530 research outputs found
Recommended from our members
Expansion of the Supergranular Magnetic Network through the Solar Atmosphere
The solar magnetic field has its footpoints in the photosphere, extends through the chromosphere, and is thought to expand through the transition region and into the corona. It is organized by fluid motions to form strong flux concentrations within the boundaries of the supergranular convection cells. These boundaries are the network lanes observed in line emission, and they display increasing width with height through the solar atmosphere. The network field concentrations are surrounded by a mixed-polarity internetwork magnetic field on the scale of granulation. We use a potential magnetic field extrapolation of synthetic photospheric magnetograms to study the magnetic network topology and the effects of a mixed-polarity background field on the network expansion with height through the solar atmosphere.We find that the expansion of the network boundary with height deviates significantly from the funnel expansion model. Moreover, we find that the background magnetic field has a considerable effect on the filling factor of the network area with height, even though the background flux is strictly equal to zero
The Role of Subsurface Flows in Solar Surface Convection: Modeling the Spectrum of Supergranular and Larger Scale Flows
We model the solar horizontal velocity power spectrum at scales larger than
granulation using a two-component approximation to the mass continuity
equation. The model takes four times the density scale height as the integral
(driving) scale of the vertical motions at each depth. Scales larger than this
decay with height from the deeper layers. Those smaller are assumed to follow a
Kolomogorov turbulent cascade, with the total power in the vertical convective
motions matching that required to transport the solar luminosity in a mixing
length formulation. These model components are validated using large scale
radiative hydrodynamic simulations. We reach two primary conclusions: 1. The
model predicts significantly more power at low wavenumbers than is observed in
the solar photospheric horizontal velocity spectrum. 2. Ionization plays a
minor role in shaping the observed solar velocity spectrum by reducing
convective amplitudes in the regions of partial helium ionization. The excess
low wavenumber power is also seen in the fully nonlinear three-dimensional
radiative hydrodynamic simulations employing a realistic equation of state.
This adds to other recent evidence suggesting that the amplitudes of large
scale convective motions in the Sun are significantly lower than expected.
Employing the same feature tracking algorithm used with observational data on
the simulation output, we show that the observed low wavenumber power can be
reproduced in hydrodynamic models if the amplitudes of large scale modes in the
deep layers are artificially reduced. Since the large scale modes have reduced
amplitudes, modes on the scale of supergranulation and smaller remain important
to convective heat flux even in the deep layers, suggesting that small scale
convective correlations are maintained through the bulk of the solar convection
zone.Comment: 36 pages, 6 figure
Recommended from our members
Understanding the Role of Small-Scale Flux in Solar Spectral Irradiance Variation
Global solar spectral irradiance variations depend on changes inmagnetic flux concentrations at the smallest scales. Modeling has focused on the contributions of magnetic structures in full disk images as those contributions have strong center-to-limb dependencies, but these dependencies have never been determined radiometrically; only the photometric intensity relative to some reference ’quiet-sun’1, themagnetic structure contrast, is measurable with ground based imagery. This is problematic because unre- solved inhomogeneities influence not only the full-disk structure intensities themselves, but also the quiet-sun background against which their contrast is measured. We thus argue that, to understand the physical causes underlying solar spectral irradiance varia- tions, two fundamental questionsmust be addressed: What is the real Iλ(µ) as a function of B in full-disk images? This can only be answered by imaging the Sun radiometrically from space, and we propose a Radiometric Solar Imager design. What governs spectral irradiance changes at sub arc-second scales? This can be addressed by a combination of high resolution ground based imaging (ATST-VBI) and three dimensional radiative magnetohydrodynamic modeling, and we propose a synoptic approach. Finally, a way to account for the variance introduced by unresolved substructure in spectral irradiance modeling must be devised. This is critical, as imaging and modeling at the highest resolutions but over the full solar disk will likely remain unattainable for some time
Dynamic Evolution of Toll-Like Receptor Multigene Families in Echinoderms
The genome sequence of the purple sea urchin, Strongylocentrotus purpuratus, a large and long-lived invertebrate, provides a new perspective on animal immunity. Analysis of this genome uncovered a highly complex immune system in which the gene families that encode homologs of the pattern recognition receptors that form the core of vertebrate innate immunity are encoded in large multigene families. The sea urchin genome contains 253 Toll-like receptor (TLR) sequences, more than 200 Nod-like receptors and 1095 scavenger receptor cysteine-rich domains, a 10-fold expansion relative to vertebrates. Given their stereotypic protein structure and simple intron-exon architecture, the TLRs are the most tractable of these families for more detailed analysis. A role for these receptors in immune defense is suggested by their similarity to TLRs in other organisms, sequence diversity, and expression in immunologically active tissues, including phagocytes. The complexity of the sea urchin TLR multigene families is largely derived from expansions independent of those in vertebrates and protostomes, although a small family of TLRs with structure similar to that of Drosophila Toll can be traced to an ancient eumetazoan ancestor. Several other echinoderm sequences are now available, including Lytechinus variegatus, as well as partial sequences from two other sea urchin species. Here, we present an analysis of the invertebrate deuterostome TLRs with emphasis on the echinoderms. Representatives of most of the S. purpuratus TLR subfamilies and homologs of the mccTLR sequences are found in L. variegatus, although the L. variegatus TLR gene family is notably smaller (68 TLR sequences). The phylogeny of these genes within sea urchins highlights lineage-specific expansions at higher resolution than is evident at the phylum level. These analyses identify quickly evolving TLR subfamilies that are likely to have novel immune recognition functions and other, more stable, subfamilies that may function more similarly to those of vertebrates
The Intensity Profile of the Solar Supergranulation
We have measured the average radial (cell center to network boundary) profile
of the continuum intensity contrast associated with supergranular flows using
data from the Precision Solar Photometric Telescope (PSPT) at the Mauna Loa
Solar Observatory (MLSO). After removing the contribution of the network flux
elements by the application of masks based on Ca II K intensity and averaging
over more than 10^5 supergranular cells, we find a ~ 0.1% decrease in red and
blue continuum intensity from the supergranular cell centers outward,
corresponding to a ~ 1.0 K decrease in brightness temperature across the cells.
The radial intensity profile may be caused either by the thermal signal
associated with the supergranular flows or a variation in the packing density
of unresolved magnetic flux elements. These are not unambiguously distinguished
by the observations, and we raise the possibility that the network magnetic
fields play an active role in supergranular scale selection by enhancing the
radiative cooling of the deep photosphere at the cell boundaries.Comment: Accepted to Ap
Peaks and Troughs in Helioseismology: The Power Spectrum of Solar Oscillations
I present a matched-wave asymptotic analysis of the driving of solar
oscillations by a general localised source. The analysis provides a simple
mathematical description of the asymmetric peaks in the power spectrum in terms
of the relative locations of eigenmodes and troughs in the spectral response.
It is suggested that the difference in measured phase function between the
modes and the troughs in the spectrum will provide a key diagnostic of the
source of the oscillations. I also suggest a form for the asymmetric line
profiles to be used in the fitting of solar power spectra.
Finally I present a comparison between the numerical and asymptotic
descriptions of the oscillations. The numerical results bear out the
qualitative features suggested by the asymptotic analysis but suggest that
numerical calculations of the locations of the troughs will be necessary for a
quantitative comparison with the observations.Comment: 18 pages + 8 separate figures. To appear in Ap
Ultrastructure of grape leaf protoplasts in comparison with the source tissue
Based on their histochemical properties, protoplasts from grape leaf mesophyll were found to be viable and devoid of remnants of the cell wall. The results were confirmed by electron microscopy of sectioned and freeze-etched protoplast preparations. The only remarkable difference between cells in the intact tissue and the protoplasts was the occurrence of electron-dense globules in the cytoplasm of some individuals. These "osmiophilic bodies" are believed to be due to inevitable plasmolytic effects during wall removal and seem not to have any influence on protoplast CO2-assimilation, as has been demonstrated earlier. lt therefore appears that the vital structures remain intact and functional, in spite of the stress situation created by the isolation procedure.Ultrastruktur von Rebenblatt-Protoplasten und Ausgangsgewebe - ein VergleichAnhand histochemischer Nachweise, welche das Weiterfunktionieren der Plasmamembran als physiologische Barriere nach dem Isolationsprozeß bestätigen, können Mesophyllprotoplasten von Vitis vinifera L. als lebensfähig betrachtet werden. Dieser Befund, sowie das vollständige Fehlen von Zellwandresten, wurde durch elektronenmikroskopische Untersuchungen an Ultradünnschnitten bzw. Gefrierätzproben von Protoplastenpräparaten erhärtet. Das vereinzelte Auftreten von elektronendichten, runden Gebilden (sog. osmiophilic bodies) im Cytoplasma ist vermutlich eine der plasmolysierenden Wirkung des Isolationsmediums zuzuschreibende Erscheinung. Wie früher gezeigt, hat aber die verwendete Isolationsmethodik keinen negativen Einfluß auf den photosynthetischen C-Stoffwechsel der Protoplasten. Daraus geht hervor, daß die lebenswichtigen Zellkomponenten, trotz der isolationsbedingten Streßsituation, keinen irreversiblen und damit nachweisbaren, strukturellen oder funktionellen Beeinträchtigungen unterworfen sind
High-resolution models of solar granulation: the 2D case
Using grid refinement, we have simulated solar granulation in 2D. The refined
region measures 1.97*2.58 Mm (vertical*horizontal). Grid spacing there is
1.82*2.84 km. The downflows exhibit strong Kelvin-Helmholtz instabilities.
Below the photosphere, acoustic pulses are generated. They proceed laterally
(in some cases distances of at least the size of our refined domain) and may be
enhanced when transversing downflows) as well as upwards where, in the
photosphere they contribute significantly to 'turbulence' (velocity gradients,
etc.) The acoustic pulses are ubiquitous in that at any time several of them
are seen in our high-resolution domain. Their possible contributions to p-mode
excitation or heating of the chromosphere needs to be investigated
Realistic Magnetohydrodynamical Simulation of Solar Local Supergranulation
Three-dimensional numerical simulations of solar surface magnetoconvection
using realistic model physics are conducted. The thermal structure of
convective motions into the upper radiative layers of the photosphere, the main
scales of convective cells and the penetration depths of convection are
investigated. We take part of the solar photosphere with size of 60x60 Mm in
horizontal direction and by depth 20 Mm from level of the visible solar
surface. We use a realistic initial model of the Sun and apply equation of
state and opacities of stellar matter. The equations of fully compressible
radiation magnetohydrodynamics with dynamical viscosity and gravity are solved.
We apply: 1) conservative TVD difference scheme for the magnetohydrodynamics,
2) the diffusion approximation for the radiative transfer, 3) dynamical
viscosity from subgrid scale modeling. In simulation we take uniform
two-dimesional grid in gorizontal plane and nonuniform grid in vertical
direction with number of cells 600x600x204. We use 512 processors with
distributed memory multiprocessors on supercomputer MVS-100k in the Joint
Computational Centre of the Russian Academy of Sciences.Comment: 6 pages, 5 figures, submitted to the proceedings of the GONG 2008 /
SOHO XXI conferenc
Numerical simulations of compressible Rayleigh-Taylor turbulence in stratified fluids
We present results from numerical simulations of Rayleigh-Taylor turbulence,
performed using a recently proposed lattice Boltzmann method able to describe
consistently a thermal compressible flow subject to an external forcing. The
method allowed us to study the system both in the nearly-Boussinesq and
strongly compressible regimes. Moreover, we show that when the stratification
is important, the presence of the adiabatic gradient causes the arrest of the
mixing process.Comment: 15 pages, 11 figures. Proceedings of II Conference on Turbulent
Mixing and Beyond (TMB-2009
- …