370 research outputs found
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
Radiative emission of solar features in Ca II K
We investigated the radiative emission of different types of solar features
in the spectral range of the Ca II K line.
We analyzed full-disk 2k x 2k observations from the PSPT Precision Solar
Photometric Telescope. The data were obtained by using three narrow-band
interference filters that sample the Ca II K line with different pass bands.
Two filters are centered in the line core, the other in the red wing of the
line. We measured the intensity and contrast of various solar features,
specifically quiet Sun (inter-network), network, enhanced network, plage, and
bright plage (facula) regions. Moreover, we compared the results obtained with
those derived from the numerical synthesis performed for the three PSPT filters
with a widely used radiative code on a set of reference semi-empirical
atmosphere models.Comment: In Proceedings of the 25th NSO Workshop: Chromospheric Structure and
Dynamic
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
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
Optimal spectral nudging for global dynamic downscaling
AbstractThis study analyzes a method to construct a homogeneous, high-resolution global atmospheric hindcast. The method is the spectral nudging technique which was applied to a state-of-the-art general circulation model (ECHAM6, T255L95). Large spatial scales of the global climate model prognostic variables were spectrally nudged towards a reanalysis data set (NCEP1, T62L28) for the last decades. The main idea is the addition of dynamically consistent regional weather details to the coarse grid NCEP1 reanalysis. A large number of sensitivity experiments were performed, using different nudging e-folding times, vertical profiles, wave numbers, and variables. Comparisons with observations and several reanalyses showed a high dependency on the variations of the nudging configuration. At the global scale, the accordance is very high for extra-tropical regions and lower in the tropics. A wave number truncation of 30, a relatively short e-folding time of 50 min and a plateau-shaped nudging profile applied only to divergence and vorticity generally yielded the best results. This is one of the first global spectral nudging hindcast studies and the first applying an altitude-dependent profile to selected prognostic variables. The method can be applied to reconstruct the history of extreme events such as intense storms in the context of ongoing climate change
Radiative emission of solar features in the Ca II K line: comparison of measurements and models
We study the radiative emission of various types of solar features, such as
quiet Sun, enhanced network, plage, and bright plage regions, identified on
filtergrams taken in the Ca II K line. We analysed fulldisk images obtained
with the PSPT, by using three interference filters that sample the Ca II K line
with different bandpasses. We studied the dependence of the radiative emission
of disk features on the filter bandpass. We also performed a NLTE spectral
synthesis of the Ca II K line integrated over the bandpass of PSPT filters. The
synthesis was carried out by utilizing both the PRD and CRD with the most
recent set of semi empirical atmosphere models in the literature and some
earlier atmosphere models. We measured the CLV of intensity values for various
solar features identified on PSPT images and compared the results obtained with
those derived from the synthesis. We find that CRD calculations derived using
the most recent quiet Sun model, on average, reproduce the measured values of
the quiet Sun regions slightly more accurately than PRD computations with the
same model. This may reflect that the utilized atmospheric model was computed
assuming CRD. Calculations with PRD on earlier quiet Sun model atmospheres
reproduce measured quantities with a similar accuracy as to that achieved here
by applying CRD to the recent model. We also find that the median contrast
values measured for most of the identified bright features, disk positions, and
filter widths are, on average, a factor 1.9 lower than those derived from PRD
simulations performed using the recent bright feature models. The discrepancy
between measured and modeled values decreases by 12% after taking into account
straylight effects on PSPT images. PRD computations on either the most recent
or the earlier atmosphere models of bright features reproduce measurements from
plage and bright plage regions with a similar accuracy.Comment: 14 pages, 18 figures, accepted by A&
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
Do quasi-regular structures really exist in the solar photosphere? I. Observational evidence
Two series of solar-granulation images -- the La Palma series of 5 June 1993
and the SOHO MDI series of 17--18 January 1997 -- are analysed both
qualitatively and quantitatively. New evidence is presented for the existence
of long-lived, quasi-regular structures (first reported by Getling and Brandt
(2002)), which no longer appear unusual in images averaged over 1--2-h time
intervals. Such structures appear as families of light and dark concentric
rings or families of light and dark parallel strips (``ridges'' and
``trenches'' in the brightness distributions). In some cases, rings are
combined with radial ``spokes'' and can thus form ``web'' patterns. The
characteristic width of a ridge or trench is somewhat larger than the typical
size of granules. Running-average movies constructed from the series of images
are used to seek such structures. An algorithm is developed to obtain, for
automatically selected centres, the radial distributions of the azimuthally
averaged intensity, which highlight the concentric-ring patterns. We also
present a time-averaged granulation image processed with a software package
intended for the detection of geological structures in aerospace images. A
technique of running-average-based correlations between the brightness
variations at various points of the granular field is developed and indications
are found for a dynamical link between the emergence and sinking of hot and
cool parcels of the solar plasma. In particular, such a correlation analysis
confirms our suggestion that granules -- overheated blobs -- may repeatedly
emerge on the solar surface. Based on our study, the critical remarks by Rast
(2002) on the original paper by Getling and Brandt (2002) can be dismissed.Comment: 21 page, 8 figures; accepted by "Solar Physics
Analysis of bacterial communities in a municipal duck pond during a phytoplankton bloom and isolation of Anatilimnocola aggregata gen. nov., sp. nov., Lacipirellula limnantheis sp. nov. and Urbifossiella limnaea gen. nov., sp. nov. belonging to the phylum Planctomycetes
Waterbodies such as lakes and ponds are fragile environments affected by human influences. Suitable conditions can result in massive growth of phototrophs, commonly referred to as phytoplankton blooms. Such events benefit heterotrophic bacteria able to use compounds secreted by phototrophs or their biomass as major nutrient source. One example of such bacteria are Planctomycetes, which are abundant on the surfaces of marine macroscopic phototrophs; however, less data are available on their ecological roles in limnic environments. In this study, we followed a cultivation-independent deep sequencing approach to study the bacterial community composition during a cyanobacterial bloom event in a municipal duck pond. In addition to cyanobacteria, which caused the bloom event, members of the phylum Planctomycetes were significantly enriched in the cyanobacteria-attached fraction compared to the free-living fraction. Separate datasets based on isolated DNA and RNA point towards considerable differences in the abundance and activity of planctomycetal families, indicating different activity peaks of these families during the cyanobacterial bloom. Motivated by the finding that the sampling location harbours untapped bacterial diversity, we included a complementary cultivation-dependent approach and isolated and characterized three novel limnic strains belonging to the phylum Planctomycetes
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