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1.
David H. Hathaway 《Solar physics》1992,137(1):15-32
The use of the spherical harmonic functions to analyse the nearly steady flows in the solar photosphere is extended to situations in which B
0, the latitude at disk center, is nonzero and spurious velocities are present. The procedures for extracting the rotation profile and meridional circulation are altered to account for the seasonal tilt of the Sun's rotation axis toward and away from the observer. A more robust and accurate method for separating the limb shift and meridional circulation signals is described. The analysis procedures include the ability to mask out areas containing spurious velocities (velocity-like signals that do not represent true flow velocities in the photosphere). The procedures are shown to work well in extracting the various flow components from realistic artificial data with a broad, continuous spectrum for the supergranulation. The presence of this supergranulation signal introduces errors of a few m s -1 in the measurements of the rotation profile, meridional circulation, and limb shift from a single Doppler image. While averaging the results of 24 hourly measurements has little effect in reducing these errors, an average of 27 daily measurements reduces the errors to well under 1 m s -1. 相似文献
2.
P. Ambrož 《Solar physics》2004,224(1-2):61-68
Large-scale magnetic field regions are evolving on a time scale of many weeks and months and are also modified during the
solar activity cycle. The position of the regions are compared in a pair of consecutive synoptic charts and the horizontal
velocity field responsible for their position changes, is inferred. Besides the axially symmetric zonal and meridional drifts,
relating to differential rotation and meridional circulation, also non-axially symmetric velocity structures were observed
during the last three solar activity cycles. Changes of the position and spatial distribution, as well as temporal variations
of the field strength, closely relate to the occurrence and variations of other forms of solar activity such as sunspots,
filaments and prominences and coronal structures. In combination with 11-yr cyclic changes of the large-scale velocity field,
a new global dynamic regime of the convection zone is described. 相似文献
3.
Francis J. Robinson † Kwing L. Chan 《Monthly notices of the Royal Astronomical Society》2001,321(4):723-732
We present the results of two simulations of the convection zone, obtained by solving the full hydrodynamic equations in a section of a spherical shell. The first simulation has cylindrical rotation contours (parallel to the rotation axis) and a strong meridional circulation, which traverses the entire depth. The second simulation has isorotation contours about mid-way between cylinders and cones, and a weak meridional circulation, concentrated in the uppermost part of the shell.
We show that the solar differential rotation is directly related to a latitudinal entropy gradient, which pervades into the deep layers of the convection zone. We also offer an explanation of the angular velocity shear found at low latitudes near the top. A non-zero correlation between radial and zonal velocity fluctuations produces a significant Reynolds stress in that region. This constitutes a net transport of angular momentum inwards, which causes a slight modification of the overall structure of the differential rotation near the top. In essence, the thermodynamics controls the dynamics through the Taylor–Proudman momentum balance . The Reynolds stresses only become significant in the surface layers, where they generate a weak meridional circulation and an angular velocity 'bump'. 相似文献
We show that the solar differential rotation is directly related to a latitudinal entropy gradient, which pervades into the deep layers of the convection zone. We also offer an explanation of the angular velocity shear found at low latitudes near the top. A non-zero correlation between radial and zonal velocity fluctuations produces a significant Reynolds stress in that region. This constitutes a net transport of angular momentum inwards, which causes a slight modification of the overall structure of the differential rotation near the top. In essence, the thermodynamics controls the dynamics through the Taylor–Proudman momentum balance . The Reynolds stresses only become significant in the surface layers, where they generate a weak meridional circulation and an angular velocity 'bump'. 相似文献
4.
Temporal variations of the subsurface meridional flow with the solar cycle have been reported by several authors. The measurements
are typically averaged over periods of time during which surface magnetic activity existed in the regions where the velocities
are calculated. The present work examines the possible contamination of these measurements due to the extra velocity fields
associated with active regions plus the uncertainties in the data obtained where strong magnetic fields are present. We perform
a systematic analysis of more than five years of GONG data and compare meridional flows obtained by ring-diagram analysis
before and after removing the areas of strong magnetic field. The overall trend of increased amplitude of the meridional flow
towards solar minimum remains after removal of large areas associated with surface activity. We also find residual circulation
toward the active belts that persists even after the removal of the surface magnetic activity, suggesting the existence of
a global pattern or longitudinally-located organized flows. 相似文献
5.
In this work we use an already-published method to infer a variation profile for the solar meridional circulation over the
last 250 years. We feed this variation profile into a numerical dynamo code, and we reconstruct a sunspot time series that
acts as a proxy for solar cycle activity. We perform three simulations with slightly different parameters, and the results
are compared with the observational data. The medium and large correlation coefficients between reconstructed and observational
time series seem to indicate that variations in meridional circulation play an important role in the modulation of solar activity. 相似文献
6.
The suspicion of Elste (1990), that telescopic stray light together with imperfect collimation of telescope and spectrograph could be a possible explanation for the systematic differences and variations found by Neckel and Labs (1987) in many limb-darkening scans, proves to be unfounded for the following reasons: (1)The collimation was performed very precisely; (2) the telescope mirrors remained fixed in position and direction during most of the observing period; (3) stray light effects depending on hour angle were not detectable; (4) in the same collimation status, also many almost symmetric scans had been recorded; (5) the observed east-west differences in the solar intensities are partly even larger than the total amount of stray light (from telescope and sky!) observed as sky-background just outside the limb; (6) any east-west differences in the sky-background near the limb are just a few 0.01% of the disk center intensity; (7) the differences of the average intensities along eastern and western radius appear to be correlated with the east-west differences of the intensity's R.M.S. 相似文献
7.
A method is described for constructing artificial data that realistically simulate photospheric velocity fields. The velocity fields include rotation, differential rotation, meridional circulation, giant cell convection, supergranulation, convective limb shift, p-mode oscillations, and observer motion. Data constructed by this method can be used for testing algorithms designed to extract and analyze these velocity fields in real Doppler velocity data. 相似文献
8.
Guided by the recent observational result that the meridional circulation of the Sun becomes weaker at the time of the sunspot
maximum, we have included a parametric quenching of the meridional circulation in solar dynamo models such that the meridional
circulation becomes weaker when the magnetic field at the base of the convection zone is stronger. We find that a flux transport
solar dynamo tends to become unstable on including this quenching of meridional circulation if the diffusivity in the convection
zone is less than about 2×1011 cm2 s−1. The quenching of α, however, has a stabilizing effect and it is possible to stabilize a dynamo with low diffusivity with sufficiently strong
α-quenching. For dynamo models with high diffusivity, the quenching of meridional circulation does not produce a large effect
and the dynamo remains stable. We present a solar-like solution from a dynamo model with diffusivity 2.8×1012 cm2 s−1 in which the quenching of meridional circulation makes the meridional circulation vary periodically with solar cycle as observed
and does not have any other significant effect on the dynamo. 相似文献
9.
This paper applies a Kuramoto model of coupled oscillators to investigate the north–south (N–S) solar asymmetry and properties of meridional circulation. We focus our study on the asymmetry of the 11-year phase, which is slight but persistent: only two changes of sign (around 1928 and 1968) are observed in the past century. We present a model of two non-linear coupled oscillators that links the hemispheric phase asymmetry of sunspots with the asymmetry of the meridional flow. We use a Kuramoto model with evolving frequencies and constant symmetric coupling to show how asymmetry in meridional circulation could produce a persistent phase lead of one solar hemisphere over the other. We associate the natural frequencies of the two oscillators with the velocities of the meridional flow cells in the northern and southern hemispheres. We assume the respective circulations to be independent and estimate the value of the relevant cross-equatorial coupling by the coupling coefficient in the Kuramoto model. We find that a persistent N–S asymmetry of sunspots and the change of the leading hemisphere could indeed both be the result of the evolving frequencies of meridional circulation; the necessary asymmetry of the meridional flow may be small; and the cross-equatorial coupling has an intermediate range value. Possible applications of these results in solar dynamo models are discussed. 相似文献
10.
David H. Hathaway 《Solar physics》1987,108(1):1-20
Steady photospheric flows can be represented by a spectrum of spherical harmonic modes. A technique is described in which full disc doppler velocity measurements are analysed using the spherical harmonic functions to determine the characteristics of this spectrum and the nature of these flows. Synthetic data is constructed for testing this technique. This data contains limb shift, rotation, differential rotation, meridional circulation, supergranules, giant cells and various levels of noise.The data is analysed in several steps. First, the limb shift is calculated by finding the average velocity in concentric rings about disc center. A polynomial representation of the limb shift is then removed from the data. Secondly, the rotation profile is calculated by finding an average slope in the velocity across the disc at each latitude position. This rotation profile is fit with Legendre polynomials and removed from the data. The third step is to find the meridional circulation by calculating the spherical harmonic transform for the axisymmetric poloidal modes and correcting for the effects of the limb shift analysis. The final step is to calculate the full spectrum of spherical harmonic components for the convective flows. Supergranules are separated from giant cells by spectral filtering for high (l >32) and low (l <32) wavenumbers, respectively.Some information about the spectrum is lost because only one hemisphere is seen, only the line-of-sight velocity is measured and the measurements contain noise. The lack of information about the motions on the backside of the Sun produces a broad smearing of the spectrum into nearby modes. The lack of information about the transverse velocity component produces a mixing between modes whose longitudinal wavenumbers differ by two and between the poloidal and toroidal components with the same wavenumber. In spite of this mode mixing much can be learned from this analysis. Solar rotation and differential rotation can be accurately measured and monitored for secular changes. Meridional circulations with small amplitudes can be measured and monitored and giant cells can be separated from supergranules. 相似文献
11.
We have analysed a large set of sunspot group data (1874 – 2004) and find that the meridional flow strongly varies with the
phase of the solar cycle, and the variation is quite different in the northern and the southern hemispheres. We also find
the existence of considerable cycle-to-cycle variation in the meridional velocity, and about a 11-year difference between
the phases of the corresponding variations in the northern and the southern hemispheres. In addition, our analysis also indicates
the following: (i) the existence of a considerable difference (about 180°) between the phases of the solar-cycle variations in the latitude-gradient terms of the northern and the southern hemispheres’
rotations; (ii) the existence of correlation (good in the northern hemisphere and weak in the southern hemisphere) between the mean solar-cycle
variations of meridional flow and the latitude-gradient term of solar rotation; (iii) in the northern hemisphere, the cycle-to-cycle variation of the mean meridional velocity leads that of the equatorial rotation
rate by about 11 years, and the corresponding variations have approximately the same phase in the southern hemisphere; and
(iv) the directions of the mean meridional velocity is largely toward the pole in the longer sunspot cycles and largely toward
the equator in the shorter cycles. 相似文献
12.
In this paper we study the interaction of rotation with convection in a deep compressible spherical shell as the Sun's convection zone. We examine how the energy transport and the large scale motions can be affected by rotation. In particular we study how a large scale meridional circulation can give rise to variations of angular velocity with latitude and depth.It is assumed that the energy transport is only due to convection and that the mixing-length theory gives an adequate representation of it. Furthermore we assume that rotation acts as a perturbation of the turbulent convective flux through its transport coefficient.The equations involved in the model are integrated numerically in the limit of large viscosity and slow rotation. After having expanded all physical quantities to the first order in terms of Legendre polynomials, the fitting with the observed solar differential rotation gives the expansion parameter, which represents the coupling constant between rotation and convection.The results show a three-cell circulation extending from the poles to the equator. The first one is located in the lower half of the convection zone with the fluid rising at the equator and sinking at the poles. In the second one the direction of the motion is reversed while the third one, located in a thin upper layer, shows the same characteristics of the first one. The meridional velocities at the surface are directed towards the poles and are about 20 cm s-1. In the other cells the meridional velocities are typically of a few cm s-1 while the radial velocities are of the order of a few tenths of cm s-1.The heat flux relative variation at the surface is about 10-4 (3 × 10-3 at the bottom) with a polar excess. The temperature variation at the surface is of the same order, with an equatorial excess however. The convection seems to be stabilized stronger at the equator. The angular velocity increases inwards and varies about 6% between the surface and the bottom of the convection zone.An attempt is made for explaining the picture which emerges. In particular the negligible flux and temperature variations at the surface are explained in terms of equalization by the particular structure of the latitudinal flow. This configuration of large scale circulation is attributed to the high stratification of the convection zone with depth. 相似文献
13.
M.S. Miesch 《Astronomische Nachrichten》2007,328(10):998-1001
In the outer envelope of the Sun and in other stars, differential rotation and meridional circulation are maintained via the redistribution of momentum and energy by convective motions. In order to properly capture such processes in a numerical model, the correct spherical geometry is essential. In this paper I review recent insights into the maintenance of mean flows in the solar interior obtained from high-resolution simulations of solar convection in rotating spherical shells. The Coriolis force induces a Reynolds stress which transports angular momentum equatorward and also yields latitudinal variations in the convective heat flux. Meridional circulations induced by baroclinicity and rotational shear further redistribute angular momentum and alter the mean stratification. This gives rise to a complex nonlinear interplay between turbulent convection, differential rotation, meridional circulation, and the mean specific entropy profile. I will describe how this drama plays out in our simulations as well as in solar and stellar convection zones. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
14.
The influence of latitudinally dependent boundary conditions on the large radius values of meridional flow in the distant solar wind is examined through a double perturbation expansion of the magnetohydrodynamic equations. A general result is derived for the meridional velocity which allows arbitrary specification of radial velocity, radial magnetic field, and mass flux, as a function of colatitude at some coronal reference surface. Three specific examples are treated, including the model of Pneuman and Kopp (1971). The latter example indicates that there may be flow toward the equator at large radii, as opposed to the pure equatorial divergence of internally generated motion due to a flow which is latitudinally uniform at the reference radius. A solar cycle effect most probably averages the boundary conditions so that only the equatorial divergence from an average spherically symmetric corona is seen in comet-tail observations. This may also explain the off-and-on-again nature of the meridional gradient in the radial velocity of the solar wind as seen in radio scintillation observations. 相似文献
15.
Elena Blanter Jean-Louis Le Mouël Mikhail Shnirman Vincent Courtillot 《Solar physics》2018,293(10):134
The solar-cycle oscillations of the toroidal and poloidal components of the solar magnetic field in the northern solar hemisphere have a persistent phase difference of about \(\pi \). We propose a symmetrical Kuramoto model with three coupled oscillators as a simple way to understand this anti-synchronization. We solve an inverse problem and reconstruct natural frequencies of the top and bottom oscillators under the conditions of a constant coupling strength and a non-delayed coupling. These natural frequencies are associated with angular velocities of the meridional flow circulation near the solar surface and in the deep layer of the solar convection zone. A relationship between our reconstructions of the shallow and the deep meridional flow speed during recent Solar Cycles 21?–?23 is in agreement with estimates obtained in helioseismology and flux-transport dynamo modeling. The reconstructed top oscillator speed presents significant solar-cycle like variations that agree with recent helioseismical reconstructions. The evolution of reconstructed natural frequencies strongly depends on the coupling strength. We find two stable regimes in the case of strong coupling with a change of regime during anomalous solar cycles. We see the onset of a new transition in Solar Cycle 24. We estimate the admitted range of coupling values and find evidence of cross-equatorial coupling between solar hemispheres not accounted for by the model. 相似文献
16.
Solar radiation acceleration imparts anti-sunward velocities to sodium atoms in the Mercury exosphere. The Earthward-directed vectors of the Sun-accelerated atom velocities can be observed from Earth as small Doppler shifts, either added to, or subtracted from the Earth-Mercury Doppler shifts. We measured these small Doppler shifts using high resolution spectrographs capable of detecting sodium velocity differences as small as 0.1 km/s. We report here four sets of observations performed at different Mercury true anomaly angles. For these measurements, the spectrograph slit was oriented first east-west, and then north-south on the planet so as to get east-west and north-south transects of the velocities. The velocity patterns in east-west transects could be explained in terms of sodium flows outwards from the subsolar point, except for unexpectedly large Earthward velocities observed above the dawn terminator, which we interpreted to be the result of evaporation of sodium as the cold surface is heated by the rising Sun. North-south transects also showed a general pattern consistent with sodium flows outwards from the subsolar point. However, in all cases, the velocities were higher in one hemisphere relative to the other. For two cases, excess sodium emission was observed in the same hemisphere as the velocity excess. We interpreted these results to mean that there existed sources of sodium at high latitudes, which could appear in either hemisphere. 相似文献
17.
Rolf Brahde 《Solar physics》1972,26(2):318-334
A numerical method for correction of stray light in solar observations has been developed. In particular a regular sunspot, where the circular contours of penumbra and umbra are projected as ellipses, has been studied. When a specified set of values for the stray light parameters is given, and also tentative values for the relative intensities of penumbra and umbra, the integration of stray light can be performed in any point. The result will be the observable intensity if the conditions were as given by these initial values.By means of limb observations the stray light parameters may be improved, and finally a variation of the penumbra- and umbra intensities in the computation, enables a determination of these quantities by comparison with observations.The method is tested on observations of the transit of Mercury, May 9, 1970. Calculation of isophotes with Mercury close to the limb shows the black drop phenomenon; which thus may be explained as an effect of stray light only.It is also shown that the Wilson effect on a sunspot cannot be produced by stray light alone. 相似文献
18.
19.
We present results of a study of photospheric horizontal motions at the initial and main phases of the solar flare which happened
on September 4, 1990, near the solar limb. The flare was accompanied by matter ejection. Spectra of the flare were obtained
using the AZU-26 horizontal solar telescope at the MAO NAS (Terskol observatory). We found variations of the matter motion
velocity’s value and direction at different stages of the photosphere during the flare development. The velocity changed in
a range from −4 to 2 km/s. Comparisons of the obtained data with variations of the chromospheric radial velocities showed
that the horizontal matter motions in the photosphere and chromosphere are mostly directed toward the observer but at particular
time moments their direction changed. At two different knots, the time shift of the photospheric velocities is different.
The highest velocities were observed at the main phase of the flare. At the initial phase of the flare, in the matter ejection
region, we note a velocity increase compared with its preflare value and at the flare knots. 相似文献
20.
Observations demonstrate a nearly 22-year periodic zonal flow superimposed on general solar differential rotation (LaBonte and Howard, 1982) and some meridional motions (e.g., Tuominen, Tuominen, and Kyrolänen, 1983). Such flows can be excited by the magnetic wave generated by the dynamo in the solar convective zone.An approximate analytical solution for the zonal and meridional flows for a given magnetic wave is constructed. This approach is justified by the fact that the magnetic field is generated by differential rotation and mean helicity, and the magnetic field in the time interval under consideration does not affect much this main flow; it can, however, strongly influence the perturbations of this flow.The density gradient in the convective zone is taken into account as an essential point in the solution construction. The solution agreed well with observational features and, in particular, it gives a phase shift between the rotational (zonal) wave and solar activity. A polar branch of the rotational wave can be described as an effect created by a poleward moving dynamo wave.Secular variations in the symmetrical part of the differential rotation and in the asymmetry between the north and south hemispheres are predicted.The alternative approaches to the explanation of the origin of the observed large-scale flows are discussed. 相似文献