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1.
We present independent observations of the solar-cycle variation of flows near the solar surface and at a depth of about 60 Mm, in the latitude range ±?45°. We show that the time-varying components of the meridional flow at these two depths have opposite sign, whereas the time-varying components of the zonal flow are in phase. This is in agreement with previous results. We then investigate whether the observations are consistent with a theoretical model of solar-cycle-dependent meridional circulation based on a flux-transport dynamo combined with a geostrophic flow caused by increased radiative loss in the active region belt (the only existing quantitative model). We find that the model and the data are in qualitative agreement, although the amplitude of the solar-cycle variation of the meridional flow at 60 Mm is underestimated by the model. 相似文献
2.
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
3.
4.
The origin of the solar wind is a long-standing issue in both observational and theoretical studies. To understand how and
where in the solar atmosphere the mass and energy of the solar wind are supplied is very important. Previous observation suggests
a scenario in which the fast solar wind originates at heights above 5 Mm in the magnetically open funnel, a process that is
accompanied by downward flow below 5 Mm, whereby the mass and energy are supplied through reconnection between the open funnel
and adjacent closed loops. Based on this scenario, we develop a fluid model to study the solar wind generation under the assumption
that mass and energy are deposited in the open funnel at 5 Mm. The mass supply rate is estimated from the mass loss rate as
given by the emptying of the side loops as a result of their assumed reconnection with the open funnel. Similarly, the energy
input rate is consistent with the energy release rate as estimated from the energy flux associated with the reconnection between
the open magnetic funnel and the closed magnetic loops. Following the observations, we not only simulate the plasma flowing
upward to form the solar wind but also calculate the downward flow back to the lower atmosphere. This model is a first attempt
to study physically the proposed scenario of solar wind origin and gives a new physical illustration of the possible initial
deposition and consequent transportation of mass and energy in the coronal funnel. 相似文献
5.
We study the temporal variation of subsurface flows of 828 active regions and 977 quiet regions. The horizontal flows cover
a range of depths from the surface to about 16 Mm and are determined by analyzing Global Oscillation Network Group high-resolution Doppler data with ring-diagram analyses. The vertical velocity component is derived from the divergence of
the measured horizontal flows using mass conservation. For comparison, we analyze Michelson Doppler Imager (MDI) Dynamics Run data covering 68 active regions common to both data sets. We determine the change in unsigned magnetic
flux during the disk passage of each active region using MDI magnetograms binned to the ring-diagram grid. 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
emerging flux has a faster rotation than the ambient fluid and pushes it up, as indicated by enhanced vertical velocity and
faster-than-average zonal flow. After active regions are formed, downflows are established within two days of emergence in
shallow layers between about 4 and 10 Mm. Emerging flux in existing active regions shows a similar scenario, where the upflows
at depths greater than about 10 Mm are enhanced and the already established downflows at shallower depths are weakened. When
active regions decay, the corresponding flow pattern disappears as well; the zonal flow slows down to values comparable to
that of quiet regions and the upflows become weaker at deeper layers. The residual meridional velocity is mainly poleward
and shows no obvious variation. The magnitude of the residual velocity, defined as the sum of the squares of the residual
velocity components, increases with increasing magnetic flux and decreases with decreasing flux. 相似文献
6.
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
7.
We study the solar-cycle variation of subsurface flows from the surface to a depth of 16 Mm. We have analyzed Global Oscillation Network Group (GONG) Dopplergrams with a ring-diagram analysis covering about 15 years and Helioseismic and Magnetic Imager (HMI) Dopplergrams covering more than 6 years. After subtracting the average rotation rate and meridional flow, we have calculated the divergence of the horizontal residual flows from the maximum of Solar Cycle 23 through the declining phase of Cycle 24. The subsurface flows are mainly divergent at quiet regions and convergent at locations of high magnetic activity. The relationship is essentially linear between divergence and magnetic activity at all activity levels at depths shallower than about 10 Mm. At greater depths, the relationship changes sign at locations of high activity; the flows are increasingly divergent at locations with a magnetic activity index (MAI) greater than about 24 G. The flows are more convergent by about a factor of two during the rising phase of Cycle 24 than during the declining phase of Cycle 23 at locations of medium and high activity (about 10 to 40 G MAI) from the surface to at least 10 Mm. The subsurface divergence pattern of Solar Cycle 24 first appears during the declining phase of Cycle 23 and is present during the extended minimum. It appears several years before the magnetic pattern of the new cycle is noticeable in synoptic maps. Using linear regression, we estimate the amount of magnetic activity that would be required to generate the precursor pattern and find that it should be almost twice the amount of activity that is observed. 相似文献
8.
We present meridional flow measurements of the Sun using a novel helioseismic approach for analyzing SOHO/MDI data in order to push the current limits in radial depth. Analyzing three consecutive months of data during solar minimum, we find that the meridional flow is as expected poleward in the upper convection zone, turns equatorward at a depth of around 40 Mm (∼ 0.95 R⊙), and possibly changes direction again in the lower convection zone. This may indicate two meridional circulation cells in each hemisphere, one beneath the other. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
9.
Recently, Duvall and Hanasoge (Solar Phys. 287, 71, 2013) found that large-distance separation [Δ] travel-time differences from a center to an annulus [δt oi] implied a model of the average supergranular cell that has a peak upflow of 240 m?s?1 at a depth of 2.3 Mm and a corresponding peak outward horizontal flow of 700 m?s?1 at a depth of 1.6 Mm. In the present work, this effect is further studied by measuring and modeling center-to-quadrant travel-time differences [δt qu], which roughly agree with this model. Simulations are analyzed that show that such a model flow would lead to the expected travel-time differences. As a check for possible systematic errors, the center-to-annulus travel-time differences [δt oi] are found not to vary with heliocentric angle. A consistency check finds an increase of δt oi with the temporal frequency [ν] by a factor of two, which is not predicted by the ray theory. 相似文献
10.
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
11.
To recover the flow information encoded in travel-time data of time?–?distance helioseismology, accurate forward modeling and a robust inversion of the travel times are required. We accomplish this using three-dimensional finite-frequency travel-time sensitivity kernels for flows along with a (2+1)-dimensional (2+1D) optimally localized averaging (OLA) inversion scheme. Travel times are measured by ridge filtering MDI full-disk Doppler data and the corresponding Born sensitivity kernels are computed for these particular travel times. We also utilize the full noise-covariance properties of the travel times, which allow us to accurately estimate the errors for all inversions. The whole procedure is thus fully consistent. Because of ridge filtering, the kernel functions separate in the horizontal and vertical directions, motivating our choice of a 2+1D inversion implementation. The inversion procedure also minimizes cross-talk effects among the three flow components, and the averaging kernels resulting from the inversion show very small amounts of cross-talk. We obtain three-dimensional maps of vector solar flows in the quiet Sun at horizontal spatial resolutions of 7?10 Mm using generally 24 hours of data. For all of the flow maps we provide averaging kernels and the noise estimates. We present examples to test the inferred flows, such as a comparison with Doppler data, in which we find a correlation of 0.9. We also present results for quiet-Sun supergranular flows at different depths in the upper convection zone. Our estimation of the vertical velocity shows good qualitative agreement with the horizontal vector flows. We also show vertical flows measured solely from f-mode travel times. In addition, we demonstrate how to directly invert for the horizontal divergence and flow vorticity. Finally we study inferred flow-map correlations at different depths and find a rapid decrease in this correlation with depth, consistent with other recent local helioseismic analyses. 相似文献
12.
We employ ring-diagram analysis to study the sub-surface thermal structure of active regions. We present results using a large number of active regions over the course of Solar Cycle 23. We present both traditional inversions of ring-diagram frequency differences, with a total sample size of 264, and a statistical study using Principal Component Analysis. We confirm earlier results on smaller samples that sound speed and adiabatic index are changed below regions of strong magnetic field. We find that sound speed is decreased in the region between approximately r=0.99?R ⊙ and r=0.995?R ⊙ (depths of 3 Mm to 7 Mm) and increased in the region between r=0.97?R ⊙ and r=0.985?R ⊙ (depths of 11 Mm to 21 Mm). The adiabatic index [Γ1] is enhanced in the same deeper layers where sound-speed enhancement is seen. A weak decrease in adiabatic index is seen in the shallower layers in many active regions. We find that the magnitudes of these perturbations depend on the strength of the surface magnetic field, but we find a great deal of scatter in this relation, implying that other factors may be relevant. 相似文献
13.
L. V. Kozak R. I. Kostyk O. K. Cheremnykh 《Kinematics and Physics of Celestial Bodies》2013,29(2):66-70
Observations obtained at the 70-cm vacuum tower telescope (VTT) at Izaña (Tenerife, Spain) are analyzed to show that turbulent processes in the solar photosphere have two distinct spectra of turbulence. The first is the well-known Kolmogorov spectrum, which describes plasmas with a zero mean magnetic field, and the second is the Kraichnan spectrum with a nonzero mean magnetic field. The transition from one spectrum type to another is found to occur at a scale of 3 Mm. This scale is consistent with the typical size of mesogranular structures, which indicates a transition to large-scale self-organizing magnetic structures. 相似文献
14.
Via the potential field extrapolation of the observed photospheric magnetic field, the structure of the photospheric magnetic fields above solar quiet regions is renewed. As revealed by the result, below 20 Mm the open magnetic lines exhibit many obvious small funnel structures. These funnels expand with height and at the height of about 20 Mm they combine into large funnel structures. By a systematic study of the tendency of change of the cross section areas of funnels, it is discovered that the cross section areas of funnels in solar quiet regions expand approximately linearly. The velocity of expansion of magnetic funnels at rather low altitudes (< 20 Mm) is larger than that at high altitudes (> 20 Mm). This phenomenon has important significance for the two-dimensional numerical simulations of the origin of solar wind and the mass flow in magnetic loops. At the same time it is found that the number of closed magnetic lines decreases in the form of exponential function. 相似文献
15.
We study the solar-cycle variation of the zonal flow in the near-surface layers of the solar convection zone from the surface to a depth of 16 Mm covering the period from mid-2001 to mid-2013 or from the maximum of Cycle 23 through the rising phase of Cycle 24. We have analyzed Global Oscillation Network Group (GONG) and Helioseismic and Magnetic Imager (HMI) Dopplergrams with a ring-diagram analysis. The zonal flow varies with the solar cycle showing bands of faster-than-average flows equatorward of the mean latitude of activity and slower-than-average flows on the poleward side. The fast band of the zonal flow and the magnetic activity appear first in the northern hemisphere during the beginning of Cycle 24. The bands of fast zonal flow appear at mid-latitudes about three years in the southern and four years in the northern hemisphere before magnetic activity of Cycle 24 is present. This implies that the flow pattern is a direct precursor of magnetic activity. The solar-cycle variation of the zonal flow also has a poleward branch, which is visible as bands of faster-than-average zonal flow near 50° latitude. This band appears first in the southern hemisphere during the rising phase of the Cycle 24 and migrates slowly poleward. These results are in good agreement with corresponding results from global helioseismology. 相似文献
16.
Johann Hirzberger Laurent Gizon Sami K. Solanki Thomas L. Duvall Jr. 《Solar physics》2008,251(1-2):417-437
Supergranulation is visible at the solar surface as a cellular pattern of horizontal outflows. Although it does not show a distinct intensity pattern, it manifests itself indirectly in, for example, the chromospheric network. Previous studies have reported significant differences in the inferred basic parameters of the supergranulation phenomenon. Here we study the structure and temporal evolution of a large sample of supergranules, measured by using local helioseismology and SOHO/MDI data from the year 2000 at solar activity minimum. Local helioseismology with f modes provides maps of the horizontal divergence of the flow velocity at a depth of about 1 Mm. From these divergence maps supergranular cells were identified by using Fourier segmentation procedures in two dimensions and in three dimensions (two spatial dimensions plus time). The maps that we analyzed contain more than 105 supergranular cells and more than 103 lifetime histories, which makes possible a detailed analysis with high statistical significance. We find that the supergranular cells have a mean diameter of 27.1 Mm. The mean lifetime is estimated to be 1.6 days from the measured distribution of lifetimes (three-dimensional segmentation), with a clear tendency for larger cells to live longer than smaller ones. The pair and mark correlation functions do not show pronounced features on scales larger than the typical cell size, which suggests purely random cell positions. The temporal histories of supergranular cells indicate a smooth evolution from their emergence and growth in the first half of their lives to their decay in the second half of their lives (unlike exploding granules, which reach their maximum size just before they fragment). 相似文献
17.
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
18.
The quiet-Sun photosphere consists of numerous magnetic flux fragments of both polarities that evolve with granular and supergranular flow fields. These concentrations give rise to a web of intermingled magnetic flux tubes which characterise the coronal magnetic field. Here, the nature of these flux tubes is studied. The photosphere is taken to be the source plane and each photospheric fragment is represented by a series of point sources. By analysing the potential field produced by these sources, it is found that the distribution of flux tube lengths obtained by (i) integrating forward from positive sources and (ii) tracing back from negative sources is highly dependent on the total flux imbalance within the region of interest. It is established that the relation between the footpoint separation of a flux tube and its height cannot be assumed to be linear. Where there is a significant imbalance of flux within a region, it is found that fragments of the dominant polarity will have noticeably more connections, on average, than the minority polarity fragments. Despite this difference, the flux from a single fragment of either polarity is typically divided such that (i) 60–70% connects to one opposite-polarity fragment, (ii) 25–30% goes to a further 1 to 2 opposite-polarity fragments, and (iii) any remaining flux may connect to as many as another 50 or more other opposite-polarity fragments. This is true regardless of any flux imbalance within the region. It is found that fragments connect preferentially to their nearest neighbours, with, on average, around 60–70% of flux closing down within 10 Mm of a typical fragment. Only 50% of the flux in a quiet region extends higher than 2.5 Mm above the solar surface and 5–10% extends higher than 25 Mm. The fragments that contribute to the field above this height cover a range of sizes, with even the smallest of fragments contributing to the field at heights of over 50 Mm. 相似文献
19.
H. J. Muthsam B. Löw-Baselli Chr. Obertscheider M. Langer P. Lenz F. Kupka 《Monthly notices of the Royal Astronomical Society》2007,380(4):1335-1340
Using advanced numerical schemes and grid refinement, we present 2D high-resolution models of solar granulation with particular emphasis on downflowing plumes. In the high-resolution portion of our simulation, a box measuring 1.97 × 2.58 Mm2 (vertical × horizontal), the grid size is 1.82 × 2.84 km2 . Calculations at the resolution usually applied in this type of simulations amount to only a few horizontal gridpoints for a downflowing plume. Due to the increased number of gridpoints in our high-resolution domain, the simulations show the development of vigorous secondary instabilities of both the plume's head and stem. The plume's head produces counterrotating vortex patches, a topology due to the 2D nature of the simulations. Below a depth of about 1 Mm, the plume's head and stem instabilities produce, in these 2D models, patches of low density, temperature, pressure and high vorticity which may last for all of our simulation time, ∼10 min, and probably considerably longer. Centrifugal forces acting in these patches counteract the strong inward pressure. Probably most importantly, the plume's instabilities give rise to acoustic pulses created predominantly down to ∼1.5 Mm. The pulses proceed laterally as well as upwards and are ubiquitous. Ultimately, most of them emerge into the photosphere. A considerable part of the photospheric 'turbulence' in these models is due to those pulses rather than to some sort of eddies. The upflows in granules are smooth where they reach the photosphere from below even in the present calculations; however, the pulses may enter in the photosphere also in granular upflows. 相似文献
20.
We study the temporal variation of subsurface flows of 788 active regions and 978 quiet regions. The vertical-velocity component
used in this study is derived from the divergence of the measured horizontal flows using mass conservation. The horizontal
flows cover a range of depths from the surface to about 16 Mm and are determined by analyzing about five years of GONG high-resolution
Doppler data with ring-diagram analysis. We determine the change in unsigned magnetic flux during the disk passage of each
active region using MDI magnetograms binned to the ring-diagram grid. We then sort the data by their flux change from decaying
to emerging flux and divide the data into five subsets of equal size. The average vertical flows of the emerging-flux subset
are systematically shifted toward upflows compared to the grand average values of the complete data set, whereas the average
flows of the decaying-flux subset show comparably more pronounced downflows especially near 8 Mm. For flux emergence, upflows
become stronger with time with increasing flux at depths greater than about 10 Mm. At layers shallower than about 4 Mm, the
flows might start to change from downflows to upflows, when flux emerges, and then back to downflows after the active regions
are established. The flows in the layers between these two depth ranges show no response to the emerging flux. In the case
of decaying flux, the flows change from strong upflows to downflows at depths greater than about 10 Mm, whereas the flows
do not change systematically at other depths. A cross-correlation analysis shows that the flows in the near-surface and the
deeper layers might change about one day before flux emerges. The flows associated with the quiet regions fluctuate with time
but do not show any systematic variation. 相似文献