22,680 research outputs found
Origin of the hemispheric asymmetry of solar activity
The frequency spectrum of the hemispheric asymmetry of solar activity shows
enhanced power for the period ranges around 8.5 years and between 30 and 50
years. This can be understood as the sum and beat periods of the superposition
of two dynamo modes: a dipolar mode with a (magnetic) period of about 22 years
and aquadrupolar mode with a period between 13 and 15 years. An updated
Babcock-Leighton-type dynamo model with weak driving as indicated by stellar
observations shows an excited dipole mode and a damped quadrupole mode in the
correct range of periods. Random excitation of the quadrupole by stochastic
fluctuations of the source term for the poloidal field leads to a time
evolution of activity and asymmetry that is consistent with the observational
results.Comment: Astronomy & Astrophysics, accepte
Are the strengths of solar cycles determined by converging flows towards the activity belts?
It is proposed that the observed near-surface inflows towards the active
regions and sunspot zones provide a nonlinear feedback mechanism that limits
the amplitude of a Babcock-Leighton-type solar dynamo and determines the
variation of the cycle strength. This hypothesis is tested with surface flux
transport simulations including converging latitudinal flows that depend on the
surface distribution of magnetic flux. The inflows modulate the build-up of
polar fields (represented by the axial dipole) by reducing the tilt angles of
bipolar magnetic regions and by affecting the cross-equator transport of
leading-polarity magnetic flux. With flux input derived from the observed
record of sunspot groups, the simulations cover the period between 1874 and
1980 (corresponding to solar cycles 11 to 20). The inclusion of the inflows
leads to a strong correlation of the simulated axial dipole strength during
activity minimum with the observed amplitude of the subsequent cycle. This in
agreement with empirical correlations and in line with what is expected from a
Babcock-Leighton-type dynamo. The results provide evidence that the latitudinal
inflows are a key ingredient in determining the amplitude of solar cycles.Comment: accepted in A&
Convectively stabilised background solar models for local helioseismology
In local helioseismology numerical simulations of wave propagation are useful
to model the interaction of solar waves with perturbations to a background
solar model. However, the solution to the equations of motions include
convective modes that can swamp the waves we are interested in. For this
reason, we choose to first stabilise the background solar model against
convection by altering the vertical pressure gradient. Here we compare the
eigenmodes of our convectively stabilised model with a standard solar model
(Model S) and find a good agreement.Comment: 3 pages, 3 figures, HELAS NA3, The Acoustic Solar Cycle, Birmingham,
6-8 January 200
Survival of Antarctic desert soil bacteria exposed to various temperatures and to three years of continuous medium-high vacuum
Survival of Antarctic dessert soil bacteria exposed to various temperatures and to three years of continuous medium-high vacuu
Specification of cell fate in the sea urchin embryo: summary and some proposed mechanisms
An early set of blastomere specifications occurs during cleavage in the sea urchin embryo, the result of both conditional and autonomous processes, as proposed in the model for this embryo set forth in 1989. Recant experimental results have greatly illuminated the mechanisms of specification in some early embryonic territories, though others remain obscure. We review the progressive process of specification within given lineage elements, and with reference to the early axial organization of the embryo. Evidence for the conditional specification of the veg(2) lineage subelement of the endoderm and other potential interblastomere signaling interactions in the cleavage-stage embryo are summarized. Definitive boundaries between mesoderm and endoderm territories of complex. the vegetal plate, and between endoderm and overlying ectoderm, are not established until later in development. These processes have been clarified by numerous observations on spatial expression of various genes, and by elegant lineage labeling studies. The early specification events depend on regional mobilization of regulatory factors resulting at once in the zygotic expression of genes encoding transcription factors, as well as downstream genes encoding proteins characteristic of the cell types that will much later arise from the progeny of the specified blastomeres. This embryo displays a maximal form of indirect development. The gene regulatory network underlying the embryonic development reflects the relative simplicity of the completed larva and of the processes required for its formation. The requirements for postembryonic adult body plan formation in the larval rudiment include engagement of a new level of genetic regulatory apparatus, exemplified by the Hox gene complex
Simulations Show that Vortex Flows could Heat the Chromosphere in Solar Plage
The relationship between vortex flows at different spatial scales and their
contribution to the energy balance in the chromosphere is not yet fully
understood. We perform three-dimensional (3D) radiation-magnetohydrodynamic
(MHD) simulations of a unipolar solar plage region at a spatial resolution of
10 km using the MURaM code. We use the swirling-strength criterion that mainly
detects the smallest vortices present in the simulation data. We additionally
degrade our simulation data to smooth-out the smaller vortices, so that also
the vortices at larger spatial scales can be detected. Vortex flows at various
spatial scales are found in our simulation data for different effective spatial
resolutions. We conclude that the observed large vortices are likely clusters
of much smaller ones that are not yet resolved by observations. We show that
the vertical Poynting flux decreases rapidly with reduced effective spatial
resolutions and is predominantly carried by the horizontal plasma motions
rather than vertical flows. Since the small-scale horizontal motions or the
smaller vortices carry most of the energy, the energy transported by vortices
deduced from low resolution data is grossly underestimated. In full resolution
simulation data, the Poynting flux contribution due to vortices is more than
adequate to compensate for the radiative losses in plage, indicating their
importance for chromospheric heating.Comment: 8 pages, 5 figures, accepted in ApJ
Observing and modeling the poloidal and toroidal fields of the solar dynamo
Context. The solar dynamo consists of a process that converts poloidal field
to toroidal field followed by a process which creates new poloidal field from
the toroidal field.
Aims. Our aim is to observe the poloidal and toroidal fields relevant to the
global solar dynamo and see if their evolution is captured by a
Babcock-Leighton dynamo.
Methods. We use synoptic maps of the surface radial field from the KPNSO/VT
and SOLIS observatories to construct the poloidal field as a function of time
and latitude, and Wilcox Solar Observatory and SOHO/MDI full disk images to
infer the longitudinally averaged surface azimuthal field. We show that the
latter is consistent with an estimate of that due to flux emergence and
therefore closely related to the subsurface toroidal field.
Results. We present maps of the poloidal and toroidal magnetic field of the
global solar dynamo. The longitude-averaged azimuthal field observed at the
surface results from flux emergence. At high latitudes this component follows
the radial component of the polar fields with a short time lag (1-3 years). The
lag increases at lower latitudes. The observed evolution of the poloidal and
toroidal magnetic fields is described by the (updated) Babcock-Leighton dynamo
model.Comment: A&
Spatial expression of Hox cluster genes in the ontogeny of a sea urchin
The Hox cluster of the sea urchin Strongylocentrous purpuratus contains ten genes in a 500 kb span of the genome. Only two of these genes are expressed during embryogenesis, while all of eight genes tested are expressed during development of the adult body plan in the larval stage. We report the spatial expression during larval development of the five 'posterior' genes of the cluster: SpHox7, SpHox8, SpHox9/10, SpHox11/13a and SpHox11/13b. The five genes exhibit a dynamic, largely mesodermal program of expression. Only SpHox7 displays extensive expression within the pentameral rudiment itself. A spatially sequential and colinear arrangement of expression domains is found in the somatocoels, the paired posterior mesodermal structures that will become the adult perivisceral coeloms. No such sequential expression pattern is observed in endodermal, epidermal or neural tissues of either the larva or the presumptive juvenile sea urchin. The spatial expression patterns of the Hox genes illuminate the evolutionary process by which the pentameral echinoderm body plan emerged from a bilateral ancestor
Penumbral structure and outflows in simulated sunspots
Sunspots are concentrations of magnetic field on the visible solar surface
that strongly affect the convective energy transport in their interior and
surroundings. The filamentary outer regions (penumbrae) of sunspots show
systematic radial outward flows along channels of nearly horizontal magnetic
field. These flows were discovered 100 years ago and are present in all fully
developed sunspots. Using a comprehensive numerical simulation of a sunspot
pair, we show that penumbral structures with such outflows form when the
average magnetic field inclination to the vertical exceeds about 45 degrees.
The systematic outflows are a component of the convective flows that provide
the upward energy transport and result from anisotropy introduced by the
presence of the inclined magnetic field.Comment: 19 pages, 8 figures, main Science article + supporting online
material combined into one fil
Hindgut specification and cell-adhesion functions of Sphox11/13b in the endoderm of the sea urchin embryo
Sphox11/13b is one of the two hox genes of Strongylocentrotus purpuratus expressed in the embryo. Its
dynamic pattern of expression begins during gastrulation, when the transcripts are transiently located in a
ring of cells at the edge of the blastopore. After gastrulation, expression is restricted to the anus–hindgut
region at the boundary between the ectoderm and the endoderm. The phenotype that results when translation
of Sphox11/13b mRNA is knocked down by treatment with morpholino antisense oligonucleotides (MASO)
suggests that this gene may be indirectly involved in cell adhesion functions as well as in the proper
differentiation of the midgut–hindgut and midgut–foregut sphincters. The MASO experiments also reveal that
Sphox11/13b negatively regulates several downstream endomesoderm genes. For some of these genes,
Sphox11/13b function is required to restrict expression to the midgut by preventing ectopic expression in the
hindgut. The evolutionary conservation of these functions indicates the general roles of posterior Hox genes
in regulating cell-adhesion, as well as in spatial control of gene regulatory network subcircuits in the
regionalizing gut
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