1,626 research outputs found
Helioseismology of Sunspots: Confronting Observations with Three-Dimensional MHD Simulations of Wave Propagation
The propagation of solar waves through the sunspot of AR 9787 is observed
using temporal cross-correlations of SOHO/MDI Dopplergrams. We then use
three-dimensional MHD numerical simulations to compute the propagation of wave
packets through self-similar magneto-hydrostatic sunspot models. The
simulations are set up in such a way as to allow a comparison with observed
cross-covariances (except in the immediate vicinity of the sunspot). We find
that the simulation and the f-mode observations are in good agreement when the
model sunspot has a peak field strength of 3 kG at the photosphere, less so for
lower field strengths. Constraining the sunspot model with helioseismology is
only possible because the direct effect of the magnetic field on the waves has
been fully taken into account. Our work shows that the full-waveform modeling
of sunspots is feasible.Comment: 21 pages, Accepted in Solar Physic
Anomalously Weak Solar Convection
Convection in the solar interior is thought to comprise structures on a
spectrum of scales. This conclusion emerges from phenomenological studies and
numerical simulations, though neither covers the proper range of dynamical
parameters of solar convection. Here, we analyze observations of the wavefield
in the solar photosphere using techniques of time-distance helioseismology to
image flows in the solar interior. We downsample and synthesize 900 billion
wavefield observations to produce 3 billion cross-correlations, which we
average and fit, measuring 5 million wave travel times. Using these travel
times, we deduce the underlying flow systems and study their statistics to
bound convective velocity magnitudes in the solar interior, as a function of
depth and spherical-harmonic degree . Within the wavenumber band
, Convective velocities are 20-100 times weaker than current
theoretical estimates. This suggests the prevalence of a different paradigm of
turbulence from that predicted by existing models, prompting the question: what
mechanism transports the heat flux of a solar luminosity outwards? Advection is
dominated by Coriolis forces for wavenumbers , with Rossby numbers
smaller than at , suggesting that the Sun may be
a much faster rotator than previously thought, and that large-scale convection
may be quasi-geostrophic. The fact that iso-rotation contours in the Sun are
not co-aligned with the axis of rotation suggests the presence of a latitudinal
entropy gradient.Comment: PNAS; 5 figures, 5 page
INITIAL CHARACTERIZATION OF MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) CLASS IIB EXON 2 IN AN ENDANGERED RATTLESNAKE, THE EASTERN MASSASAUGA (SISTRURUS CATENATUS)
Genes of the major histocompatibility complex (MHC) play an important role in the vertebrate immune system and exhibit remarkably high levels of polymorphism, maintained by strong balancing selection. While the conservation implications of MHC variation have been explored in a variety of vertebrates, non-avian reptiles (most notably snakes) have received less attention. To address this gap and take the first steps toward more extensive population-level analyses, we cloned and sequenced MHC IIB exon 2 in an endangered rattlesnake, the Eastern Massasauga (Sistrurus catenatus). Based on three individuals, we found evidence of at least four putatively functional loci. These sequences exhibited relatively high levels of variation and significantly higher rates of nonsynonymous to synonymous substitutions, especially within the antigen-binding sites, indicating strong positive selection. Phylogenetic analysis revealed a pattern of trans-species polymorphism, also suggesting positive selection. These results contribute to our understanding of MHC variation in non-avian reptiles and form a basis for more studies of MHC variation in snakes of conservation concern
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&
Space-time segmentation method for study of the vertical structure and evolution of solar supergranulation from data provided by local helioseismology
Solar supergranulation remains a mystery in spite of decades of intensive
studies. Most of the papers about supergranulation deal with its surface
properties. Local helioseismology provides an opportunity to look below the
surface and see the vertical structure of this convective structure. We present
a concept of a (3+1)-D segmentation algorithm capable of recognising individual
supergranules in a sequence of helioseismic 3-D flow maps. As an example, we
applied this method to the state-of-the-art data and derived descriptive
statistical properties of segmented supergranules -- typical size of 20--30 Mm,
characteristic lifetime of 18.7 hours, and estimated depth of 15--20 Mm. We
present preliminary results obtained on the topic of the three-dimensional
structure and evolution of supergranulation. The method has a great potential
in analysing the better data expected from the helioseismic inversions, which
are being developed.Comment: 6 pages, 4 figures, accepted in New Astronom
Acoustic wave propagation in the solar sub-photosphere with localised magnetic field concentration: effect of magnetic tension
Aims: We analyse numerically the propagation and dispersion of acoustic waves in the solar-like sub-photosphere with localised non-uniform magnetic field concentrations, mimicking sunspots with various representative magnetic field configurations.
Methods: Numerical simulations of wave propagation through the solar sub-photosphere with a localised magnetic field concentration are carried out using SAC, which solves the MHD equations for gravitationally stratified plasma. The initial equilibrium density and pressure stratifications are derived from a standard solar model. Acoustic waves are generated by a source located at the height corresponding approximately to the visible surface of the Sun. By means of local helioseismology we analyse the response of vertical velocity at the level corresponding to the visible solar surface to changes induced by magnetic field in the interior.
Results: The results of numerical simulations of acoustic wave propagation and dispersion in the solar sub-photosphere with localised magnetic field concentrations of various types are presented. Time-distance diagrams of the vertical velocity perturbation at the level corresponding to the visible solar surface show that the magnetic field perturbs and scatters acoustic waves and absorbs the acoustic power of the wave packet. For the weakly magnetised case, the effect of magnetic field is mainly thermodynamic, since the magnetic field changes the temperature stratification. However, we observe the signature of slow magnetoacoustic mode, propagating downwards, for the strong magnetic field cases
The Chloroplast Genome of Anomochloa Marantoidea (Anomochlooideae; Poaceae) Comprises a Mixture of Grass-like and Unique Features
Features in the complete plastome of Anomochloa marantoidea (Poaceae) were investigated. This species is one of four of
Anomochlooideae, the crown node of which diverged before those of any other grass subfamily. The plastome was sequenced
from overlapping amplicons using previously designed primers. The plastome of A. marantoidea is 138 412 bp long with a typical
gene content for Poaceae. Five regions were examined in detail because of prior surveys that identifi ed structural alterations among
graminoid Poales. Anomochloa marantoidea was found to have an intron in rpoC1 , unlike other Poaceae. The insertion region of
rpoC2 is unusually short in A. marantoidea compared with those of other grasses, but with atypically long subrepeats. Both ycf1
and ycf2 are nonfunctional as is typical in grasses, but A. marantoidea has a uniquely long ψ ycf1. Finally, the rbcL - psaI spacer in
A. marantoidea is atypically short with no evidence of the ψ rpl23 locus found in all other Poaceae. Some of these features are of
noteworthy dissimilarity between A . marantoidea and those crown grasses for which entire plastomes have been sequenced. Complete
plastome sequences of other Anomochlooideae and outgroups will further advance our understanding of the evolutionary
events in the plastome that accompanied graminoid diversifi cation
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