the thickness of the tachocline to the
amplitude of the magnetic field or to the Maxwell stress. The dynamo model is
found to be robust against the nonlinearity introduced by this simplified fast
tachocline mechanism. Solar-like butterfly diagrams are found to persist and,
even without any parameter fitting, the overall thickness of the tachocline is
well within the range admitted by helioseismic constraints. In the most
realistic case of a time and latitude dependent tachocline thickness linked to
the value of the Maxwell stress, both the thickness and its latitude dependence
are in excellent agreement with seismic results. In the nonparametric models,
cycle related temporal variations in tachocline thickness are somewhat larger
than admitted by helioseismic constraints; we find, however, that introducing a
further parameter into our feedback formula readily allows further fine tuning
of the thickness variations.Comment: Accepted in Solar Physic
The Origin of Solar Activity in the Tachocline
Solar active regions, produced by the emergence of tubes of strong magnetic
field in the photosphere, are restricted to within 35 degrees of the solar
equator. The nature of the dynamo processes that create and renew these fields,
and are therefore responsible for solar magnetic phenomena, are not well
understood. We analyze the magneto-rotational stability of the solar tachocline
for general field geometry. This thin region of strong radial and latitudinal
differential rotation, between the radiative and convective zones, is unstable
at latitudes above 37 degrees, yet is stable closer to the equator. We propose
that small-scale magneto-rotational turbulence prevents coherent magnetic
dynamo action in the tachocline except in the vicinity of the equator, thus
explaining the latitudinal restriction of active regions. Tying the magnetic
dynamo to the tachocline elucidates the physical conditions and processes
relevant to solar magnetism.Comment: 10 pages, 1 figure, accepted for publication in ApJ
Solar Polar Fields During Cycles 21 --- 23: Correlation with Meridional Flows
We have examined polar magnetic fields for the last three solar cycles,
{}, cycles 21, 22 and 23 using NSO Kitt Peak synoptic magnetograms.
In addition, we have used SoHO/MDI magnetograms to derive the polar fields
during cycle 23. Both Kitt Peak and MDI data at high latitudes
(78--90) in both solar hemispheres show a significant
drop in the absolute value of polar fields from the late declining phase of the
solar cycle 22 to the maximum of the solar cycle 23. We find that long term
changes in the absolute value of the polar field, in cycle 23, is well
correlated with changes in meridional flow speeds that have been reported
recently. We discuss the implication of this in influencing the extremely
prolonged minimum experienced at the start of the current cycle 24 and in
forecasting the behaviour of future solar cycles.Comment: 4 Figures 11 pages; Revised version under review in Solar Physic
Magnetic helicity fluxes in interface and flux transport dynamos
Dynamos in the Sun and other bodies tend to produce magnetic fields that
possess magnetic helicity of opposite sign at large and small scales,
respectively. The build-up of magnetic helicity at small scales provides an
important saturation mechanism. In order to understand the nature of the solar
dynamo we need to understand the details of the saturation mechanism in
spherical geometry. In particular, we want to understand the effects of
magnetic helicity fluxes from turbulence and meridional circulation. We
consider a model with just radial shear confined to a thin layer (tachocline)
at the bottom of the convection zone. The kinetic alpha owing to helical
turbulence is assumed to be localized in a region above the convection zone.
The dynamical quenching formalism is used to describe the build-up of mean
magnetic helicity in the model, which results in a magnetic alpha effect that
feeds back on the kinetic alpha effect. In some cases we compare with results
obtained using a simple algebraic alpha quenching formula. In agreement with
earlier findings, the magnetic alpha effect in the dynamical alpha quenching
formalism has the opposite sign compared with the kinetic alpha effect and
leads to a catastrophic decrease of the saturation field strength with
increasing magnetic Reynolds numbers. However, at high latitudes this quenching
effect can lead to secondary dynamo waves that propagate poleward due to the
opposite sign of alpha. Magnetic helicity fluxes both from turbulent mixing and
from meridional circulation alleviate catastrophic quenching.Comment: 9 pages, 14 figures, submitted to A &
Magnetic Cycles in a Convective Dynamo Simulation of a Young Solar-type Star
Young solar-type stars rotate rapidly and many are magnetically active; some
undergo magnetic cycles similar to the 22-year solar activity cycle. We conduct
simulations of dynamo action in rapidly rotating suns with the 3D MHD anelastic
spherical harmonic (ASH) code to explore dynamo action achieved in the
convective envelope of a solar-type star rotating at 5 times the current solar
rotation rate. Striking global-scale magnetic wreaths appear in the midst of
the turbulent convection zone and show rich time-dependence. The dynamo
exhibits cyclic activity and undergoes quasi-periodic polarity reversals where
both the global-scale poloidal and toroidal fields change in sense on a roughly
1500 day time scale. These magnetic activity patterns emerge spontaneously from
the turbulent flow and are more organized temporally and spatially than those
realized in our previous simulations of the solar dynamo. We assess in detail
the competing processes of magnetic field creation and destruction within our
simulations that contribute to the global-scale reversals. We find that the
mean toroidal fields are built primarily through an -effect, while the
mean poloidal fields are built by turbulent correlations which are not
necessarily well represented by a simple -effect. During a reversal the
magnetic wreaths propagate towards the polar regions, and this appears to arise
from a poleward propagating dynamo wave. The primary response in the convective
flows involves the axisymmetric differential rotation which shows variations
associated with the poleward propagating magnetic wreaths. In the Sun, similar
patterns are observed in the poleward branch of the torsional oscillations, and
these may represent poleward propagating magnetic fields deep below the solar
surface. [abridged]Comment: 20 pages, 14 figures, emulateapj format; accepted for publication in
ApJ. Expanded and published version of sections 5-6 from
http://arxiv.org/abs/0906.240
Can catastrophic quenching be alleviated by separating shear and alpha effect?
The small-scale magnetic helicity produced as a by-product of the large-scale
dynamo is believed to play a major role in dynamo saturation. In a mean-field
model the generation of small-scale magnetic helicity can be modelled by using
the dynamical quenching formalism. Catastrophic quenching refers to a decrease
of the saturation field strength with increasing Reynolds number. It has been
suggested that catastrophic quenching only affects the region of non-zero
helical turbulence (i.e. where the kinematic alpha operates) and that it is
possible to alleviate catastrophic quenching by separating the region of strong
shear from the alpha layer. We perform a systematic study of a simple
axisymmetric two-layer alpha-omega dynamo in a spherical shell for Reynolds
numbers in the range 1 < Rm < 10^5. In the framework of dynamical quenching we
show that this may not be the case, suggesting that magnetic helicity fluxes
would be necessary.Comment: 8 pages, 5 figures (Accepted in Geophysical and Astrophysical Fluid
Dynamics
Outstanding Issues in Solar Dynamo Theory
The magnetic activity of the Sun, as manifested in the sunspot cycle,
originates deep within its convection zone through a dynamo mechanism which
involves non-trivial interactions between the plasma and magnetic field in the
solar interior. Recent advances in magnetohydrodynamic dynamo theory have led
us closer towards a better understanding of the physics of the solar magnetic
cycle. In conjunction, helioseismic observations of large-scale flows in the
solar interior has now made it possible to constrain some of the parameters
used in models of the solar cycle. In the first part of this review, I briefly
describe this current state of understanding of the solar cycle. In the second
part, I highlight some of the outstanding issues in solar dynamo theory related
to the the nature of the dynamo -effect, magnetic buoyancy and the
origin of Maunder-like minima in activity. I also discuss how poor constraints
on key physical processes such as turbulent diffusion, meridional circulation
and turbulent flux pumping confuse the relative roles of these vis-a-vis
magnetic flux transport. I argue that unless some of these issues are
addressed, no model of the solar cycle can claim to be ``the standard model'',
nor can any predictions from such models be trusted; in other words, we are
still not there yet.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
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