19,028 research outputs found
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&
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
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
Surface flux transport modeling for solar cycles 15--21: effects of cycle-dependent tilt angles of sunspot groups
We model the surface magnetic field and open flux of the Sun from 1913 to
1986 using a surface flux transport model, which includes the observed
cycle-to-cycle variation of sunspot group tilts. The model reproduces the
empirically derived time evolution of the solar open magnetic flux, and the
reversal times of the polar fields. We find that both the polar field and the
axial dipole moment resulting from this model around cycle minimum correlate
with the strength of the following cycle.Comment: Accepted for publication by Ap
Constructing and Characterising Solar Structure Models for Computational Helioseismology
In this paper, we construct background solar models that are stable against
convection, by modifying the vertical pressure gradient of Model S
(Christensen-Dalsgaard et al., 1996, Science, 272, 1286) relinquishing
hydrostatic equilibrium. However, the stabilisation affects the eigenmodes that
we wish to remain as close to Model S as possible. In a bid to recover the
Model S eigenmodes, we choose to make additional corrections to the sound speed
of Model S before stabilisation. No stabilised model can be perfectly
solar-like, so we present three stabilised models with slightly different
eigenmodes. The models are appropriate to study the f and p1 to p4 modes with
spherical harmonic degrees in the range from 400 to 900. Background model CSM
has a modified pressure gradient for stabilisation and has eigenfrequencies
within 2% of Model S. Model CSM_A has an additional 10% increase in sound speed
in the top 1 Mm resulting in eigenfrequencies within 2% of Model S and
eigenfunctions that are, in comparison with CSM, closest to those of Model S.
Model CSM_B has a 3% decrease in sound speed in the top 5 Mm resulting in
eigenfrequencies within 1% of Model S and eigenfunctions that are only
marginally adversely affected. These models are useful to study the interaction
of solar waves with embedded three-dimensional heterogeneities, such as
convective flows and model sunspots. We have also calculated the response of
the stabilised models to excitation by random near-surface sources, using
simulations of the propagation of linear waves. We find that the simulated
power spectra of wave motion are in good agreement with an observed SOHO/MDI
power spectrum. Overall, our convectively stabilised background models provide
a good basis for quantitative numerical local helioseismology. The models are
available for download from http://www.mps.mpg.de/projects/seismo/NA4/.Comment: 35 pages, 23 figures Changed title Updated Figure 1
- …