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1.
Results of a detailed study on supergranule lifetime and velocity fields are presented. We show the correlation between the observed downdraft velocity and the network magnetic flux elements on the quiet sun. After excluding areas with magnetic flux density 25 G, we find that the upper limit of the supergranule vertical speed is 0.1 km s–1 for both downdraft and updraft, and the r.m.s. speed is 0.03 km s–1. By observing the evolution of individual supergranules, we find that the average lifetime of supergranules might be 50 hours. We describe different ways of formation and decay of supergranular cells. New cells usually form in an area containing no pre-existing supergranule velocity fields. Cells may disappear in two ways: fragmentation and fading away. 相似文献
2.
We have measured the proper motion of magnetic elements on the quiet Sun by means of local correlation tracking. The existence of a pattern in the intranetwork (IN) flow is confirmed. This velocity field is consistent with the direct Doppler measurement of the horizontal component of the supergranular velocity field. The IN elements generally move toward the network boundaries. By tracking test points we confirm that the magnetic elements converge in areas corresponding to the magnetic network. But because the IN elements are of random polarity, they cannot contribute to the growth or maintenance of the magnetic network.By calculating the cross correlation between the magnetogram and Dopplergram, we confirm that the supergranule boundaries and the magnetic network are roughly correlated. 相似文献
3.
Several series of coordinated observations on decaying active regions and enhanced magnetic network regions have been carried out jointly at Big Bear Solar Observatory (BBSO) and Huairou Solar Observing Station of the Bejing Astronomical Observatory in China. The evolution of magnetic fields in several regions was followed closely for 3 to 7 days. The transport of magnetic flux from the remnants of decayed active regions was studied. Three related topics are included in this paper. (1) We studied the evolution and lifetime of the magnetic network which defines the boundaries of supergranules. The results are consistent with our earlier studies: network cells have an average lifetime of about 70 hours; 68% of new cells appeared by growing from a single network magnetic element; 50% of decaying cells disappeared by contracting to a network element. (2) We studied the magnetic flux transport in an enhanced network region in detail, and found the diffusion rate to be negative, i.e., there was more flux moving towards the decayed active region than away from it. We found several other cases where the magnetic diffusion rate does not agree with Leighton's model. The slow diffusion rate is likely due to the fact that the average velocity of larger magnetic elements, which carry most of the magnetic flux, is less than 0.1 km s–1; their average lifetime is longer than 100 hours. (3) We briefly described some properties of Moving Magnetic Features (MMFs) around a sunspot (detailed discussion on MMFs will be presented in a separate paper). In this particular case, the MMFs did not carry net flux away from the central spot. Instead, the polarities of MMFs were essentially mixed so that outflowing positive and negative fluxes were roughly balanced. During the 3-day period, there was almost no net flux accumulation to form a moat. The cancellation of MMFs of opposite polarities at the boundary of the super-penumbra caused quite a few surges and H brightenings. 相似文献
4.
We have measured the motion of facular points and granules in the same region near a decaying sunspot. It is found that both features move away across the moat surrounding the sunspot. The mean speed of facular points is larger than that of granules: 0.65 km s–1 and 0.4 km s–1, respectively. These results are consistent with previous measurements of the speed of bright network features and moving magnetic fields, as well as of non-magnetic photospherical material. They support models in which a decaying sunspot is at the center of a supergranule, whose horizontal motions sweep out granules and magnetic flux tubes associated to the facular points. It is also found that granules are dragged by supergranular motions away of the moat.Contributions from the Kwasan and Hida Observatories, University of Kyoto.A part of this work was done while one of the authors (R.M.) was staying at the Kwasan and Hida Observatories, University of Kyoto, Japan, as a JSPS research fellow. 相似文献
5.
We analyzed two sequences of quiet-Sun magnetograms obtained on June 4, 1992 and July 28, 1994. Both were observed during excellent seeing conditions such that the weak intranetwork (IN) fields are observed clearly during the entire periods. Using the local correlation tracking technique, we derived the horizontal velocity fields of IN and network magnetic fields. They consist of two components: (1) radial divergence flows which move IN fields from the network interior to the boundaries, and (2) lateral flows which move along the network boundaries and converge toward stronger magnetic elements. Furthermore, we constructed divergence maps based on horizonal velocities, which are a good representation of the vertical velocities of supergranules. For the June 4, 1992 data, the enhanced network area in the field of view has twice the flux density, 10% higher supergranular velocity and 20% larger cell sizes than the quiet, unenhanced network area. Based on the number densities and flow velocities of IN fields derived in this paper and a previous paper (Wang et al., 1995), we estimate that the lower limit of total energy released from the recycling of IN fields is 1.2 × 1028 erg s–1, which is comparable to the energy required for coronal heating. 相似文献
6.
Supergranule revolution rate and lifetime can be measured by cross-correlating pairs of Doppler-velocity maps that have been filtered (by Hathaway's method) to remove other flows. As a conceptual framework for that analysis, this exploratory paper develops an idealized, phenomenological model of supergranule flows. Assumptions made about supergranule cells on the Sun's photosphere include: random location in space and time, and horizontal flows with circular symmetry and having a Simon–Weiss velocity function. Each supergranule is stable for a time, dies, and after a while, a daughter is born at a nearby position determined by a random walk. The effect on the cross-correlations of changing projection onto the line-of-sight as the Sun rotates is analyzed. The total cross-correlation for strips of constant latitude depends on two generic, slowly-varying projection functions. Effects of differential rotation and time-evolution are also considered. GONG observations of June 1994 show systematic variations in the width and shape of correlation peaks with latitude; our model suggests that projection effects alone can account for these without invoking any intrinsic variations of the supergranules. 相似文献
7.
R. N. Singh 《Astrophysics and Space Science》1992,191(1):125-130
The acoustic waves generated in the solar atmosphere propagate globally as well as upwards. These waves interact with the solar magnetic field structures and are ducted upwards. The velocity of these modified acoustic waves is shown to vary in a modelled solar atmosphere. The solar plasma propagating upwards with these waves are likely to alter the observed features of spicules, granules, and supergranules during changing phases of sunspot regions. 相似文献
8.
Haimin Wang 《Solar physics》1988,116(1):1-16
To obtain quantitative temporal and spatial information on the network magnetic fields, we applied auto- and cross-correlation techniques to the Big Bear videomagnetogram (VMG) data. The average size of the network magnetic elements derived from the auto-correlation curve is about 5700 km. The distance between the primary and secondary peak in the auto-correlation curve is about 17000 km, which is half of the size of the supergranule as determined from the velocity map. The correlation time is about 10 to 20 hours. The diffusion constant derived from the cross-correlation curve is 150 km2 s-1. We also found that in the quiet regions the total magnetic flux in a window 3 × 4 changes very little in nearly 10 hours. That means the emergence and the disappearance of magnetic flux are in balance. The cancelling features and the emergence of ephemeral regions are the major sources for the loss and replenishment of magnetic flux on the quiet Sun. 相似文献
9.
Alfred Clark Jr. 《Solar physics》1968,4(4):386-400
Some theoretical models are given which illustrate the structure of chromospheric magnetic fields associated with supergranulation. It is found that the chromospheric fields depend critically on whether or not there are large-scale vertical motions at the level where the horizontal supergranule motions are observed. In the absence of such motions, the concentration of field produced in the photosphere does not persist more than a few scale heights into the chromosphere; however, the chromospheric mass density is increased above the supergranule boundaries in this case. Completely different results-such as a chromospheric potential field-may be obtained by the inclusion of vertical motions. It is concluded that a rather wide range of chromospheric-field structures is consistent with present observational knowledge of the supergranulation. 相似文献
10.
Transverse and longitudinal magnetic field scans together with K232 spectroheliograms that cover the early phases of active region formation reveal the following:
- The new active region forms near the periphery of an old magnetic region. There is evidence that the new region forms an interrelated system with the old magnetic structures on the sun.
- Noticeable changes in the background magnetic field are seen nearly 3 days prior to the appearance of the sunspot. Magnetic hills of the longitudinal component appear along with bright localized K232 emission. Subsequently the K232 emission spreads along the boundary of one or two adjacent supergranules and at the time of sunspot formation occupies the whole supergranular cell.
- Transverse fields with strengths of 100–150 gauss form closed regions in the area of the longitudinal component hills, in the very early phases of the region. These fields stretch and link up the two areas later, at which time the peak transverse fields with values near 250 gauss coincide with the zero line of the longitudinal field. When subsequently the spots appear in the new region, the transverse fields are located about the hills of the longitudinal field. The total field vectors just prior to sunspot formation are pressed to the surface. These are inclined about 45° to the surface after the spot appears. The findings indicate that the magnetic field of a new region emerges from the sub-photospheric layers. It is highly likely that the dynamics of a supergranule influences only the emergence of the magnetic field into the upper layers of the solar atmosphere.
11.
Center-limb observations of line-center intensity and velocity fluctuations in the Magnesium b lines are described. Autocorrelation and power spectral analyses indicate small scale brightness structures having periodicities of 3000 km and 8000 km and large scale structures of 22000 km. Corresponding velocity structures are 6000 km and 30000 km.The relative rms fluctuation amplitude for the small scale bright features is found to be of order 12% and for the large scale features 8%. The variation of these rms values with heliocentric angle is also shown.At disk center some weak correlation is found between bright features and downward velocities in the large scale structures. Towards the limb there is a strong correlation in all three lines between line of sight motions and bright features. This indicates that the large scale bright features are closely associated with the supergranule motions.By inspecting the actual brightness and velocity fluctuation tracings it can be seen that, in some regions, the small scale structures show a significant negative correlation over a range of about 25000 km. Beyond this characteristic length, however, the correlation may decrease abruptly or even become positive for a similar distance. There is some evidence which suggests that this behaviour may also be related to the supergranule motions. 相似文献
12.
To study the evolution of large convective cells known as supergranules, a solar telescope was set up at Maitri, Indian permanent station in Antarctica region, during the local summer months (December 1989 through March 1990). A continuous sequence of calcium K-line filtergrams for 106 hours spaced at intervals of about 10 min was obtained. The analysis of the data indicates that the most probable lifetime of the calcium-K network is about 22 hours. The lifetime depends upon the size of the cell and is larger for bigger cells. The data also show that cells (of a given size) associated with remnant magnetic field regions live longer than those in the field-free region. This may mean that the magnetic field plays an important role in the confinement of these structures. 相似文献
13.
K-line observations of enhanced network taken with the NASA/SPO Multichannel Spectrometer on 28 September 1975 in support of OSO-8 are discussed. The data show a correlation between core brightness and asymmetry for spatial scans which cross enhanced network boundaries. The implications of this result concerning mass flow in and near supergranule boundaries are discussed. 相似文献
14.
Brynildsen N. Maltby P. Brekke P. Haugan S.V.H. Kjeldseth-Moe O. 《Solar physics》1999,186(1-2):141-191
We present results from a study of the spatial distributions of line emission and relative line-of-sight velocity in the atmosphere above 17 sunspot regions, from the chromosphere, through the transition region and into the corona, based on simultaneous observations of ten EUV emission lines with the Coronal Diagnostic Spectrometer – CDS on SOHO. We find that the spatial distributions are nonuniform over the sunspot region and introduce the notation 'sunspot loop' to describe an enhanced transition region emission feature that looks like a magnetic loop, extending from inside the sunspot to the surrounding regions. We find little evidence for the siphon flow. Attention is given to the time variations since we observe both a rapid variation with a characteristic time of a few to several minutes and a slow variation with a time constant of several hours to 1 day. The most prominent features in the transition region intensity maps are the sunspot plumes. We introduce an updated criterion for the presence of plumes and find that 15 out of 17 sunspots contain a plume in the temperature range logT5.2–5.6. The relative line-of-sight velocity in sunspot plumes is high and directed into the Sun in the transition region. Almost all the sunspot regions contain one or a few prominent, strongly redshifted velocity channels, several of the channels extend from the sunspot plume to considerable distances from the sunspot. The flow appears to be maintained by plasmas at transition region temperatures, moving from regions located at a greater height outside the sunspots and towards the sunspot. The spatial correlation is high to moderate between emission lines formed in the transition region lines, but low between the transition region lines and the coronal lines. From detailed comparisons of intensity and velocity maps we find transition region emission features without any sign of coronal emission in the vicinity. A possible explanation is that the emission originates in magnetic flux tubes that are too cold to emit coronal emission. The comparisons suggest that gas at transition region temperature occur in loops different from loops with coronal temperature. However, we cannot exclude the presence of transition region temperatures close to the footpoints of flux tubes emitting at coronal temperatures. Regions with enhanced transition region line emission tend to be redshifted, but the correlation between line emission and relative line-of-sight velocity is weak. We extend our conditional probability studies and confirm that there is a tendency for line profiles with large intensities and red shifts (blue shifts) above the average to constitute an increasing (decreasing) fraction of the profiles as the wavelength shift increases. 相似文献
15.
Observational aspects of the previously found quasi-hourly oscillations of magnetic fragments in sunspot polar coordinates are investigated. The orientation of the oscillations is shown to be azimuthally anisotropic, with their amplitude reaching a maximum in penumbra at a distance of ～0.8 sunspot radius (the maximum amplitude is estimated to be 3700 km). Based on the detected deviations of the oscillations from the radial direction, we numerically simulate the horizontal configuration of field lines in the region of the major spots in bipolar groups. 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.
The evolution of the velocity and magnetic fields associated with supergranulation has been investigated using the Sacramento Peak Observatory Diode Array Magnetograph. The observations consist of time sequences of simultaneous velocity, magnetic field, and chromospheric network measurements. From these data it appears that the supergranular velocity cells may have lifetimes in excess of the accepted value of 24 hours. Magnetic field motions associated with supergranulation were infrequent and seem to be accompanied by changes in the velocity field. More prevalent were the slow dissipation and diffusion of stationary flux points. Vertical velocity fields of 200 m s–1 appear to be confined to downflows in magnetic field regions at supergranular boundaries. These downflows are only observed using certain absorption lines. Corresponding upflows in the center of supergranules of less than 50 m s–1 may be present but cannot be confirmed. 相似文献
18.
Supergranulation is a component of solar convection that manifests itself on the photosphere as a cellular network of around
35 Mm across, with a turnover lifetime of 1 – 2 days. It is strongly linked to the structure of the magnetic field. The horizontal,
divergent flows within supergranule cells carry local field lines to the cell boundaries, while the rotational properties
of supergranule upflows may contribute to the restoration of the poloidal field as part of the dynamo mechanism, which controls
the solar cycle. The solar minimum at the transition from cycle 23 to 24 was notable for its low level of activity and its
extended length. It is of interest to study whether the convective phenomena that influence the solar magnetic field during
this time differed in character from periods of previous minima. This study investigates three characteristics (velocity components,
sizes and lifetimes) of solar supergranulation. Comparisons of these characteristics are made between the minima of cycles
22/23 and 23/24 using MDI Doppler data from 1996 and 2008, respectively. It is found that whereas the lifetimes are equal
during both epochs (around 18 h), the sizes are larger in 1996 (35.9 ± 0.3 Mm) than in 2008 (35.0 ± 0.3 Mm), while the dominant
horizontal velocity flows are weaker (139 ± 1 m s−1 in 1996; 141 ± 1 m s−1 in 2008). Although numerical differences are seen, they are not conclusive proof of the most recent minimum being inherently
unusual. 相似文献
19.
P. R. Wilson 《Solar physics》1973,32(2):435-439
The coordinates of the cooling cycle described in Paper I are re-defined in order to provide an account in which the part played by the cycle in cooling the sunspot is separated from the role of the supergranule cells in transporting energy away from it.More recent observations of velocity fields and magnetic outflow near sunspots are discussed. A model suggested by Harvey and Harvey to explain the observed magnetic flux transported across the moat region is refined and extended using the cooling cycle model. 相似文献
20.
We compare horizontal flow fields in the photosphere and in the subphotosphere (a layer 0.5 Mm below the photosphere) in two solar active regions: AR?11084 and AR?11158. AR?11084 is a mature, simple active region without significant flaring activity, and AR?11158 is a multipolar, complex active region with magnetic flux emerging during the period studied. Flows in the photosphere are derived by applying the Differential Affine Velocity Estimator for Vector Magnetograms (DAVE4VM) on HMI-observed vector magnetic fields, and the subphotospheric flows are inferred by time–distance helioseismology using HMI-observed Dopplergrams. Similar flow patterns are found for both layers for AR?11084: inward flows in the sunspot umbra and outward flows surrounding the sunspot. The boundary between the inward and outward flows, which is slightly different in the photosphere and the subphotosphere, is within the sunspot penumbra. The area having inward flows in the subphotosphere is larger than that in the photosphere. For AR?11158, flows in these two layers show great similarities in some areas and significant differences in other areas. Both layers exhibit consistent outward flows in the areas surrounding sunspots. On the other hand, most well-documented flux-emergence-related flow features seen in the photosphere do not have counterparts in the subphotosphere. This implies that the horizontal flows caused by flux emergence do not extend deeply into the subsurface. 相似文献