387 research outputs found
Time series of high resolution photospheric spectra in a quiet region of the Sun. I. Analysis of global and spatial variations of line parameters
A 50 min time series of one-dimensional slit-spectrograms, taken in quiet sun
at disk center, observed at the German Vacuum Tower Telescope (Observatorio del
Teide), was used to study the global and spatial variations of different line
parameters. In order to determine the vertical structure of the photosphere two
lines with well separated formation heights have been considered. The data have
been filtered of p-modes to isolate the pure convective phenomenon. From our
studies of global correlation coefficients and coherence and phase shift
analyzes between the several line parameters, the following results can be
reported. The convective velocity pattern preserves structures larger than 1.0"
up to the highest layers of the photosphere (~ 435 km). However, at these
layers, in the intensity pattern only structures larger than 2.0" are still
connected with those at the continuum level although showing inverted
brightness contrast. This confirms an inversion of temperature that we have
found at a height of ~140 km. A possible evidence of gravity waves superimposed
to the convective motions is derived from the phase shift analysis. We
interpret the behavior of the full width at half maximum and the equivalent
width as a function of the distance to the granular borders, as a consequence
of enhanced turbulence and/or strong velocity gradients in the intergranular
lanes.Comment: 16 pages, 15 figures, 5 tables; Astronomy & Astrophysics, Volume 408,
p.363-378, 200
Excitation of Oscillations in the Magnetic Network on the Sun
We examine the excitation of oscillations in the magnetic network of the Sun
through the footpoint motion of photospheric magnetic flux tubes located in
intergranular lanes. The motion is derived from a time series of
high-resolution G band and continuum filtergrams using an object-tracking
technique. We model the response of the flux tube to the footpoint motion in
terms of the Klein-Gordon equation, which is solved analytically as an initial
value problem for transverse (kink) waves. We compute the wave energy flux in
upward propagating transverse waves. In general we find that the injection of
energy into the chromosphere occurs in short-duration pulses, which would lead
to a time variability in chromospheric emission that is incompatible with
observations. Therefore, we consider the effects of turbulent convective flows
on flux tubes in intergranular lanes. The turbulent flows are simulated by
adding high-frequency motions (periods 5-50 s) with an amplitude of 1 km
s^{-1}. The latter are simulated by adding random velocity fluctuations to the
observationally determined velocities. In this case we find that the energy
flux is much less intermittent and can in principle carry adequate energy for
chromospheric heating.Comment: 11 pages, 5 figures, figure 1 is in color, all files gzippe
Where the granular flows bend
Based on IMaX/Sunrise data, we report on a previously undetected phenomenon
in solar granulation. We show that in a very narrow region separating granules
and intergranular lanes the spectral line width of the Fe I 5250.2 A line
becomes extremely small. We offer an explanation of this observation with the
help of magneto-convection simulations. These regions with extremely small line
widths correspond to the places where the granular flows bend from mainly
upflow in granules to downflow in intergranular lanes. We show that the
resolution and image stability achieved by IMaX/Sunrise are important
requisites to detect this interesting phenomenon.Comment: Accepted for the Sunrise Special Issue of ApJ
Distribution, stock structure, and growth of the squid Berryteuthis magister (Berry, 1913) (Cephalopoda, Gonatidae) during summer and fall in the western Bering Sea
Distribution, abundance, and length-frequency composition of schoolmaster gonate squid, Berryteuthis magister, were studied during seven trawl surveys in the western Bering Sea between June and November 1993. Statolith age analysis was undertaken for 1,381 B. magister to estimate age, stock structure, and both growth and maturation rates. Three kinds of growth increments were revealed in B. magister statoliths. Daily periodicity of the second-order increments was confirmed by two independent, indirect methods. According to our data, B. magister live >1 yr; the oldest specimen was about 16 months old. Berryteuthis magister is a slow-growing and slow-maturing squid, and males exhibit slower growth and earlier maturation than do females. Growth in length was best described by a logistic curve, with a larger asymptotic parameter for females. In summer, concentrations of B. magister were low within the whole region, whereas in September-October squids aggregated into dense shoals over the continental slope of the Navarin-Olyutorsky region and Olyutorsky Bay. Stock structure of B. magister was complicated: each month, from 5 to 12 (usually 7-8) monthly classes of squid were encountered in the western Bering Sea. Three seasonal groups of B. magister occurred in the region: winter-, summer- and fall-hatched squids utilizing resources of the continental slope in different ways. A possible life cycle for the B. magister fall-hatched group includes a longevity of ca. 2 yr: 6 mo of embryonic development and 18 mo of postembryonic growth
Time series of high resolution photospheric spectra in a quiet region of the Sun. II. Analysis of the variation of physical quantities of granular structures
From the inversion of a time series of high resolution slit spectrograms
obtained from the quiet sun, the spatial and temporal distribution of the
thermodynamical quantities and the vertical flow velocity is derived as a
function of logarithmic optical depth and geometrical height. Spatial coherence
and phase shift analyzes between temperature and vertical velocity depict the
height variation of these physical quantities for structures of different size.
An average granular cell model is presented, showing the granule-intergranular
lane stratification of temperature, vertical velocity, gas pressure and density
as a function of logarithmic optical depth and geometrical height. Studies of a
specific small and a specific large granular cell complement these results. A
strong decay of the temperature fluctuations with increasing height together
with a less efficient penetration of smaller cells is revealed. The T -T
coherence at all granular scales is broken already at log tau =-1 or z~170 km.
At the layers beyond, an inversion of the temperature contrast is revealed.
Vertical velocities are in phase throughout the photosphere and penetrate into
the highest layers under study.Comment: 13 pages, 12 figures, 1 table; Astronomy & Astrophysics, Volume 441,
Issue 3, pp.1157-1169, 200
Granulation signatures in the spectrum of the very metal-poor red giant HD122563
A very high resolution (R=200,000), high signal-to-noise ratio (S/N=340)
blue-green spectrum of the very metal-poor ([Fe/H]=-2.6) red giant star
HD122563 has been obtained by us at McDonald Observatory. We measure the
asymmetries and core wavelengths of a set of unblended FeI lines covering a
wide range of line strength. Line bisectors exhibit the characteristic C-shape
signature of surface convection (granulation) and they span from about 100 m/s
in the strongest FeI features to 800 m/s in the weakest ones. Core wavelength
shifts range from about -100 to -900 m/s, depending on line strength. In
general, larger blueshifts are observed in weaker lines, but there is
increasing scatter with increasing residual flux. Assuming local thermodynamic
equilibrium (LTE), we synthesize the same set of spectral lines using a
state-of-the-art three-dimensional hydrodynamic simulation for a stellar
atmosphere of fundamental parameters similar to those of HD122563. We find good
agreement between model predictions and observations. This allows us to infer
an absolute zero-point for the line shifts and radial velocity. Moreover, it
indicates that the structure and dynamics of the simulation are realistic, thus
providing support to previous claims of large 3D-LTE corrections, based on the
hydrodynamic model used here, to elemental abundances and fundamental
parameters of very metal-poor red giant stars obtained with standard 1D-LTE
spectroscopic analyses.Comment: ApJL, in pres
Zur Anregung thermomechanischer Schwingungen: Wärmekonvektion, Wärmeübertrag und Modulationsformen
Wir untersuchen thermomechanische Schwingungen, bei denen eine eingespannte Saite durch einen Gleichstrom erwärmt und so angeregt wird, dass mechanische Oszillationen mit Temperaturvariationen gekoppelt sind. Ursache des Phänomens ist der geschwindigkeitsabhängige Wärmeübertrag an die Umgebungsluft. Wir untersuchen die Frage, wie es zur resonanten Anregung kommt und inwieweit sich das Anregungsspektrum von dem einer rein mechanischen Resonanz unterscheidet. Dazu stellen wir auch vergleichende Experimente vor, bei denen eine Saite mit einer Lautsprechermembran verbunden und durch diese zur parametrischen Resonanz gebracht wird (eine Variation des Experiments von Melde)
Finite-size effects of dimensional crossover in quasi-two-dimensional three-state Potts model
A nearest neighbour spin pair of the quasi-two-dimensional three-state Potts
model interacts with the strength in the -plane and with in the -axis. The phase transition is of
second-order when and is of first-order when . The
dimensional crossover occurs with a change of the order of the phase
transition. We study the finite-size effects of the phenomenon by using a Monte
Carlo method with a multi-spin coding technique. The prediction of the
finite-size scaling theory is consistent with the Monte Carlo results.Comment: 12 pages(two LaTeX figures included), EDRUSBA-94101
Decorrelation Times of Photospheric Fields and Flows
We use autocorrelation to investigate evolution in flow fields inferred by
applying Fourier Local Correlation Tracking (FLCT) to a sequence of
high-resolution (0.3 \arcsec), high-cadence ( min) line-of-sight
magnetograms of NOAA active region (AR) 10930 recorded by the Narrowband Filter
Imager (NFI) of the Solar Optical Telescope (SOT) aboard the {\em Hinode}
satellite over 12--13 December 2006. To baseline the timescales of flow
evolution, we also autocorrelated the magnetograms, at several spatial
binnings, to characterize the lifetimes of active region magnetic structures
versus spatial scale. Autocorrelation of flow maps can be used to optimize
tracking parameters, to understand tracking algorithms' susceptibility to
noise, and to estimate flow lifetimes. Tracking parameters varied include: time
interval between magnetogram pairs tracked, spatial binning applied
to the magnetograms, and windowing parameter used in FLCT. Flow
structures vary over a range of spatial and temporal scales (including
unresolved scales), so tracked flows represent a local average of the flow over
a particular range of space and time. We define flow lifetime to be the flow
decorrelation time, . For , tracking results represent
the average velocity over one or more flow lifetimes. We analyze lifetimes of
flow components, divergences, and curls as functions of magnetic field strength
and spatial scale. We find a significant trend of increasing lifetimes of flow
components, divergences, and curls with field strength, consistent with Lorentz
forces partially governing flows in the active photosphere, as well as strong
trends of increasing flow lifetime and decreasing magnitudes with increases in
both spatial scale and .Comment: 48 pages, 20 figures, submitted to the Astrophysical Journal;
full-resolution images in manuscript (8MB) at
http://solarmuri.ssl.berkeley.edu/~welsch/public/manuscripts/flow_lifetimes_v2.pd
Spatial variations in the field of velocities and real solar granulation
In this paper, the physical conditions within the inhomogeneous solar
atmosphere have been reconstructed by means of solving the inverse problem of
Non Local Thermodynamic Equilibrium (NLTE) radiative transfer. The profiles of
nm FeI spectral line of high spatial and time resolution were
used as observational data. The velocity field has been studied for the real
solar granulation in superadiabatic layer and overshooting convection region.
Also, we investigate the vertical structure of inhomogeneous solar photosphere
and consider penetration of granules from convective region into upper layers
of stable atmosphere. The microturbulent velocity appears to be minimal at the
bottom of overshooting convection region and increases sharply through
superadiabatic layer and upper photosphere. High-turbulent layers emerge either
in the central part of a flow or at the boundary of an incipient flow with
following drift toward the centre of the flow. Wide descending flows tend to
disintegrate into structures having turbulence augmented, these structures
correspond to the flows of matter. High microturbulence of the intensive flows
provokes steep temperature depression in upper photosphere leading to the
second inversion of temperature for the intergranules. The inversion of
vertical velocities is observed to be frequent in the solar granulation. Some
of the convective flows reach the minimum temperature region. Vertical
convective velocities of the matter flows were found to be smaller in the
middle and upper photosphere. Also, the effect of finite resolution on the
spacial variations of the velocities in solar photosphere has been estimated.Comment: 8 pages, 7 figure
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