共查询到20条相似文献，搜索用时 31 毫秒
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 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
3.
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
4.
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
5.
In the standard model of the solar convective zone, turbulent eddies transport entropy rather than temperature. We consider the turbulent mean field equations for the convective zone, including entropy transport, and show that the zone can be unstable to larger scale motion which we identify with the supergranulation and giant cells. 相似文献
6.
7.
The characteristics of the most unstable fundamental mode and the first harmonic excited in the convection zone of a variety
of solar envelope models are shown to be in reasonable agreement with the observed features of granulation and supergranulation.
On leave of absence from Government Digvijai College, Rajnandgaon 491441 相似文献
8.
Noboru Takeichi 《Celestial Mechanics and Dynamical Astronomy》2010,108(4):405-416
The parametric excitation of a gravity gradient stabilized spacecraft induced by the periodic solar pressure torque is discussed.
The solar pressure torque in the linearized equations of motion appears as linear terms with periodic coefficients. The attitude
stability is analyzed numerically through the calculation of the Floquet multiplier. The perturbation method is also applied
to identify the instability condition analytically. It is made clear that the periodic solar pressure torque can destabilize
the coupled roll and yaw attitude motion of the spacecraft. It is also shown that the conditions of parametric resonance are
included in the gravity gradient stability condition. Nonlinear simulations are also carried out to verify the effect of the
parametric resonance. The numerical simulation using actual parameters shows that the spacecraft inevitably experiences a
large amplitude attitude motion due to the periodic solar pressure torque even if the gravity gradient stability condition
is satisfied. 相似文献
9.
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
10.
Turbulent convection models (TCMs) based on hydrodynamic moment equations are compared with the classical mixing-length theory (MLT) in solar models. The aim is to test the effects of some physical processes on the structure of the solar convection zone, such as the dissipation, diffusion and anisotropy of turbulence that have been ignored in the MLT. Free parameters introduced by the TCMs are also tested in order to find appropriate values for astrophysical applications. It is found that the TCMs usually give larger convective heat fluxes than the MLT does, and the heat transport efficiency is sensitively related to the dissipation parameters used in the TCMs. As a result of calibrating to the present solar values, our solar models usually have rather smaller values of the mixing length to local pressure scaleheight ratio than the standard solar model. The turbulent diffusion is found to have important effects on the structure of the solar convection zone. It leads to significantly lowered and expanded profiles for the Reynolds correlations, and a larger temperature gradient in the central part of the superadiabatic convection region but a smaller one near the boundaries of the convection zone. It is interesting to note that, due to a careful treatment of turbulence developing towards isotropic state, our non-local TCM results in radially dominated motion in the central part and horizontally dominated motion near the boundaries of the convection zone, just as what has been observed in many 3D numerical simulations. Our solar models with the TCMs give small but meaningful differences in the temperature and sound speed profiles compared with the standard solar model using the MLT. 相似文献
11.
The power spectra of temperature and vertical velocity variations in the solar photosphere are calculated using the data obtained through observations of a nonperturbed region near the solar disk center in the neutral iron line λ ≈ 639.3 nm conducted at the 70 cm German Vacuum Tower Telescope (VTT) located in the Canary Islands (Spain). The variations of these spectra with altitude are analyzed. It is found that the primary power in the lower photosphere is localized in the range of frequencies that correspond to granulation with a peak at the λ ≈ 1.5–2.0 Mm scale and is reduced with altitude, the power spectrum maximum in the upper photospheric layers is shifted towards larger scales (Δλ ≤ 1 Mm), and the power of variations of the vertical supergranulation velocity (λ ≈ 20–30 Mm) virtually does not change with altitude. An isolated mesogranulation regime (λ ≈ 5–12 Mm) is not found at any of the studied altitudes. The obtained results suggest that the convective structure of the solar photosphere at mesogranulation scales behaves like granulation: the mesostructures are a part of an extended distribution of granulation scales. It is shown that the supergranulation flows are stable throughout the entire photosphere and reach much higher altitudes than the granulation flows. 相似文献
12.
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
13.
J. J. Podesta 《Solar physics》2005,232(1-2):1-23
The solar convection zone is modeled as a horizontally stratified atmosphere with a constant gravitational field and an adiabatic
temperature gradient (a neutrally stratified polytrope). At equilibrium, the gas pressure and density decreases to zero at
the solar surface so that the solar surface is treated as a free surface which is bounded by vacuum. The evolution of small
amplitude perturbations about the equilibrium state is described by the linearized Euler equations for an inviscid compressible
fluid with an adiabatic equation of state. A sunquake is initiated at time zero by means of an initial perturbation with a
Gaussian velocity profile and the exact solution of the initial value problem is obtained in terms of a Fourier integral.
Comparisons between theory and observations indicate that this highly simplified model is able to predict the propagation
of sunquake waves across the solar surface with an error of approximately 10% or 20%. 相似文献
14.
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
15.
Supergranulation is one of the most visible length scales of solar convection and has been studied extensively by local helioseismology. We use synthetic data computed with the Seismic Propagation through Active Regions and Convection (SPARC) code to test regularized-least squares (RLS) inversions of helioseismic-holography measurements for a supergranulation-like flow. The code simulates the acoustic wavefield by solving the linearized three-dimensional Euler equations in Cartesian geometry. We model a single supergranulation cell with a simple, axisymmetric, mass-conserving flow. The use of simulated data provides an opportunity for direct evaluation of the accuracy of measurement and inversion techniques. The RLS technique applied to helioseismic-holography measurements is generally successful in reproducing the structure of the horizontal-flow field of the model supergranule cell. The errors are significant in horizontal-flow inversions near the top and bottom of the computational domain as well as in vertical-flow inversions throughout the domain. We show that the errors in the vertical velocity are due largely to cross talk from the horizontal velocity. 相似文献
16.
The Rayleigh-Taylor instability of the plane interface separating the two partially-ionized superposed fluids through porous medium is analysed. The effect of variable horizontal magnetic field, surface tension and rotation along the vertical axis are also incorporated. The relevant linearized perturbation equations are taken and using normal mode analysis the general relation is obtained from which the dispersion relation for two superposed fluids of different densities is derived. It is found that the surface tension and horizontal magnetic field have the stabilizing effect on the R-T-instability. The condition of instability remains unaffected by the permeability of porous medium, presence of neutral particles in the fluids and rotation.It is concluded that the system is unstable only for those positive wave numbers which are less than certain critical value in case of an adverse density gradient. 相似文献
17.
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
18.
Jacques M. Beckers 《Solar physics》1968,5(3):309-322
The large scale (> 5000 km) intensity structure of the photosphere has been examined. The power per frequency unit indicates a continuous increase towards smaller spatial frequency. No excess power exists at wavelengths near the size of the supergranulation (30000 km) or at any other wavelength between 5000 and 100000 km. However, direct measurement of the intensity distribution in 1652 supergranulation cells shows a very small increase of the intensity towards the cell boundary. The amount of this increase is larger near the solar limb. It is probably due to a weak continuum emission associated with the chromospheric network. Any temperature difference arising from the supergranulation convection is obscured by this emission and is probably less than 1 K. 相似文献
19.
We investigate the excitation of magnetoacoustic–gravity waves generated from localized pulses in the gas pressure as well as in the vertical component of velocity. These pulses are initially launched at the top of the solar photosphere, which is permeated by a weak magnetic field. We investigate three different configurations of the background magnetic field lines: horizontal, vertical, and oblique to the gravitational force. We numerically model magnetoacoustic–gravity waves by implementing a realistic (VAL-C) model of the solar temperature. We solve the two-dimensional ideal magnetohydrodynamic equations numerically with the use of the FLASH code to simulate the dynamics of the lower solar atmosphere. The initial pulses result in shocks at higher altitudes. Our numerical simulations reveal that a small-amplitude initial pulse can produce magnetoacoustic–gravity waves, which are later reflected from the transition region due to the large-temperature gradient. The cavities in the lower solar atmosphere are found to have the best conditions to act as a resonator for various oscillations, including their trapping and leakage into the higher atmosphere. Our numerical simulations successfully model the excitation of such wave modes, their reflection and trapping, as well as the associated plasma dynamics. 相似文献
20.
V. Bumba 《Solar physics》1970,14(1):80-88
Using the Mt. Wilson magnetic synoptic charts from the recent solar activity cycles the dynamics of the formation of giant regular structures formed in the plus (leading) polarity of older more extended magnetic fields are studied. Although their diameters are about one order greater than those of supergranules, the processes of their development go analogously to those of supergranulation and granulation.The close relation of the minus (following) polarity distribution to the new formed active regions is demonstrated and the possible different mode of distribution and different role of both opposite polarities on activity processes is discussed. 相似文献