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1.
Del Moro  D.  Berrilli  F.  Duvall  T.L.  Kosovichev  A.G. 《Solar physics》2004,221(1):23-32
In this paper we investigate the temporal evolution and geometric properties of solar supergranular features. For this purpose we apply an automatic feature-tracking algorithm to a 6-day time series of 18 near-surface flowmaps containing 548 target objects. Lifetimes are calculated by measuring the time elapsing between the birth and death of each target. Using an exponential fit on the lifetime distribution of single supergranules we derived a mean lifetime of 22 hours. Based on the application of segmentation numerical procedures, we estimated characteristic geometric parameters such as area distributions of supergranular cells. We also derive the relationship between measured lifetime and the area of the supergranules.  相似文献   

2.
We employ fractal analysis to study the complexity of supergranulation structure using the Solar and Heliospheric Observatory (SOHO) dopplergrams. Our data consists of 200 visually selected supergranular cells, for which we find a broad, slightly asymmetric dispersion in the size distribution, with the most probable size around 31.9 Mm. From the area–perimeter relation, we deduce a fractal dimension D of about 1.25. This is consistent with that for isobars, and suggests a possible turbulent origin of supergranulation. By relating this to the variances of kinetic energy, temperature and pressure, it is concluded that the supergranular network is close to being isobaric and that it has a possible turbulent origin.  相似文献   

3.
Supergranulation is visible at the solar surface as a cellular pattern of horizontal outflows. Although it does not show a distinct intensity pattern, it manifests itself indirectly in, for example, the chromospheric network. Previous studies have reported significant differences in the inferred basic parameters of the supergranulation phenomenon. Here we study the structure and temporal evolution of a large sample of supergranules, measured by using local helioseismology and SOHO/MDI data from the year 2000 at solar activity minimum. Local helioseismology with f modes provides maps of the horizontal divergence of the flow velocity at a depth of about 1 Mm. From these divergence maps supergranular cells were identified by using Fourier segmentation procedures in two dimensions and in three dimensions (two spatial dimensions plus time). The maps that we analyzed contain more than 105 supergranular cells and more than 103 lifetime histories, which makes possible a detailed analysis with high statistical significance. We find that the supergranular cells have a mean diameter of 27.1 Mm. The mean lifetime is estimated to be 1.6 days from the measured distribution of lifetimes (three-dimensional segmentation), with a clear tendency for larger cells to live longer than smaller ones. The pair and mark correlation functions do not show pronounced features on scales larger than the typical cell size, which suggests purely random cell positions. The temporal histories of supergranular cells indicate a smooth evolution from their emergence and growth in the first half of their lives to their decay in the second half of their lives (unlike exploding granules, which reach their maximum size just before they fragment).  相似文献   

4.
Beck  John G.  Schou  Jesper 《Solar physics》2000,193(1-2):333-343
Simple convection models estimate the depth of supergranulation at approximately 7500 km which suggests that supergranules would rotate at the rate of the plasma in the outer 1% of the solar radius. The supergranulation rotation obtained from MDI dopplergrams shows that supergranules rotate faster than the outer 5% of the convection zone and show zonal flows matching results from inversions of f-mode splittings. Additionally, the rotation rate depends on the size scale of the features.  相似文献   

5.
Spatial and temporal variations in thermodynamic and kinematic parameters of structural elements of solar granulation are investigated by solving the inverse nonequilibrium radiative transfer problem using the observational data from the Vacuum Tower Telescope (duration of observations 2.6 h). In the lower photosphere, we have detected long-living (with lifetime up to 1.5 h) structures—trees of fragmenting granules. They occur as a result of the division of an ascending granular flow into several fragments, which can be repeated multiple times. We have found that approximately 67% of the regions with the highest positive variations of pressure correspond to the time and place of fragmentation of granular flows; approximately 12% of the regions correspond to the approach of adjacent structures.  相似文献   

6.
Surface granulation of the Sun is primarily a consequence of thermal transport in the outer 1 % of the radius. Its typical scale of about 1?–?2 Mm?is set by the balance between convection, free-streaming radiation, and the strong density stratification in the surface layers. The physics of granulation is well understood, as demonstrated by the close agreement between numerical simulation, theory, and observation. Superimposed on the energetic granular structure comprising high-speed flows, are larger-scale long-lived flow systems (≈?300 m?s?1) called supergranules. Supergranulation has a typical scale of 24?–?36 Mm. It is not clear if supergranulation results from the interaction of granules or is causally linked to deep convection or a consequence of magneto–convection. Other outstanding questions remain: how deep are supergranules? How do they participate in global dynamics of the Sun? Further challenges are posed by our lack of insight into the dynamics of larger scales in the deep convection region. Recent helioseismic constraints have suggested that convective-velocity amplitudes on large scales may be overestimated by an order of magnitude or more, implying that Reynolds stresses associated with large-scale convection, thought to play a significant role in the sustenance of differential rotation and meridional circulation, might be two orders of magnitude weaker than theory and computation predict. While basic understanding on the nature of convection on global scales and the maintenance of global circulations is incomplete, progress is imminent, given substantial improvements in computation, theory, and helioseismic inferences.  相似文献   

7.
Recently, Duvall and Hanasoge (Solar Phys. 287, 71, 2013) found that large-distance separation [Δ] travel-time differences from a center to an annulus [δt oi] implied a model of the average supergranular cell that has a peak upflow of 240 m?s?1 at a depth of 2.3 Mm and a corresponding peak outward horizontal flow of 700 m?s?1 at a depth of 1.6 Mm. In the present work, this effect is further studied by measuring and modeling center-to-quadrant travel-time differences [δt qu], which roughly agree with this model. Simulations are analyzed that show that such a model flow would lead to the expected travel-time differences. As a check for possible systematic errors, the center-to-annulus travel-time differences [δt oi] are found not to vary with heliocentric angle. A consistency check finds an increase of δt oi with the temporal frequency [ν] by a factor of two, which is not predicted by the ray theory.  相似文献   

8.
A summary of the most recent observational results on the solar granulation phenomenon is given and its physical interpretation with respect to theoretical and numerical models is briefly discussed. Special attention is paid to unsolved questions concerning the relation between convective motions and related solar features.  相似文献   

9.
10.
High-resolution white-light pictures are analyzed to study the differences between the granular size near sunspot penumbrae and in light-bridges presenting granular structure and that of the quiet photosphere. No difference is found between the mean granular diameter in light-bridges and the quiet photosphere. The dispersion found in the results corresponding to different zones around the sunspots indicates that the size of the granulation may vary from place to place near the sunspots, its mean value not differing significantly from that of the quiet photosphere. A possible systematic bias in the selection of the granules by Macris (1979) is found.  相似文献   

11.
An excellent series of images of solar granulation was taken during the partial solar eclipse of 1994 May 10 at the Swedish Vacuum Solar Telescope at Roque de los Muchachos Observatory. Making use of the lunar limb profile, images at different heliocentric positions were corrected for instrumental and atmospheric effects. The centre-to-limb variation of the granulation contrast was calculated and compared with previous determinations. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

12.
The stability of linear convective modes in the solar convection zone is investigated by incorporating the mechanical and thermal effects of turbulence through the eddy transport coefficients. The inclusion of turbulent thermal conductivity and viscosity, calculated in the framework of the mixing length approximation, is demonstrated to have a profound influence on the convective growth rates. The solar envelope model of Spruit (1977) is used to show that that most rapidly growing fundamental mode and the first harmonic are in reasonable accord with the observed features of granulation and supergranulation, respectively.On leave of absence from Govt. Digvijai College, Rajnandgaon 491441, India.  相似文献   

13.
Observations indicate that in plage areas (i.e. in active regions outside sunspots) acoustic waves travel faster than in the quiet Sun, leading to shortened travel times and higher p-mode frequencies. Coupled with the 11-year variation of solar activity, this may also explain the solar cycle variation of oscillation frequencies. While it is clear that the ultimate cause of any difference between the quiet Sun and plage is the presence of magnetic fields of order 100 G in the latter, the mechanism by which the magnetic field exerts its influence has not yet been conclusively identified. One possible such mechanism is suggested by the observation that granular motions in plage areas tend to be slightly “abnormal”, dampened compared to the quiet Sun. In this paper we consider the effect that abnormal granulation observed in active regions should have on the propagation of acoustic waves. Any such effect is found to be limited to a shallow surface layer where sound waves propagate nearly vertically. The magnetically suppressed turbulence implies higher sound speeds, leading to shorter travel times. This time shift Δ τ is independent of the travel distance, while it shows a characteristic dependence on the assumed plage field strength. As a consequence of the variation of the acoustic cutoff with height, Δ τ is expected to be significantly higher for higher frequency waves within the observed regime of 3 – 5 mHz. The lower group velocity near the upper reflection point further leads to an increased envelope time shift, as compared to the phase shift. p-mode frequencies in plage areas are increased by a corresponding amount, Δ ν/ν=ν Δ τ. These characteristics of the time and frequency shifts are in accordance with observations. The calculated overall amplitudes of the time and frequency shifts are comparable to, but still significantly less than (by a factor of 2 to 5), those suggested by measurements.  相似文献   

14.
A. Anđić 《Solar physics》2007,243(2):131-141
High-frequency velocity oscillations were observed in the spectral lines Fe i 543.45 nm and 543.29 nm, by using 2D spectroscopy with a Fabry – Perot and speckle reconstruction, at the Vacuum Tower Telescope in Tenerife. We investigate the radial component of waves with frequencies in the range 8 – 22 mHz in the internetwork, network, and a pore. We find that the occurrence of waves does not show any location preference and that they are equally distributed over downflows and upflows, regardless of the activity of the observed area in the line of Fe i 543.45 nm. The waves observed in the lower formed line of Fe i 543.29 nm seem to appear preferentially over downflows.  相似文献   

15.
We have developed a new pattern-recognition algorithm based on multiple intensity clips which assures an optimal adaptation to the solar structure under study. The shapes found at higher clip levels are repeatedly extended to lower levels, thus filling more and more of the observed intensity contours. Additionally, at each intensity threshold new shapes, exceeding the level, are integrated. The number and height of the levels can be optimized making this `multiple level tracking' algorithm (MLT) superior to commonly used Fourier-based recognition techniques (FBR). The capability of MLT is demonstrated by application to the intensity structure of solar granulation near the disk center, both speckle reconstructed and not. Comparisons with Doppler maps prove its reliability. The granular pattern recognized by MLT differs essentially from that obtained with FBR. Elongated `snake-like' granules do not occur with MLT and, consequently, the perimeter-area distribution exhibits only a marginal `second branch' of higher fractal dimension, which dramatically diminishes the better the MLT pattern matches the granular structure. The final distribution obtained with optimized parameters has a single fractal dimension near 1.1, making the question of a `critical size', a `second branch', and the often discussed dimension of 4/3; highly questionable. This result is equally obtained from application of MLT to the corresponding Doppler velocity map of granular up-flows. In contrast, the pattern of down-flows contains some elongated `snake-like' structures with higher fractal dimension. We also use the new algorithm to recognize speckle-reconstructed limb faculae, which MLT separates from their granular surroundings, and compare both, granules and faculae, using large statistical samples. The facular grains near cos=57° exhibit a slightly different ellipticity than the (geometrically foreshortened) adjacent granules. However, small facular grains are more round than small granules and larger grains are more similar to granules.  相似文献   

16.
Résumé Huit photographies monochromatiques (=2000 Å) au bord du soleil sont analysées en vue de mettre en évidence la granulation dans le domaine ultra-violet lointain. La fenêtre spectrale est définie par deux filtres interférentiels qui donnent une bande passante à mi-hauteur de 125 Å. Ces clichés de haute résolution ont été obtenus en ballon stratosphérique à une altitude de 30 500 m en profitant d'une fenêtre de transparence atmosphérique et en utilisant un télescope Cassegrain de 20 cm d'ouverture dont la résolution atteint 1. Ce télescope était porté par un dispositif pointeur de soleil dont la stabilité à court terme était meilleure que 2. Les clichés ont été pris à intervalles de 210 secondes avec un temps de pose de 0.25 seconde. On observe d'une part des variations de brillance grossières de largeur comprise entre 10 et 20 qui sont interprétées comme microfacules chromosphériques de durée de vie supérieure à 1/2 heure. On observe d'autre part une granulation dont la distribution densitométrique pour laquelle on a calculé les fonctions de corrélation et de structure présente une corrélation limitée à une distance angulaire de 4.
Summary Eight ultra-violet monochromatic pictures at the limb of the sun are analyzed for evidence of granulation in this spectral range ( = 2000 Å). The spectral window (125 Å) is defined by two interference filters. These high-resolution pictures were obtained with a 20 cm Cassegrain telescope carried at 30 500 m of altitude by a stratospheric balloon. The resolution of the telescope is 1. The instrument is mounted on a sunpointing control of excellent short time stability (2). The pictures were taken at intervals of 210 seconds with a 0.25 second exposure. Large brilliant irregularities 10 to 20 in diameter are interpreted as chromospheric microfaculae with a half-hour lifetime. On the other hand, the smaller irregularities are interpreted as ultra-violet granulation. Using the correlation and structure functions, the analysis of the density distribution shows that spatial correlation of these irregularities is limited to a 4 range.
  相似文献   

17.
The composition and structure of interstellar dust are important and complex for the study of the evolution of stars and the interstellar medium(ISM).However,th...  相似文献   

18.
We have characterized the internal structure of two close-to-round starless cores in Taurus, L1498 and L1517B, setting constraints on the initial conditions of star formation and on models of core condensation. Our analysis is based on high angular resolution observations in at least two transitions of NH3, N2H+, CS, C34S, C18O, and C17O, together with maps of the 1.2 mm continuum. For both cores, we derive radial profiles of constant temperature and constant turbulence, and density distributions close to those of non-singular isothermal spheres. Using a Monte Carlo radiative transfer model, we derive abundance profiles for all species and find a pattern of strong chemical differentiation. NH3 has a higher abundance toward the core centers while N2H+ has a constant abundance over most of the cores. Both C18O and CS (and isotopomers) are strongly depleted in the core interiors, most likely due to their freeze out onto cold dust grains. Concerning the kinematics of the dense gas, we find (in addition to constant turbulence) a pattern of internal motions at the level of 0.05 km s?1. These motions seem correlated with asymmetries in the pattern of molecular depletion, and we interpret them as residuals of core contraction. Their distribution and size suggest that core formation happens in rather irregular manner. A comparison with supersonic turbulence models of core formation shows that our observed cores are much more quiescent than allowed by these models.  相似文献   

19.
Solar System Research - The peculiarities of the composition and internal structure of chondrite NWA 12370, petrological type H5 S1 W1, were studied by means of Raman spectroscopy, XRF, and...  相似文献   

20.
束成钢 《天文学进展》2001,19(2):249-249
从星系形成和演化的角度出发,对星系结构和动力学进行的粗略的评述,内容包括:(1)初步描述了星系中各主要成分的物理特征(空间分布,运行学和化学)及其形成和演化,(2)Damped Lyman-alpha systems(DLAs)是本地星系的化石,对其进行观测研究是HST的主要任务之一,对DLAs宽的谱线轮廓的物理机制和其恒星形成,化学演化进行了讨论,(3)目前已证明Lyman Break方法是发现高红移高恒星形成星系的有效手段,讨论了Lyman Break Galaxies的动力学过程和恒星形象,(4)旋涡星系和椭圆星系的Scaling Law是星系形成和演化所必须解释的问题,对近期该方面的研究结果作了介绍,(5)整体超星的反馈作用在星系形成和演化中起了重要作用,评述了该物理过程对星系演化的影响;(6)随着观测资料的不断积累,各种物体对河外背景辐射的贡献已成了一个重要的研究方向,讨论了宇宙整体的星形成历史和化学演化,(7)银河系是进行星系形成和演化研究的归算零点,介绍了银河系的结构,动力学及演化。  相似文献   

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