998 research outputs found
Solar differential rotation and meridional flow: The role of a subadiabatic tachocline for the Taylor-Proudman balance
We present a simple model for the solar differential rotation and meridional
circulation based on a mean field parameterization of the Reynolds stresses
that drive the differential rotation. We include the subadiabatic part of the
tachocline and show that this, in conjunction with turbulent heat conductivity
within the convection zone and overshoot region, provides the key physics to
break the Taylor-Proudman constraint, which dictates differential rotation with
contour lines parallel to the axis of rotation in case of an isentropic
stratification. We show that differential rotation with contour lines inclined
by 10 - 30 degrees with respect to the axis of rotation is a robust result of
the model, which does not depend on the details of the Reynolds stress and the
assumed viscosity, as long as the Reynolds stress transports angular momentum
toward the equator. The meridional flow is more sensitive with respect to the
details of the assumed Reynolds stress, but a flow cell, equatorward at the
base of the convection zone and poleward in the upper half of the convection
zone, is the preferred flow pattern.Comment: 15 pages, 7 figure
Observation and Modeling of the Solar-Cycle Variation of the Meridional Flow
We present independent observations of the solar-cycle variation of flows
near the solar surface and at a depth of about 60 Mm, in the latitude range
. We show that the time-varying components of the meridional flow
at these two depths have opposite sign, while the time-varying components of
the zonal flow are in phase. This is in agreement with previous results. We
then investigate whether the observations are consistent with a theoretical
model of solar-cycle dependent meridional circulation based on a flux-transport
dynamo combined with a geostrophic flow caused by increased radiative loss in
the active region belt (the only existing quantitative model). We find that the
model and the data are in qualitative agreement, although the amplitude of the
solar-cycle variation of the meridional flow at 60 Mm is underestimated by the
model.Comment: To be published in Solar Physcis Topical Issue "Helioseismology,
Asteroseismology, and MHD Connections
Origin of solar torsional oscillations
Helioseismology has revealed many details of solar differential rotation and
also its time variation, known as torsional oscillations. So far there is no
generally accepted theoretical explanation for torsional oscillations, even
though a close relation to the solar activity cycle is evident. On the
theoretical side non-kinematic dynamo models (including the Lorentz force
feedback on differential rotation) have been used to explain torsional
oscillations. In this paper we use a slightly different approach by forcing
torsional oscillations in a mean field differential rotation model. Our aim is
not a fully self-consistent model but rather to point out a few general
properties of torsional oscillations and their possible origin that are
independent from a particular dynamo model. We find that the poleward
propagating high latitude branch of the torsional oscillations can be explained
as a response of the coupled differential rotation / meridional flow system to
periodic forcing in mid-latitudes, of either mechanical (Lorentz force) or
thermal nature. The speed of the poleward propagation sets constraints on the
value of the turbulent viscosity in the solar convection zone to be less than
3x10^8 m^2/s. We also show that the equatorward propagating low latitude branch
is very unlikely a consequence of mechanical forcing (Lorentz force) alone, but
rather of thermal origin due to the Taylor-Proudman theorem.Comment: 11 pages, 7 figures. accepted by Astrophys.
Flux-transport dynamos with Lorentz force feedback on differential rotation and meridional flow: Saturation mechanism and torsional oscillations
In this paper we discuss a dynamic flux-transport dynamo model that includes
the feedback of the induced magnetic field on differential rotation and
meridional flow. We consider two different approaches for the feedback:
meanfield Lorentz force and quenching of transport coefficients such as
turbulent viscosity and heat conductivity. We find that even strong feedback on
the meridional flow does not change the character of the flux-transport dynamo
significantly; however it leads to a significant reduction of differential
rotation. To a large degree independent from the dynamo parameters, the
saturation takes place when the toroidal field at the base of the convection
zone reaches between 1.2 an 1.5 T, the energy converted intomagnetic energy
corresponds to about 0.1 to 0.2% of the solar luminosity. The torsional
oscillations produced through Lorentz force feedback on differential rotation
show a dominant poleward propagating branch with the correct phase relation to
the magnetic cycle. We show that incorporating enhanced surface cooling of the
active region belt (as proposed by Spruit) leads to an equatorward propagating
branch in good agreement with observations.Comment: 15 pages, 12 figures, Accepted for publication in ApJ August 10
issue; corrected typos, corrected referenc
Chaotic saddles in nonlinear modulational interactions in a plasma
A nonlinear model of modulational processes in the subsonic regime involving
a linearly unstable wave and two linearly damped waves with different damping
rates in a plasma is studied numerically. We compute the maximum Lyapunov
exponent as a function of the damping rates in a two-parameter space, and
identify shrimp-shaped self-similar structures in the parameter space. By
varying the damping rate of the low-frequency wave, we construct bifurcation
diagrams and focus on a saddle-node bifurcation and an interior crisis
associated with a periodic window. We detect chaotic saddles and their stable
and unstable manifolds, and demonstrate how the connection between two chaotic
saddles via coupling unstable periodic orbits can result in a crisis-induced
intermittency. The relevance of this work for the understanding of modulational
processes observed in plasmas and fluids is discussed.Comment: Physics of Plasmas, in pres
Lagrangian chaos in an ABC--forced nonlinear dynamo
The Lagrangian properties of the velocity field in a magnetized fluid are
studied using three-dimensional simulations of a helical magnetohydrodynamic
dynamo. We compute the attracting and repelling Lagrangian coherent structures,
which are dynamic lines and surfaces in the velocity field that delineate
particle transport in flows with chaotic streamlines and act as transport
barriers. Two dynamo regimes are explored, one with a robust coherent mean
magnetic field and one with intermittent bursts of magnetic energy. The
Lagrangian coherent structures and the statistics of finite--time Lyapunov
exponents indicate that the stirring/mixing properties of the velocity field
decay as a linear function of the magnetic energy. The relevance of this study
for the solar dynamo problem is discussed
Hybrid nanoparticles based on sulfides, oxides, and carbides
The methods for synthesis of hybrid nanoparticles based on sulfides, oxides, and carbides of heavy and transition metals were considered. The problem of the influence of the method of synthesis of the hybrid nanoparticles on their atomic structure, morphology of the nanomaterials, and functional properties was analyzed. The areas of practical use of the hybrid nanoparticles were proposed. © 2013 Springer Science+Business Media New York
Local helioseismology of sunspot regions: comparison of ring-diagram and time-distance results
Local helioseismology provides unique information about the subsurface
structure and dynamics of sunspots and active regions. However, because of
complexity of sunspot regions local helioseismology diagnostics require careful
analysis of systematic uncertainties and physical interpretation of the
inversion results. We present new results of comparison of the ring-diagram
analysis and time-distance helioseismology for active region NOAA 9787, for
which a previous comparison showed significant differences in the subsurface
sound-speed structure, and discuss systematic uncertainties of the measurements
and inversions. Our results show that both the ring-diagram and time-distance
techniques give qualitatively similar results, revealing a characteristic
two-layer seismic sound-speed structure consistent with the results for other
active regions. However, a quantitative comparison of the inversion results is
not straightforward. It must take into account differences in the sensitivity,
spatial resolution and the averaging kernels. In particular, because of the
acoustic power suppression, the contribution of the sunspot seismic structure
to the ring-diagram signal can be substantially reduced. We show that taking
into account this effect reduces the difference in the depth of transition
between the negative and positive sound-speed variations inferred by these
methods. Further detailed analysis of the sensitivity, resolution and averaging
properties of the local helioseismology methods is necessary for consolidation
of the inversion results. It seems to be important that both methods indicate
that the seismic structure of sunspots is rather deep and extends to at least
20 Mm below the surface, putting constraints on theoretical models of sunspots.Comment: 10 pages, 10 figures, submitted to Journal of Physics: Conference
Series (JPCS) GONG 2010 - SoHO 24 "A new era of seismology of the Sun and
solar-like stars", June 27 - July 2, 2010 Aix-en-Provence, Franc
Newly identified properties of surface acoustic power
The cause of enhanced acoustic power surrounding active regions, the acoustic
halo, is not as yet understood. We explore the properties of the enhanced
acoustic power observed near disk center from 21 to 27 January 2002, including
AR 9787. We find that (i) there exists a strong correlation of the enhanced
high frequency power with magnetic-field inclination, with greater power in
more horizontal fields, (ii) the frequency of the maximum enhancement increases
along with magnetic field strength, and (iii) the oscillations contributing to
the halos show modal ridges which are shifted to higher wavenumber at constant
frequency in comparison to the ridges of modes in the quiet-Sun.Comment: 16 pages, 10 figures, submitted to solar physic
Validated helioseismic inversions for 3-D vector flows
According to time-distance helioseismology, information about internal fluid
motions is encoded in the travel times of solar waves. The inverse problem
consists of inferring 3-D vector flows from a set of travel-time measurements.
Here we investigate the potential of time-distance helioseismology to infer 3-D
convective velocities in the near-surface layers of the Sun. We developed a new
Subtractive Optimally Localised Averaging (SOLA) code suitable for pipeline
pseudo-automatic processing. Compared to its predecessor, the code was improved
by accounting for additional constraints in order to get the right answer
within a given noise level. The main aim of this study is to validate results
obtained by our inversion code. We simulate travel-time maps using a snapshot
from a numerical simulation of solar convective flows, realistic Born
travel-time sensitivity kernels, and a realistic model of travel-time noise.
These synthetic travel times are inverted for flows and the results compared
with the known input flow field. Additional constraints are implemented in the
inversion: cross-talk minimization between flow components and spatial
localization of inversion coefficients. Using modes f, p1 through p4, we show
that horizontal convective flow velocities can be inferred without bias, at a
signal-to-noise ratio greater than one in the top 3.5 Mm, provided that
observations span at least four days. The vertical component of velocity (v_z),
if it were to be weak, is more difficult to infer and is seriously affected by
cross-talk from horizontal velocity components. We emphasise that this
cross-talk must be explicitly minimised in order to retrieve v_z in the top 1
Mm. We also show that statistical averaging over many different areas of the
Sun allows for reliably measuring of average properties of all three flow
components in the top 5.5 Mm of the convection zone.Comment: 14 pages main paper, 9 pages electronic supplement, 28 figures.
Accepted for publication in Astronomy & Astrophysic
- …