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1.
Structure of horizontal convective currents in the solar atmosphere has been investigated using profiles of the λ ≈ 532.42
nm neutral iron line which were observed at the solar limb with high spatial resolution. The asymmetry of the observed line
was shown to arise when approaching the solar limb. The spatial and time velocity variations were simulated using the λ-meter
technique. Acoustic waves were removed using the k-ω filters. The convection currents on various spatial scales were distinguished, namely, those connected with granulation,
mesogranulation, and supergranulation. The spatial and time distribution of the convection velocities in the photosphere and
in the low chromosphere has been analyzed. The horizontal currents were shown to exist on granulation, mesogranulation, and
supergranulation scales as low as h ≈ 250 km, and the granulation and mesogranulation horizontal velocities increase with height. In the photospheric layers,
the supergranulation vertical-velocity field appears almost invariable, while the supergranulation horizontal-velocity field
can vary with height. The horizontal velocity distribution within large convection currents is found to be asymmetric on granulation,
mesogranulation, and supergranulation scales. 相似文献
2.
The theoretical power spectrum of velocity fields and flux fluctuations at the solar photosphere is calculated using a quasi-nonlinear framework of superposition of unstable convective eigenmodes excited in the solar convection zone. It is demonstrated that this power spectrum exhibits at least three distinct peaks corresponding to granulation, mesogranulation and supergranulation. The vertical velocity and the brightness fluctuation at the solar surface are found to be correlated. The theoretical framework can be adopted for application to other types of stars in order to predict the dominant length scales in the power spectrum of convection in these stars. 相似文献
3.
We investigate the structure of convective flows in the solar photosphere on subgranulation scales. The solar granulation pattern is reproduced by solving the inverse problem of nonequilibrium radiation transfer on the basis of the profiles of the neutral iron line λ 523.42 nm. The wave motions are excluded by the k-ω filtration. The line-of-sight velocity has an asymmetric distribution inside the convective flows in large granules (1.5″ and larger) in the lower photosphere and at the bottom of the middle photosphere. This asymmetry is weaker in the upper photosphere. For smaller flows the distribution is more symmetric at all heights. The asymmetry of the temperature distribution is less pronounced. Large convective flows were found to have a fine structure: they are fragmentized into several smaller flows. The fine structure of large flows and spatial smearing are responsible for the observed asymmetry of the convection velocity distribution inside flows. 相似文献
4.
As large-distance rays (say, 10?–?24°) approach the solar surface approximately vertically, travel times measured from surface pairs for these large separations are mostly sensitive to vertical flows, at least for shallow flows within a few Mm of the solar surface. All previous analyses of supergranulation have used smaller separations and have been hampered by the difficulty of separating the horizontal and vertical flow components. We find that the large-separation travel times associated with supergranulation cannot be studied using the standard phase-speed filters of time–distance helioseismology. These filters, whose use is based upon a refractive model of the perturbations, reduce the resultant travel-time signal by at least an order of magnitude at some distances. More effective filters are derived. Modeling suggests that the center–annulus travel-time difference [δt oi] in the separation range Δ=10?–?24° is insensitive to the horizontally diverging flow from the centers of the supergranules and should lead to a constant signal from the vertical flow. Our measurement of this quantity, 5.1±0.1 seconds, is constant over the distance range. This magnitude of the signal cannot be caused by the level of upflow at cell centers seen at the photosphere of 10 m?s?1 extended in depth. It requires the vertical flow to increase with depth. A simple Gaussian model of the increase with depth implies a peak upward flow of 240 m?s?1 at a depth of 2.3 Mm and a peak horizontal flow of 700 m?s?1 at a depth of 1.6 Mm. 相似文献
5.
Laurence J. November 《Solar physics》1994,154(1):1-17
The 2D horizontal velocity field determined from local correlation tracking of granulation and its divergence have remarkably different appearances. The 2D horizontal velocity shows the classical 32 Mm supergranular cellular outflow bounded by the chromospheric network, whereas the divergence is dominated by distinct long-lived sources and sinks of about 7 Mm size. The 2D horizontal velocity shows no obvious evidence for 7 Mm cells, and the divergence exhibits little power with the 32 Mm scale. However, by mass continuity for a steady 3D flow in a stratified atmosphere, the divergence of the 2D horizontal component is equal to the vertical velocity divided by a height scale. Thus the 3D steady solar flow field at the bottom of the photosphere has a vertical component consisting primarily of 7 Mm sources and sinks, which define the 2D cellular-like 32 Mm continuous horizontal outflows.Simultaneous Doppler vertical velocity measurements verify the mass-continuity relation, and give a height scale equal to the density scale height in the photosphere within observational error. The observational result is consistent with our theoretical expectation. Any height scale other than the density scale height would indicate a vertical velocity thate-folds on a scale comparable to or smaller than the density scale height, which we argue is unphysical near the top of the convection zone. The continuity relation indicates that vortex-free steady horizontal velocities seen at the solar surface, i.e., the horizontal supergranular flow, must diminish with depth due to the increasing density scale height. We estimate that the horizontal supergranular flow cannot extend much more than onee-fold increase in the density scale height below the visible solar surface, about 2.4 Mm. Therefore the convection below the solar surface should be characterized by the scale of the principal steady vertical velocity component, i.e., by vertical plumes having a dimension of 7 Mm - what we have called mesogranulation - rather than closed 32 Mm cells as is widely believed.Operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with National Science Foundation. 相似文献
6.
The Sun is a non-equilibrium, dissipative system subject to an energy flow that originates in its core. Convective overshooting motions create temperature and velocity structures that show a temporal and spatial multiscale evolution. As a result, photospheric structures are generally considered to be a direct manifestation of convective plasma motions. The plasma flows in the photosphere govern the motion of single magnetic elements. These elements are arranged in typical patterns, which are observed as a variety of multiscale magnetic patterns. High-resolution magnetograms of the quiet solar surface revealed the presence of multiscale magnetic underdense regions in the solar photosphere, commonly called voids, which may be considered to be a signature of the underlying convective structure. The analysis of such patterns paves the way for the investigation of all turbulent convective scales, from granular to global. In order to address the question of magnetic structures driven by turbulent convection at granular and mesogranular scales, we used a voids-detection method. The computed distribution of void length scales shows an exponential behavior at scales between 2 and 10 Mm and the absence of features at mesogranular scales. The absence of preferred scales of organization in the 2?–?10 Mm range supports the multiscale nature of flows on the solar surface and the absence of a mesogranular convective scale. 相似文献
7.
Below the scale of supergranules we find that cellular flows are present in the solar photosphere at two distinct size scales, approximately 2 Mm and 4 Mm, with distinct characteristic times. Simultaneously present in the flow is a non-cellular component, with turbulent scaling properties and containing 30% of the flow energy. These results are obtained by means of wavelet spectral analysis and modeling of vertical photospheric motions in a 2-hour sequence of 120 SOHO/MDI, high-resolution, Doppler images near disk center. The wavelets permit detection of specific local flow patterns corresponding to convection cells. 相似文献
8.
A method is presented for the direct measurement of the heights of the radio emission of solar active regions when they are located at the limb in order to reconstruct the vertical structure of the magnetic field in solar active regions. The method involves an analysis of radio source positions in the scans based on high frequency resolution one-dimensional centimeter-wave measurements performed on the RATAN-600 radio telescope. Radio sources are difficult to identify at many frequencies when observed at the limb at zero position angle because of abrupt signal variations at the solar limb. To eliminate edge effects on the scan, special observing periods are used (near vernal and autumnal equinoxes), when the source at the limb is located far from the scan edge because of the large position angle of the Sun. As a result of these observations, the spectra of relative heights are constructed for a number of sources for the period from 2007 through 2012. Source heights are shown to generally increase with wavelength. The height difference between the 5 and 2 cm emission is equal to 5.2 ± 2.0 Mm, and the corresponding height difference between the 8 and 2 cm emission is equal to 9.6 ± 3.0 Mm. It is shown that such characteristics can be obtained for a field generated by a dipole submerged under the photosphere at a depth of 17 Mm irrespective of the possible reduction of relative altitudes to absolute altitudes. 相似文献
9.
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
10.
Supergranulation is a component of solar convection that manifests itself on the photosphere as a cellular network of around
35 Mm across, with a turnover lifetime of 1 – 2 days. It is strongly linked to the structure of the magnetic field. The horizontal,
divergent flows within supergranule cells carry local field lines to the cell boundaries, while the rotational properties
of supergranule upflows may contribute to the restoration of the poloidal field as part of the dynamo mechanism, which controls
the solar cycle. The solar minimum at the transition from cycle 23 to 24 was notable for its low level of activity and its
extended length. It is of interest to study whether the convective phenomena that influence the solar magnetic field during
this time differed in character from periods of previous minima. This study investigates three characteristics (velocity components,
sizes and lifetimes) of solar supergranulation. Comparisons of these characteristics are made between the minima of cycles
22/23 and 23/24 using MDI Doppler data from 1996 and 2008, respectively. It is found that whereas the lifetimes are equal
during both epochs (around 18 h), the sizes are larger in 1996 (35.9 ± 0.3 Mm) than in 2008 (35.0 ± 0.3 Mm), while the dominant
horizontal velocity flows are weaker (139 ± 1 m s−1 in 1996; 141 ± 1 m s−1 in 2008). Although numerical differences are seen, they are not conclusive proof of the most recent minimum being inherently
unusual. 相似文献
11.
Berrilli F. Del Moro D. Consolini G. Pietropaolo E. Duvall T.L. Kosovichev A.G. 《Solar physics》2004,221(1):33-45
We investigate spatial dislocation ordering of the solar structures associated with supergranulation and granulation scales.
The supergranular and granular structures are automatically segmented from time-distance divergence maps and from broad-band
images, respectively. The spatial dislocation ordering analysis is accomplished by applying the statistical method of Pair
Correlation Function, g
2(r), to segmented features in the solar fields. We compare the computed g
2(r) functions obtained from both single and persistent, i.e., time-averaged, fields associated with supergranulation and granulation.
We conclude that supergranulation and granulation patterns present a different topological order both in single and persistent
fields. The analysis carried out on single fields suggests that the granulation behaves as an essentially random distribution
of soft plasma features with a very broad distribution in size, while supergranulation behaves as a random distribution of
close packed, coherent stiff features with a rather defined mean size. 相似文献
12.
High resolution spectra of the coronal emission line Fe xiv at 530.3 nm obtained at the 30 May 1965 total solar eclipse are analyzed and interpreted. Deconvolution techniques that preserve the line intensity vs wavelength profile shape are developed to obtain further resolution improvement. The west limb coronal enhancement is determined to have temperatures less than 3 MK and turbulent velocities of ～25 km s-1 decreasing with altitude. Temperature gradients provide evidence for marginal solar wind flow from this enhancement. Above the quiet photosphere in the southwest quadrant the comparison of line and continuum intensities and consideration of line width suggest to us the coronal region is filled with inhomogeneous plasma, dense enough in localized regions to maintain collisional excitation. Solar wind flow from this region obtains when turbulent velocities are assumed to contribute to the line broadening. We identify this region as a coronal hole and suggest that coronal material is heated by the quiet photosphere below. 相似文献
13.
The Sun emits radiation at several wavelengths of the electromagnetic spectrum. In the optical band, the solar radius is 695?700 km, and this defines the photosphere, which is the visible surface of the Sun. However, as the altitude increases, the electromagnetic radiation is produced at other frequencies, causing the solar radius to change as a function of wavelength. These measurements enable a better understanding of the solar atmosphere, and the radius dependence on the solar cycle is a good indicator of the changes that occur in the atmospheric structure. We measure the solar radius at the subterahertz frequencies of 0.212 and 0.405 THz, which is the altitude at which these emissions are primarily generated, and also analyze the radius variation over the 11-year solar activity cycle. For this, we used radio maps of the solar disk for the period between 1999 and 2017, reconstructed from daily scans made by the Solar Submillimeter-wave Telescope (SST), installed at El Leoncito Astronomical Complex (CASLEO) in the Argentinean Andes. Our measurements yield radii of \(966.5'' \pm2.8''\) for 0.2 THz and \(966.5'' \pm2.7''\) for 0.4 THz. This implies a height of \(5.0 \pm2.0 \times10^{6}\) m above the photosphere. Furthermore, we also observed a strong anticorrelation between the radius variation and the solar activity at both frequencies. 相似文献
14.
A sequence of Ca-K images obtained in a period of minimum solar activity, from July to November 1996, at the Rome Observatory with the PSPT (Precision Solar Photometric Telescope) prototype instrument have been used to analyze the geometrical properties of cells identified by the chromospheric network. In particular, we used 256 × 256 sub-arrays of the calibrated full-disk PSPT images. These sub-arrays, centered on the solar disk, are reduced to two-levels (binary) images by means of a suitable threshold after an FFT high-pass filtering. A medial axis transform, better known as skeleton, combined with a cellular automaton, is applied to the two-level images, in order to derive the cell boundaries. The regions corresponding to the cells are then filled by a growing algorithm. In this way we can derive a set of output parameters describing the cells geometry. The size distribution of the identified cells shows a continuous increase toward the smaller scales, rather than a small dispersion around a characteristic scale. Nevertheless the analysis of the inter-cell distances and of the area distribution pointed out a characteristic scale (square root of the area) of ± 24 Mm. To describe the cells irregularity and to probe the nature of solar turbulence, we apply a Mandelbrot fractal analysis to such irregularly shaped features. Examining the cell perimeter–area relationship we found the existence of a critical area at which a change in the geometrical properties occurs. This area corresponds to the scale of ± 24 Mm. The estimated fractal dimension for cells with area greater than the critical one is 1.35. This value, close to that predicted for isobars in the Kolmogorov 3-D turbulent theory, does not exclude a turbulent origin for such cells. The analysis seems to point to a common origin for solar granulation and supergranulation. 相似文献
15.
The stability of linear convective and acoustic modes in solar envelope models is investigated by incorporating the thermal and mechanical effects of turbulence through the eddy transport coefficients. With a reasonable value of the turbulent Prandtl number it is possible to obtain the scales of motion corresponding to granulation, supergranulation and the five-minute oscillations. Several of the acoustic modes trapped in the solar convection zone are found to be overstable and the most unstable modes, spread over a region centred predominantly around a period of 300 s with a wide range of horizontal length scales, are in reasonable accord with the observed power-spectrum of the five-minute oscillations. It is demonstrated that these oscillations are driven by a simultaneous action of the -mechanism and the radiative and turbulent conduction mechanisms operating in the strongly superadiabatic region in the hydrogen ionization zone, the turbulent transport being the dominant process in overstabilizing the acoustic modes. 相似文献
16.
George Driver Nelson 《Solar physics》1978,60(1):5-18
We present a two-dimensional, nearly flux constant solar atmosphere which includes a physical model of granulation. If the inhomogeneous character of the solar photosphere is neglected in the construction of empirical models, the result is an underestimate of the temperature gradient in the layers below 0.5 and an overestimate of the convective flux penetration into the observable layers. The two-dimensional model adequately reproduces the mean limb darkening, the magnitude and center-to-limb variation of the intensity fluctuations, and the rms vertical and horizontal convective velocities. 相似文献
17.
We compare horizontal flow fields in the photosphere and in the subphotosphere (a layer 0.5 Mm below the photosphere) in two solar active regions: AR?11084 and AR?11158. AR?11084 is a mature, simple active region without significant flaring activity, and AR?11158 is a multipolar, complex active region with magnetic flux emerging during the period studied. Flows in the photosphere are derived by applying the Differential Affine Velocity Estimator for Vector Magnetograms (DAVE4VM) on HMI-observed vector magnetic fields, and the subphotospheric flows are inferred by time–distance helioseismology using HMI-observed Dopplergrams. Similar flow patterns are found for both layers for AR?11084: inward flows in the sunspot umbra and outward flows surrounding the sunspot. The boundary between the inward and outward flows, which is slightly different in the photosphere and the subphotosphere, is within the sunspot penumbra. The area having inward flows in the subphotosphere is larger than that in the photosphere. For AR?11158, flows in these two layers show great similarities in some areas and significant differences in other areas. Both layers exhibit consistent outward flows in the areas surrounding sunspots. On the other hand, most well-documented flux-emergence-related flow features seen in the photosphere do not have counterparts in the subphotosphere. This implies that the horizontal flows caused by flux emergence do not extend deeply into the subsurface. 相似文献
18.
A model of diffusion induced by the joint action of random cells of two different sizes (granulation and supergranulation) is developed. The basic properties of the model are defined by the dimensionless ratio = (the life time)/(characteristic size/velocity) constructed for these two types of cells. An analytical expression for the diffusivity as a function of is derived. It is estimated that for the standard parameters used the contribution of granulation to the joint diffusion is small. A comparison between the model and numerical simulations of the kinematic diffusion of magnetic fields on the solar surface is made. 相似文献
19.
R. I. Kostyk 《Kinematics and Physics of Celestial Bodies》2013,29(1):32-36
Results are presented of observations of the facula area near the solar disc center. Observations were performed at the German Vacuum Tower Telescope of the Observatorio del Teide (Tenerife) with the simultaneous use of two instruments, i.e., TESOS in the Ba IIλ 455.4 nm line to measure intensity variations in the photosphere and, at the same time, TIP in the Fe I (λλ 1564.3–1565.8 nm) line to measure Stokes parameters. Using the Fourier filtering technique, we separated the convective and wave components of the intensity field. Stokes parameters Fe I λ 1564.8 nm and λ 1565.2 nm were inverted by the SIR inversion code to estimate the magnetic field strength. We found that the contrast of intergranular lines of the facula in the continuum is almost independent of the magnetic field strengh (in the range from 30 to 160 mT). This result casts doubt on the assertion that solar faculae are a cluster of magnetic flux tubes. Most likely, due to the decrease of transparency of the matter in a strong (approximately 1 kilogauss) magnetic field, we can see the hot walls of granules. 相似文献
20.
Three radial-velocity fluctuation arrays V(Δλ, Y) and line-formation fluctuation arrays L(Δλ, Y),where Δλ is wavelength displacement from the center of Nai D1 and Y is displacement on the Sun's surface along the spectrograph slit, were obtained from Sacramento Peak Observatory spectrograms. The variations of these line profile fluctuations are qualitatively described. The RMSυ's, coherences, and power spectra shapes for V(Δλ, Y) fluctuations are examined at different Δλ with the corresponding effective heights of formation calculated with Mein weighting functions. Results include: (a) possible anticorrelation between continuum fluctuations and those near line center; (b) RMS υ (cr) 's, which are root-mean-square values of the radial velocity corrected for instrumental and atmospheric blurring, are large (1.5 to 4.0 km s?1) primarily due to large corrections for atmospheric blurring; (c) RMS υ (cr) minima at effective heights of formation above 350 km suggest penetration of granulation velocities into the upper photosphere; (d) very rough determinations of RMS υ (cr) 's, which are additionally corrected for line-of-sight averaging, range from around 5 km s?1 in the low chromosphere to a sharp minimum ≤ 0.5 km s?1 located in the upper photosphere; (e) power spectra shapes reflect decreasing average fluctuation scales above the temperature minimum (possibly high-frequency oscillations) and in the low and middle photosphere (possibly penetration of granulation); and (f) RMS υ (cr) 's and average fluctuation scales suggest changes in the resolvable velocity field occurring near the temperature minimum. 相似文献