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1.
We investigate the regime of chromospheric oscillations at the bases of coronal holes and compare them with the oscillations in the quiet chromosphere outside coronal holes using time series of spectrograms taken at different times in eight quiet regions on the Sun. As the oscillation parameter being studied, we have chosen the central intensity of the chromospheric Ca II K and H and 849.8-nm lines. The intensity measurements at all spatial points (along the spectrograph slit) have been subjected to a standard Fourier analysis. For the identified areas of the networks, cells, and network boundaries, we have calculated the integrated oscillation powers in several frequency bands. For all frequency bands, the powers of the intensity oscillations at the formation level of the Ca II resonance doublet line cores have been found to be enhanced at the bases of coronal holes approximately by a factor of 1.5. For the “three-minute” band, this enhancement is more pronounced in the network than in the cell, while the opposite is true for the “five-minute” band. The power in the five-minute band is higher than that in the three-minute one both at the bases of coronal holes and outside them, but this ratio in the network for a coronal hole is higher (1.40 ± 0.25 and 1.30 ± 0.10). We interpret this fact and the fact that the power of the three-minute oscillations for nonmagnetic regions changes with height differently at the base of a coronal hole and outside it as an increase in the importance of magnetoacoustic portals at the chromospheric base of the coronal hole.  相似文献   

2.
We study the Ca ii K, H, and λ 849.8 nm line profiles in two regions of the quiet Sun, one being located in the extensive low-latitude coronal hole observed on 3 through 5 August 2003, and the other being located outside the coronal hole. Comparison of the profiles was carried out separately for cells and cell boundaries of the chromospheric network. Our principal result is that space- and time-averaged profiles of the central self-reversal in the coronal hole sites differ from those outside of the hole: Intensities of the K3 and H3 central depressions are increased in the cells but are unchanged in the network; the height of the K2 peaks is reduced in the cells and particularly in the network; the central self-reversal asymmetry is intensified in the network. Distinctions appear at a high confidence level. Line wings as well as average characteristics of the infrared line remain practically unchanged. We discuss probable causes for this behavior of the lower chromosphere lines.  相似文献   

3.
The oscillatory processes in the relatively quiet solar atmosphere, at the base of an extensive coronal hole, have been investigated. The properties of the oscillations in a number of parameters related mainly to the Ca II line intensity have been analyzed in areas belonging to various chromospheric network structures (cells, networks, flocculi, etc.). The goal of this study was to reveal peculiarities of the oscillatory process in the spatial areas located (in projection) at the center of a coronal hole, near its boundary, and at a bright coronal point at various heights of the solar atmosphere (from the photosphere to the middle chromosphere). In most structural elements, the low- and high-frequency components of the spectrum have been found to increase and decrease, respectively, with height. The oscillatory power of the low-frequency oscillations is at a maximum in the areas bordering the bright magnetic network elements. The power of the three-minute, five-minute, and low-frequency oscillations decreases at the centers of the bright chromospheric network. The phase relations point to the propagation of waves mainly at the boundaries of the bright chromospheric network and intermediate (in brightness) network elements. In two of the three investigated regions, the power of the five-minute oscillations (2.4–4.0 mHz) in cells is higher than that of the three-minute ones (5.2–6.8 mHz) at the investigated levels of the quiet solar atmosphere.  相似文献   

4.
The locations of coronal holes are usually based on equivalent-width images in the He i 1083 nm line. However, it is difficult to differentiate coronal holes from the centers of quiet chromospheric network without complementary data and the skill of an experienced observer. Analysis of imaging spectroscopy shows that line half-width and central intensity are correlated differently in coronal holes and a quiet Sun. This fact can be used to form linear combinations of these images in which coronal holes are better separated from the quiet Sun. Coronal hole borders agree well with SOHO/EIT data but can show significant differences from National Solar Observatory maps.  相似文献   

5.
Activity in the chromosphere-corona transition region of the quiet Sun is found both at network boundaries and in cell interiors using a time series of the EUV spectroheliograms obtained with the Harvard experiment on Skylab. We identify time-varying sources by subtracting the minimum count at each pixel in the time series from the counts at any time. Larger flux enhancements in emission lines occur only at the network boundary, though the cell interiors also have variable intensities. Time-varying sources in the cell interior appear often in the shape of streaks which seem to originate from sources at the network boundary, or as expanding network boundary sources. It is likely that the sources in the cell interior come from the transition sheaths of chromospheric inhomogenities. A multi-temperature analysis shows that two types of sources occur in the quiet Sun. One is due to heating of cool chromospheric inhomogenities like dark mottles. Sometimes cool matter is heated to coronal temperatures. The typical mass of the coronal material produced is 1011-1012g. The other type seems to be due to draining of transition region material at the network boundary as the result of thermal instabilities. This quiet Sun activity is compatible with the time-varying sources at 6 cm wavelength.  相似文献   

6.
The behavior of oscillations in the quite solar chromosphere under a coronal hole at several heights has been investigated. The properties of oscillations in cell, network, and weak-floccule areas have been analyzed. A time series of spectrograms in three ionized calcium lines, the Ca II K and H resonance doublet lines and the infrared Ca II 849.8-nm triplet line, was used. The observations were carried out at the horizontal solar telescope of the Sayan Observatory. The goal of this study was to compare the distributions of spectral power in various frequency ranges and their variations for selected spatial areas at different heights of the chromosphere. Particular attention was paid to the weak floccule due to a noticeable difference in the central intensity distribution between the K and H lines and the 849.8-nm line. A spectral Fourier analysis was used. The central intensities of the observed spectral lines, the K-index, and the equivalent width (the latter for the 849.8-nm line) were chosen as oscillation parameters. The studies have shown that the main intensity oscillation power at both atmospheric levels is concentrated at frequencies below 9 mHz. In the distribution of intensity oscillation power at different chromospheric levels, there are differences clearly distinguishable in the floccule. Powerful five-minute oscillations whose main peak frequency decreases with height, while the amplitude increases have been detected in the central part of the floccule. This result confirms the assumptions recently pointed out in the literature that vertical magnetic field concentrations can serve as a channel for the passage of low-frequency oscillations from the photosphere to the chromosphere in faculae. The intensity oscillation power in the frequency ranges under consideration has turned out to decrease with height, on average, for the entire observed spatial area. This may be related to the loss of part of the wave energy through the reflection, dissipation, and transformation of wave modes in the magnetic canopy layer. An area with a low brightness but powerful oscillations at about 3.3 mHz covering a considerable range of heights probably pertaining to “magnetic flashers” has been isolated in the telescope’s field of view.  相似文献   

7.
In order to investigate the high chromosphere and the low transition region in a coronal hole, we have analysed Ca ii, Mg ii and hydrogen resonance lines, recorded by the OSO-8 spectrometer in 1975. We present the comparison between average profiles observed in and out of the equatorial coronal hole which was at the center of the solar disk between 27 and 29 November, 1975. We separate internetwork and quiet-Sun (network+internetwork) profiles: for the internetwork, we observe that the hydrogen and Mg ii profiles recorded in the hole are stronger than the profiles recorded out of the hole; a similar result, but with a much lower contrast, is found for the quiet Sun. We discuss this surprising result.  相似文献   

8.
Intensity distributions of the EUV network and the cell interior in the solar atmosphere have been obtained in fourteen emission lines from Solar and Heliospheric Observatory (SOHO)/Coronal Diagnostic Spectrometer (CDS) observations. The formation temperature of the observed lines is in the range log T=4.90 – 6.06 (T in Kelvin), and hence they represent increasing heights in the solar atmosphere from the upper chromosphere and the transition region to the low corona. Intensity distributions of the cell interior have been found to be different in the quiet Sun and the coronal hole even at the lower transition region, which is at variance with some earlier results. The intensity contrast of the network with respect to the cell interior has been obtained for each line, and differences in the quiet Sun and the coronal hole have been examined. The network contrast, in general, is lower for the coronal hole as compared to the quiet Sun, but becomes equal to it in the upper transition region. The maximum contrast for both the regions is at about log T=5.3. Also obtained are the relative contributions of the network and the cell interior to the total intensity. The implications of the results for models of the transition region are briefly mentioned.  相似文献   

9.
Using the correlation between the radiance or Doppler velocity and the extrapolated magnetic field, we determined the emission heights of a set of solar transition region lines in an equatorial coronal hole and in the surrounding quiet Sun region. We found that for all of the six lower-transition-region lines, the emission height is about 4-5 Mm in the equatorial coronal hole, and around 2 Mm in the quiet Sun region. This result confirms the previous findings that plasma with different temperature can coexist at the same layer of transition region. In the quiet Sun region, the emission height of the upper-transition-region line Ne viii is almost the same that of the lower-transition-region line, but in the coronal hole, it is twice as high. This difference reveals that the outflow of Ne Ⅷ is a signature of solar wind in the coronal hole and is just a mass supply to the large loops in the quiet Sun.  相似文献   

10.
We present a preliminary analysis of spectral lines obtained with the SUMER instrument (Solar Ultraviolet Measurements of Emitted Radiation) onboard the Solar and Heliospheric Observatory (SOHO), as observed during three observing campaigns. From the 70 observed spectral lines, we selected 12, representing 9 ions or atoms, in order to analyse line intensities, shifts and widths in polar coronal holes as well as in the normal quiet Sun. We find that coronal lines show a distinct blueshift in coronal holes relative to the quiet Sun at equal heliospheric angle, while there is no evidence for such a shift for lines formed at temperatures below 105K. The widths of lines formed at temperatures above 3 – 104K are slightly increased inside the coronal hole, but unaffected for lower temperatures. Intensity measurements clearly show the center-to-limb variation, as well as an intensity diminution inside the coronal hole for lines formed above approximately 105K. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

11.
The height of the source region of Si II emission lines characterizes the height of the bottom layer of solar atmosphere's transition region. The correlation analysis of the intensities of ultraviolet spectral lines and the threedimensional structure of magnetic field yielded by force-free extrapolation is a new method for determining the height of ultraviolet emission lines’ source region. It has been found that the height thus obtained is larger than that given by traditional viewpoint. Because the existing numerical analyses with this method are scarce, this result has to be further verified with more observational materials. In this work, this method is applied to the Si II emission lines observed by SOHO/SUMER for the solar surface region beneath the solar coronal hole at southern pole and to the magnetic fields measured by National Solar Observatory/Kitt Peak (NSO/KP) in U.S.A. The correlation height of the source region of Si II emission lines in coronal hole region is approximately 5.0 Mm. This result supports the conclusion that the height of the bottom layer of transition region in coronal hole region is larger than that in quiet regions. Moreover, some new phenomena have been discovered and their causes are probed.  相似文献   

12.
Observations made at the quiet Sun-centre with the Coronal Diagnostic Spectrometer (CDS) and Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instruments on the Solar and Heliospheric Observatory ( SOHO ) have shown that the intensities of the resonance lines of He  i and He  ii are significantly larger than predicted by emission measure distributions found from other transition region lines. The intensities of the helium lines are observed to be lower in coronal holes than in the quiet Sun. Any theory proposed to account for the behaviour of the helium lines must explain the observations of both the quiet Sun and coronal holes. We use observations made with SOHO to find the physical conditions in a polar coronal hole. The electron pressure is found using the C  iii 1175-Å and N  iii 991.5-Å lines, as the C  iii line at 977.0 Å becomes optically thick in some regions at high latitudes. The mean electron pressure is a factor of ≃2 lower than that at the quiet Sun-centre. The mean coronal electron temperature is     . The helium lines are enhanced with respect to other transition region lines but by factors which are ≃ 30 per cent smaller than at the quiet Sun-centre. The mean ratios of the intensities of the He  i 537.0- and 584.3-Å lines and of the He  i and He  ii 303.8-Å lines vary little with the type of region studied. These ratios are compared with those predicted by models of the transition region, taking into account the radiative transfer in the helium lines. No significant variation is found in the relative abundances of carbon and silicon.  相似文献   

13.
We have investigated 15 time series of Ca II line spectrograms in quiet-Sun regions located at various distances from the disk center. Our goal is to reveal the center-to-limb variation of the brightness oscillations. The residual intensities at the centers of the Ca II K and 849.806-nm lines and the K index have been analyzed. We have considered separately two components of the chromospheric network. Our main result is that the power of the brightness oscillations in the chromosphere of the average quiet Sun decreases to the limb. This change for the boundaries of supergranulation cells (networks) is considerably larger than that for their inner parts (cells). It is mainly determined by the 5-min oscillations; the 3-min oscillations show virtually no center-to-limb variation. In addition to studying the dependence of the oscillation power on the heliocentric angle, we also consider other characteristics of the oscillatory regime of the chromosphere. For example, the low-frequency oscillations with periods longer than 700 s, which are inherent predominantly in the K line core in networks, have been separated into an isolated mode. No center-to-limb variation has been revealed for them. As a result of our discussion of the patterns found based on present-day publications on the chromosphere dynamics, we conclude that different mechanisms and sources of its heating can simultaneously make their contributions.  相似文献   

14.
We report on studies of the 1393 line of Si iv, formed in the transition region at about 80 000 K, made using the Colorado experiment on OSO-8. Results indicate that the line width is somewhat greater in coronal holes compared to the quiet Sun, implying a difference in the broadening mechanism. There is no evidence that the line is Doppler shifted in coronal holes relative to the quiet Sun implying there is no mass flow in holes, at the 80 000 K level, greater than 4.3 km s–1. Within the uncertainty of our experiment the integrated line intensities are the same in a coronal hole as in the quiet Sun.  相似文献   

15.
We have observed the solar Caii H and K lines to obtain well-calibrated ratios of their core residual intensities. From three independent calibrations, one using a standard lamp, we conclude that the residual intensity ratio r(K3)/r(H3) is 1.048 ± 0.03 in the quiet chromosphere and 1.20 ± 0.03 in a plage region. These ratios correspond closely to those observed in stars with quiet and active chromospheres, respectively. For a chromospheric model suggested by the calcium lines and a four-level Caii ion, we compute H and K line profiles varying the direct collisional coupling and indirect radiative and collisional coupling via the 3 2 D level. We conclude that enhanced chromospheric activity in the sun and late-type stars results more from a steepening of the chromospheric thermal gradient than from a change in density.Kitt Peak National Observatory Contribution No. 530.Of the University of Colorado and the National Bureau of Standards.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.  相似文献   

16.
Coronal holes (CH) emit significantly less at coronal temperatures than quiet-Sun regions (QS), but can hardly be distinguished in most chromospheric and lower transition region lines. A key quantity for the understanding of this phenomenon is the magnetic field. We use data from SOHO/MDI to reconstruct the magnetic field in coronal holes and the quiet Sun with the help of a potential magnetic model. Starting from a regular grid on the solar surface we then trace field lines, which provide the overall geometry of the 3D magnetic field structure. We distinguish between open and closed field lines, with the closed field lines being assumed to represent magnetic loops. We then try to compute some properties of coronal loops. The loops in the coronal holes (CH) are found to be on average flatter than in the QS. High and long closed loops are extremely rare, whereas short and low-lying loops are almost as abundant in coronal holes as in the quiet Sun. When interpreted in the light of loop scaling laws this result suggests an explanation for the relatively strong chromospheric and transition region emission (many low-lying, short loops), but the weak coronal emission (few high and long loops) in coronal holes. In spite of this contrast our calculations also suggest that a significant fraction of the cool emission in CHs comes from the open flux regions. Despite these insights provided by the magnetic field line statistics further work is needed to obtain a definite answer to the question if loop statistics explain the differences between coronal holes and the quiet Sun.  相似文献   

17.
K. P. Raju 《Solar physics》2009,255(1):119-129
Relative Doppler velocities and spectral linewidths in a coronal hole and in the quiet Sun region outside have been obtained from Solar and Heliospheric Observatory (SOHO)/Coronal Diagnostic Spectrometer (CDS) observations. Five strong emission lines in the CDS wavelength range (namely, O? iii 599 Å, O?v 630 Å, Ne?vi 562.8 Å, He?ii 304 Å, and Mg?ix 368 Å), whose formation temperatures represent different heights in the solar atmosphere from the lower transition region to the inner corona, have been used in the study. As reported earlier, relative velocities in the coronal hole are generally blueshifted with respect to the quiet Sun, and the magnitude of the blueshifts increases with height. It has been found that the polar coronal hole has larger relative velocities than the equatorial extension in the inner corona. Several localized velocity contours have been found mainly on network brightenings and in the vicinity of the coronal hole boundary. The presence of velocity contours on the network may represent network outflows whereas the latter could be due to localized jets probably arising from magnetic reconnection at the boundary. All spectral lines have larger widths in the coronal hole than in the quiet Sun. In O?v 630 Å an extended low-linewidth region is seen in the coronal hole?–?quiet Sun boundary, which may indicate fresh mass transfer across the boundary. Also polar coronal holes have larger linewidths in comparison with the equatorial extension. Together with larger relative velocities, this suggests that the solar wind emanating from polar hole regions is faster than that from equatorial hole regions.  相似文献   

18.
Brynildsen  N.  Brekke  P.  Fredvik  T.  Haugan  S. V. H.  Kjeldseth-Moe  O.  Maltby  P.  Harrison  R. A.  Wilhelm  K. 《Solar physics》1998,181(1):23-50
We present high spatial and spectral resolution observations of one active and one quiet-Sun region, obtained with CDS and SUMER on SOHO. The connections between the line profile parameters are studied and a systematic wavelength shift towards the red with increasing peak line intensity (line broadening) is detected. The large scatter in the data calls for another approach. We apply conditional probability analysis to a series of EUV emission lines and find significant correlations between line profile parameters. For a given interval in wavelength shift we find that: (1) line profiles with large intensities (line widths) and red shifts above the average constitute an increasing fraction of the profiles as the relative wavelength shift increases, (2) line profiles with large intensities (line widths) and blue shifts compared to the average, on the other hand, constitute a decreasing fraction of the profiles as the relative wavelength shift increases. These results extend the findings of an earlier quiet-Sun study from one to several emission lines and expand the validity to include the active region. Interestingly, the active region observations show correlations between peak line intensity and wavelength shift in the coronal lines.The tendency for red-shifted profiles to be more intense than blue-shifted profiles will shift line profiles derived by integrating along the slit towards the red. From the present observations we are not able to determine the fraction of the net red shift that emerges from this correlation. We suggest that the same mechanism is responsible both for the correlation between the line profile parameters and for the differential red shift between the transition region and chromospheric emission lines.  相似文献   

19.
The relative Doppler velocities and linewidths in a polar coronal hole and the nearby quiet-Sun region have been obtained from the Solar and Heliospheric Observatory (SOHO)/Coronal Diagnostic Spectrometer (CDS) observations using emission lines originating at different heights in the solar atmosphere from the lower transition region (TR) to the low solar corona. The observed region is separated into the network and the cell interior, and the behavior of the above parameters were examined in the different regions. It has been found that the histograms of Doppler velocity and width are generally broader in the cell interior than in the network. The histograms of Doppler velocities of the network and cell interior do not show significant differences in most cases. However, in the case of the quiet Sun, the Doppler velocities of the cell interior are more blueshifted than those of the network for the lowermost line He?ii 304 Å, and an opposite behavior is seen for the uppermost line Mg?ix 368 Å. The linewidth histograms show that the network–cell difference is more prominent in the coronal hole. The network has a significantly larger linewidth than the cell interior for the lowermost TR line He?ii 304 Å for the quiet Sun. For the coronal hole, this is true for the three lower TR lines: He?ii 304 Å, O?iii 599 Å, and O?v 630 Å. We also obtained the correlations between the relative Doppler velocity and the width. A mild positive correlation is found for the lowermost transition-region line He?ii 304 Å, which decreases even more or become insignificant for the intermediate lines. For the low coronal line Mg?ix 368 Å, the correlation becomes strongly negative. This might be caused by standing waves or waves propagating from the lower to the upper solar atmosphere. The results may have implications for the generation of the fast solar wind and coronal heating.  相似文献   

20.
The close relation between the mass of the central black hole of galaxy and the stellar velocity dispersion of bulge indicates that it is of especial importance to accurately measure the stellar velocity dispersion for determining the mass of the central black hole of galaxy. A method which uses the spectra of SDSS (Sloan Digital Sky Survey) to measure the velocity dispersion and its uncertainty is provided in this paper. Through fitting four different spectral regions which contain remarkable characteristic absorption lines in pixel space, the spectral regions used to accurately measure the stellar velocity dispersion σ are obtained. In this paper, the absorption lines which are mainly contained in these four fitted bands are Ca II K, Mg I b triplet (with wavelengths of 5 167.5, 5 172.7, 5 183.6 Å) and CaT (Ca II triplet with wavelengths of 8 498.0, 8 542.1, 8 662.1 Å). As indicated by the results in different regions, the values of σ obtained by fitting the Mg I b region are small because this region is affected by the emission lines of iron group; the spectral line in the Ca II K line region is easily restricted to the searching algorithm of least square method because its strength is very weak; the stellar velocity dispersions obtained in the combined region of CaT and Ca II K are equivalent to the results given by calculating only the CaT region. This method is used to test a sample of Seyfert galaxies whose redshifts are less than 0.05. It is found that the CaT region is the best spectral region for measuring the stellar velocity dispersion.  相似文献   

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