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1.
Approximate expressions are derived for the perturbations in solar p- and f-mode oscillation eigenfunctions, due to large-scale, meridional flows which are symmetric about the equator. The essential signature of the perturbed eigenfunctions in global helioseismic data is derived and the prospects for detecting meridional flow using global seismic techniques are discussed. 相似文献
2.
We have derived the velocities of meridional flows by measuring the latitudinal motions (or drifts) of umbrae of spot groups
classified into three categories of area: 0 – 5 μ, 5 – 10 μ, and >10 μ (μ area in millionths of the solar hemisphere). The latitudinal drifts (or the meridional flows) in all three categories are
directed equatorward in both the northern and southern hemispheres. By sorting the spot groups into three area classes, we
are able to relate the respective latitudinal drifts with the three depths in the convection zone where the footpoints of
the flux loops of the spot groups of each area class are anchored. We obtain estimates of the anchor depths through a comparison
of the rotation rates of the spot groups of each area class with the rotation-rate profiles from helioseismic inversions.
The equatorward drifts obtained provide estimates of the meridional flows at the three depths in the convection zone and thereby
suggest the presence of return meridional flows as envisaged in the flux-transport dynamo models, which have remained undetected
so far. The data sources for this study are measurements of positions and areas of umbrae of sunspots from the photographic
white-light images of the Sun of the Kodaikanal Observatory archives for the period 1906 – 1987 and a very similar, but independent,
data set from the Mt. Wilson Observatory archives for the period 1917 – 1985. 相似文献
3.
We study the North–South asymmetry of zonal and meridional components of horizontal, solar subsurface flows during the years
2001–2004, which cover the declining phase of solar cycle 23. We measure the horizontal flows from the near-surface layers
to 16 Mm depth by analyzing 44 consecutive Carrington rotations of Global Oscillation Network Group (GONG) Doppler images
with a ring-diagram analysis technique. The meridional flow and the errors of both flow components show an annual variation
related to the B
0-angle variation, while the zonal flow is less affected by the B
0-angle variation. After correcting for this effect, the meridional flow is mainly poleward but it shows a counter cell close
to the surface at high latitudes in both hemispheres. During the declining phase of the solar cycle, the meridional flow mainly
increases with time at latitudes poleward of about 20˚, while it mainly decreases at more equatorward latitudes. The temporal variation of the zonal flow in both hemispheres is
significantly correlated at latitudes less than about 20˚. The zonal flow is larger in the southern hemisphere than the northern one, and this North–South asymmetry increases with
depth. Details of the North–South asymmetry of zonal and meridional flow reflect the North–South asymmetry of the magnetic
flux. The North–South asymmetries of the flows show hints of a variation with the solar cycle. 相似文献
4.
The directions and velocities of meridional plasma motions are investigated using Doppler shifts of the magnetically non-split line Fe 557.6 nm. Possible drifts of the spectrograph were controlled by measuring nearly iodine lines from a laboratory source. The scattered light was kept low by using the Capri Coudé refractor mainly around local noon.There is a general scatter of about ±20 to ±50 m s–1 in the yearly mean results. The scatter is up to –200 m s–1 for the year 1985. The results are compared with published data. Although some systematic meridional plasma motions could be detected from the average of the meridional flows within the whole observing period 1982 until 1986, the final analysis suggests, that all meridional motions averaged over half a solar cycle are below ±10 m s–1.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.This paper has also been presented at the IAU Colloquium No. 121Inside the Sun andAstronomy and Astrophysics
229, 224 (1990). 相似文献
5.
R. Komm I. González Hernández F. Hill R. Bogart M. C. Rabello-Soares D. Haber 《Solar physics》2013,287(1-2):85-106
We have determined the meridional flows in subsurface layers for 18 Carrington rotations (CR 2097 to 2114) analyzing high-resolution Dopplergrams obtained with the Helioseismic and Magnetic Imager (HMI) instrument onboard the Solar Dynamics Observatory (SDO). We are especially interested in flows at high latitudes up to 75° in order to address the question whether the meridional flow remains poleward or reverses direction (so-called counter cells). The flows have been determined in depth from near-surface layers to about 16 Mm using the HMI ring-diagram pipeline. The measured meridional flows show systematic effects, such as a variation with the B 0-angle and a variation with central meridian distance (CMD). These variations have been taken into account to lead to more reliable flow estimates at high latitudes. The corrected average meridional flow is poleward at most depths and latitudes with a maximum amplitude of about $20~\mathrm{m\,s}^{-1}$ near 37.5° latitude. The flows are more poleward on the equatorward side of the mean latitude of magnetic activity at 22° and less poleward on the poleward side, which can be interpreted as convergent flows near the mean latitude of activity. The corrected meridional flow is poleward at all depths within ±?67.5° latitude. The corrected flow is equatorward only at 75° latitude in the southern hemisphere at depths between about 4 and 8 Mm and at 75° latitude in the northern hemisphere only when the B 0 angle is barely large enough to measure flows at this latitude. These counter cells are most likely the remains of an insufficiently corrected B 0-angle variation and not of solar origin. Flow measurements and B 0-angle corrections are difficult at the highest latitude because these flows are only determined during limited periods when the B 0 angle is sufficiently large. 相似文献
6.
We study the solar-cycle variation of subsurface flows from the surface to a depth of 16 Mm. We have used ring-diagram analysis to analyze Dopplergrams obtained with the Michelson Doppler Imager (MDI) Dynamics Program, the Global Oscillation Network Group (GONG), and the Helioseismic and Magnetic Imager (HMI) instrument. We combined the zonal and meridional flows from the three data sources and scaled the flows derived from MDI and GONG to match those from HMI observations. In this way, we derived their temporal variation in a consistent manner for Solar Cycles 23 and 24. We have corrected the measured flows for systematic effects that vary with disk positions. Using time-depth slices of the corrected subsurface flows, we derived the amplitudes and times of the extrema of the fast and slow zonal and meridional flows during Cycles 23 and 24 at every depth and latitude. We find an average difference between maximum and minimum amplitudes of \(8.6 \pm0.4~\mbox{m}\,\mbox{s}^{-1}\) for the zonal flows and \(7.9 \pm0.3~\mbox{m}\,\mbox{s}^{-1}\) for the meridional flows associated with Cycle 24 averaged over a depth range from 2 to 12 Mm. The corresponding values derived from GONG data alone are \(10.5 \pm0.3~\mbox{m}\,\mbox{s}^{-1}\) for the zonal and \(10.8 \pm0.3~\mbox{m}\,\mbox{s}^{-1}\) for the meridional flow. For Cycle 24, the flow patterns are precursors of the magnetic activity. The timing difference between the occurrence of the flow pattern and the magnetic one increases almost linearly with increasing latitude. For example, the fast zonal and meridional flow appear \(2.1 \pm 0.6\) years and \(2.5\pm 0.6\) years, respectively, before the magnetic pattern at \(30^{\circ}\) latitude in the northern hemisphere, while in the southern hemisphere, the differences are \(3.2 \pm 1.2\) years and \(2.6 \pm 0.6\) years. The flow patterns of Cycle 25 are present and have reached \(30^{\circ}\) latitude. The amplitude differences of Cycle 25 are about 22% smaller than those of Cycle 24, but are comparable to those of Cycle 23. Moreover, polynomial fits of meridional flows suggest that equatorward meridional flows (counter-cells) might exist at about \(80^{\circ}\) latitude except during the declining phase of the solar cycle. 相似文献
7.
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
8.
We study the temporal variation of the vorticity of subsurface flows of 828 active regions and 977 quiet regions. The vorticity
of these flows is derived from measured subsurface velocities. The horizontal flows are determined by analyzing high-resolution
Global Oscillation Network Group Doppler data with ring-diagram analysis covering a range of depths from the surface to about 16 Mm. The vertical velocity
component is derived from the divergence of the measured horizontal flows using mass conservation. We determine the change
in unsigned magnetic flux density during the disk passage of each active region using Michelson Doppler Imager (MDI) magnetograms binned to the ring-diagram grid with centers spaced by 7.5° ranging ± 52.5° in latitude and central meridian
distance with an effective diameter of 15° after apodization. We then sort the data by their flux change from decaying to
emerging flux and divide the data into five subsets of equal size. We find that the vorticity of subsurface flows increases
during flux emergence and decreases when active regions decay. For flux emergence, the absolute values of the zonal and meridional
vorticity components show the most coherent variation with activity, while for flux decrease the strongest signature is in
the absolute values of the meridional and vertical vorticity components. The temporal variation of the enstrophy (residual
vorticity squared) is thus a good indicator for either flux increase or decrease. There are some indications that the increase
in vorticity during flux emergence happens about a day later at depths below about 8 Mm compared to layers shallower than
about 4 Mm. This timing difference might imply that the vorticity signal analyzed here is caused by the interaction between
magnetic flux and turbulent flows near the solar surface. There are also hints that the vorticity decrease during flux decay
begins about a day earlier at layers deeper than about 8 Mm compared to shallower ones. However, the timing difference between
the change at different depths is comparable to the time step of the analysis. 相似文献
9.
Digitized synoptic charts of photospheric magnetic fields were analyzed for the past 4 incomplete solar activity cycles (1969–2000).
The zonal structure and cyclic evolution of large-scale solar magnetic fields were investigated using the calculated values
of the radial B
r, |B
r|, meridional B
θ, |B
θ|, and azimuthal B
φ, |B
φ| components of the solar magnetic field averaged over a Carrington rotation (CR). The time–latitude diagrams of all 6 parameters
and their correlation analysis clearly reveal a zonal structure and two types of the meridional poleward drift of magnetic
fields with the characteristic times of travel from the equator to the poles equal to ∼16–18 and ∼2–3 years. A conclusion
is made that we observe two different processes of reorganization of magnetic fields in the Sun that are related to generation
of magnetic fields and their subsequent redistribution in the process of emergence from the field generation region to the
solar surface. Redistribution is supposed to be caused by some external forces (presumably, by sub-surface plasma flows in
the convection zone). 相似文献
10.
We study the meridional flow of small magnetic features, using high-resolution magnetograms taken from 1978 to 1990 with the NSO Vacuum Telescope on Kitt Peak. Latitudinal motions are determined by a two-dimensional crosscorrelation analysis of 514 pairs of consecutive daily observations from which active regions are excluded. We find a meridional flow of the order of 10 m s–1, which is poleward in each hemisphere, increases in amplitude from 0 at the equator, reaches a maximum at mid-latitude, and slowly decreases poleward. The average observed meridional flow is fit adequately by an expansion of the formM () = 12.9(±0.6) sin(2) + 1.4(±0.6) sin(4), in m s–1 where is the latitude and which reaches a maximum of 13.2 m s–1 at 39°. We also find a solar-cycle dependence of the meridional flow. The flow remains poleward during the cycle, but the amplitude changes from smaller-than-average during cycle maximum to larger-than-average during cycle minimum for latitudes between about 15° and 45°. The difference in amplitude between the flows at cycle minimum and maximum depends on latitude and is about 25% of the grand average value. The change of the flow amplitude from cycle maximum to minimum occurs rapidly, in about one year, for the 15–45° latitude range. At the highest latitude range analyzed, centered at 52.5°, the flow is more poleward-than-average during minimumand maximum, and less at other times. These data show no equatorward migration of the meridional flow pattern during the solar cycle and no significant hemispheric asymmetry. Our results agree with the meridional flow and its temporal variation derived from Doppler data. They also agree on average with the meridional flow derived from the poleward migration of the weak large-scale magnetic field patterns but differ in the solar-cycle dependence. Our results, however, disagree with the meridional flow derived from sunspots or plages.Operated by the Association of Universities for Research in Astronomy, Inc. under cooperative agreement with the National Science Foundation. 相似文献
11.
We have analyzed data on sunspot groups compiled during 1874–1981 and investigated the following: (i) dependence of the `initial' meridional motion (v
ini()) of sunspot groups on the life span () of the groups in the range 2–12 days, (ii) dependence of the meridional motion (v(t)) of sunspot groups of life spans 10–12 days on the age (t) of the spot groups, and (iii) variations in the mean meridional motion of spot groups of life span 2–12 days during the solar cycle. In each of the latitude intervals 0°–10°, 10°–20° and 20°–30°, the values of both v
ini() and v(t) often differ significantly from zero. In the latitude interval 20°–30°, the forms of v
ini() and v(t) are largely systematic and mutually similar in both the north and south hemispheres. The form of v(t) suggests existence of periodic variation in the solar meridional motion with period of 4 days and amplitude 10–20 m s–1. Using the anchoring depths of magnetic structures for spot groups of different and testimated earlier, (Javaraiah and Gokhale, 1997), we suggest that the forms of v
ini() and v(t) may represent radial variation of meridional flow in the Sun's convection zone, rather than temporal variation of the flow. The meridional flows (v
e(t)) determined from the data during the last few days (i.e., age t: 10–12 days) of spot groups of life spans of 10–12 days are found to have magnitudes (10–20 m s–1) and directions (poleward) similar to the those of the surface meridional plasma flows determined from the Dopplergrams and magnetograms. The mean meridional velocity of sunspot groups living 2–12 days seems to vary during the solar cycle. The velocity is not significantly different from zero during the rising phase of the cycle and there is a suggestion of equatorward motion (a few m s–1at lower latitudes and 10 m s–1at higher latitudes) during the declining phase (last few years) of the cycle. The variation during the odd numbered cycles seems to anticorrelate with the variation during the even numbered cycles, suggesting existence of 22-year periodicity in the solar meridional flow. The amplitude of the anticorrelation seems to be depending on latitude and the cycle phase. In the latitude interval 20°–30° the `surface plasma meridional motion', v
e(t), is found to be poleward during maximum years (v
e(t) 20 m s–1at 4th year) and equatorward during ending years of the cycle (v
e(t) –17 m s–1at 10th year). 相似文献
12.
Travel times measured for the f mode have been used to study flows near the solar surface in conjunction with simultaneous measurements of the magnetic field. Previous flow measurements of Doppler surface rotation, small magnetic feature rotation, supergranular pattern rotation, and surface meridional circulation have been confirmed. In addition, the flow in supergranules due to Coriolis forces has been measured. The spatial and temporal power spectra for a six-day observing sequence have been measured. 相似文献
13.
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
14.
We have analyzed the effects that differential rotation and a hypothetical meridional flow would have on the evolution of the Sun's mean line-of-sight magnetic field as seen from Earth. By winding the large-scale field into strips of alternating positive and negative polarity, differential rotation causes the mean-field amplitude to decay and the mean-field rotation period to acquire the value corresponding to the latitude of the surviving unwound magnetic flux. For a latitudinally broad two-sector initial field such as a horizontal dipole, the decay is rapid for about 5 rotations and slow with a t
–1/2 dependence thereafter. If a poleward meridional flow is present, it will accelerate the decay by carrying the residual flux to high latitudes where the line-of-sight components are small. The resulting decay is exponential with an e-folding time of 0.75 yr (10 rotations) for an assumed 15 m s–1 peak meridional flow speed.E.O. Hulburt Center for Space Research.Laboratory for Computational Physics. 相似文献
15.
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
16.
By introducing an asymmetry between the two hemispheres, we study whether the solar dynamo solutions in the two hemispheres
remain coupled with each other. Our calculations are based on the solar dynamo code SURYA, which incorporates the helioseismically-determined
solar-rotation profile, a Babcock–Leighton α effect concentrated near the surface, and a meridional circulation. When the magnetic coupling between the hemispheres is
enhanced by either increasing the diffusion or introducing an α effect distributed throughout the convection zone, we find that the solutions in the two hemispheres evolve together with
a single period even when we make the meridional circulation or the α effect different in the two hemispheres. On the other hand, when the hemispheric coupling is weaker for other values of parameters,
an asymmetry between the hemispheres can make solutions in the two hemispheres evolve independently with different periods. 相似文献
17.
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
18.
The standard methods for determining the meridional flow on the Sun from the motions of tracers are shown to give an error related to the latitudinal nonuniformity of the tracer distribution. We suggest a simple method for eliminating this error. Using this method to determine the meridional circulation from the motions of sunspots brings the result into agreement with helioseismological data on the meridional flow. The discussed effects can be important for observations of meridional flows on stars. 相似文献
19.
The behavioural features of the IMF Bz component for different solar wind velocity regimes have been studied. The study revealed a significant difference in variations of the Bz component between high-speed and low-speed regimes. Formation mechanisms for the IMF meridional component as well as the relationship of Bz with dynamical properties of the large-scale magnetic fields on the Sun are discussed. 相似文献
20.
The influence of the basic rotation on anisotropic and inhomogeneous turbulence is discussed in the context of differential rotation theory. An improved representation for the original turbulence leads to a Λ‐effect which complies with the results of 3D numerical simulations. The resulting rotation law and meridional flow agree well with both the surface observations (∂Ω/∂r < 0 and meridional flow towards the poles) and with the findings of helioseismology. The computed equatorward flow at the bottom of convection zone has an amplitude of about 10 m/s and may be significant for the solar dynamo. The depth of the meridional flow penetration into the radiative zone is proportional to ν0.5core, where νcore is the viscosity beneath the convection zone. The penetration is very small if the tachocline is laminar. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献