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We have obtained the temporal correlation function, Q(t), from time sequences of Caii K filtergrams and Dopplergrams from Antarctica, Taiwan Oscillation Network (TON) and Solar and Heliospheric Observatory (SOHO). Q(t) gives the time evolution of the pattern under examination, supergranulation in this case. It has been found that Q(t) shows oscillatory signals of both 5-min and long-term periods. The 5-min oscillations are suppressed by averaging the images over 10 min. An exponential decay curve which represents the lifetime trend of supergranules, is fitted to Q(t) and subtracted out. The Q(t) residuals thus obtained contain the oscillatory component and are then subjected to a periodogram analysis. Significant periodicities in the range of 1.4–10 hours have been noted. The causes of these oscillations are not fully known at present, but the instrumental and atmospheric factors can be ruled out, pointing to solar origin. Various possibilities are discussed. Some of the observed periodicities may be considered as probable candidates for long-term oscillations in the Sun, such as the elusive gravity modes. 相似文献
2.
Acoustic imaging is a new method to construct the acoustic signal at a point on the solar surface or in the solar interior with the signals measured at the solar surface. The constructed signals contain both intensity information and phase information. The intensity is computed by summing the squared amplitude of the constructed signal over time. The phase of constructed signals can be studied by the cross-correlation function between the time series constructed with ingoing waves and outgoing waves. The location of the envelope peak of the cross-correlation function and the phase of the cross-correlation function contain different information on the physical conditions of the plasma along the wave path. From the constructed signals, one can form the two-dimensional outgoing intensity map, absorption map, phase-shift map, and envelope-shift map of a target region at different focal depths. The perturbed physical conditions caused by the magnetic fields of active regions manifest in these maps. The outgoing intensity is lower in magnetic regions than the quiet Sun. The group travel time and phase travel time are smaller in magnetic regions than in the quiet Sun. In this paper, we review the studies of active regions, including emerging flux regions, with acoustic imaging. 相似文献
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We use five-day helioseimic data from the Taiwan Oscillation Network to study the flow around a sunspot, NOAA 7887. The p-mode oscillations in an annular region centered at the sunspot are decomposed into the modes propagating toward and away from the sunspot. We find that the frequency of an outgoing mode is greater than that of the corresponding incoming mode. This indicates that the plasma is flowing outward from the sunspot. The outflow velocity is estimated to be about 40–80 m s-1. 相似文献
4.
The continuous high spatial resolution Doppler observation of the Sun by the Solar Dynamics Observatory/Helioseismic and Magnetic Imager allows us to compute a helioseismic k–ω power-spectrum diagram using only oscillations inside a sunspot. Individual modal ridges can be clearly seen with reduced power in the k–ω diagram that is constructed from a 40-hour observation of a stable and round sunspot. Comparing this with the k–ω diagram obtained from a quiet-Sun region, one sees that inside the sunspot the f-mode ridge is more reduced in power than the p-mode ridges, especially at high wavenumbers. The p-mode ridges all shift toward lower wavenumber (or higher frequency) for a given frequency (or wavenumber), implying an increase of phase velocity beneath the sunspot. This is probably because the acoustic waves travel across the inclined magnetic field of the sunspot penumbra. Line-profile asymmetries exhibited in the p-mode ridges are more significant in the sunspot than in the quiet Sun. Convection inside the sunspot is also highly suppressed, and its characteristic spatial scale is substantially larger than the typical convection scale of the quiet Sun. These observational facts demand a better understanding of magnetoconvection and interactions of helioseismic waves with magnetic field. 相似文献
5.
Hui Zhao Dean-Yi Chou Ming-Hsu Yang Zhi-Chao Liang Ming-Tsung Sun 《Solar physics》2011,268(2):363-376
The cross-correlation function is a useful tool in helioseismology. The magnitude of the cross-correlation function has been used to represent the power of wave packets. Dispersion causes a decrease in amplitude and an increase in width of wave packets. This leads to a decrease in magnitude and an increase in width of cross-correlation functions. The effect of dispersion on the magnitude of cross-correlation functions needs to be adequately corrected for in order to use the magnitude of cross-correlation functions to represent the power of wave packets. In this study, we investigate how the magnitude of cross-correlation functions changes with the number of skips owing to dispersion and the method to correct it. Our study, using simulated and observational data, indicates that the correction should be three dimensional instead of the one-dimensional correction adopted in previous studies. Using the three-dimensional correction, the measured dissipation rate in the quiet Sun is smaller than the value of previous studies. 相似文献
6.
We study the properties of power maps of solar acoustic waves filtered with direction filters and phase-velocity filters.
A direction filter is used to isolate acoustic waves propagating in a narrow range of directions. The acoustic-power map of
the waves filtered with a direction filter shows extended reduced-power features behind magnetic regions with respect to the
wave direction. A phase-velocity filter is further applied to isolate waves with similar wave paths. In the power maps of
the waves filtered with both a direction filter and a phase-velocity filter, a reduced-power image of a sunspot appears behind
the sunspot with respect to the wave direction. The distance between the sunspot and the secondary image is consistent with
the one-skip travel distance of the wave packet associated with the phase-velocity filter. The waves filtered with direction
and phase-velocity filters at the location of the secondary image could be used to probe the sunspot. In the quiet Sun, spatial
fluctuations exist in any acoustic-power map. These fluctuations are mainly caused by interference among modes with the same
frequency. The fluctuations are random with two properties: They change rapidly with time, and their magnitude decreases with
the square root of the number of frames used in computing the acoustic-power map. 相似文献
7.
The cross-correlation function of solar p modes in a time – distance analysis changes with travel distance (or travel time). The exponential decrease in the amplitude
of the cross-correlation function with travel distance has been interpreted as the dissipation of solar p-mode power and used to determine the lifetimes of high-degree p modes. It is found that the width of the cross-correlation function increases with travel distance. We interpret the increase
in width as the dispersion of the wave packet in a time – distance analysis. The dispersion would also cause a decrease in
amplitude and affect the determination of lifetimes. We include the dispersion effect in the determination of lifetimes of
high-degree p modes in a time – distance analysis and find that the derived lifetime increases significantly compared with the previous
study for degree less than 400. 相似文献
8.
Dean-Yi Chou Ming-Tsung Sun Teng-Yi Huang Shih-Ping Lai Pi-Jen Chi Knight-Tien Ou Chang-Chi Wang Jui-Yang Lu An-Li Chu Chi-Seng Niu et al. 《Solar physics》1995,160(2):237-243
The Taiwan Oscillation Network (TON) is a ground-based network to measure solar intensity oscillations to study the internal structure of the Sun. K-line full-disk images of 1000 pixels diameter are taken at a rate of one image per minute. Such data would provide information onp-modes withl as high as 1000. The TON will consist of six identical telescope systems at proper longitudes around the world. Three telescope systems have been installed at Teide Observatory (Tenerife), Huairou Solar Observing Station (near Beijing), and Big Bear Solar Observatory (California). The telescopes at these three sites have been taking data simultaneously since October of 1994. Anl – v diagram derived from 512 images is included to show the quality of the data. 相似文献
9.
We measure the separation velocity of opposite poles from 24 new bipoles on the Sun. We find that the measured velocities range from about 0.2 to 1 km s–1. The fluxes of the bipoles range over more than two orders of magnitude, and the mean field strength and the sizes range over one order of magnitude. The measured separation velocity is not correlated with the flux and the mean field strength of the bipole. The separation velocity predicted by the present theory of magnetic buoyancy is between 7.4Ba
–1/4 cot and 13 cot km s–1, where is the elevation angle of the flux tube at the photosphere (see Figure 9), B is the mean field strength, and a is the radius of the observed bipole. The rising velocity of the top of flux tubes predicted by the theory of magnetic buoyancy is between 3.7Ba
–1/4 and 6.5 km s–1. The predicted separation velocity is about one order of magnitude higher than those measured, or else the flux tubes are almost vertical at the photosphere. There is no correlation between the measured separation velocity and the theoretical value, 7.4Ba
–1/4. The predicted rising velocity is also higher than the vertical velocity near the line of inversion in emerging flux regions observed by other authors. 相似文献
10.
The three-dimensional distribution of change in phase travel time of an active region below the solar surface can be constructed
with the technique of acoustic imaging. The interpretation of the distribution of measured phase travel time perturbation
suffers from the finite spatial resolution of the acoustic lenses. In the ray approximation, the phase travel time perturbation
measured in acoustic imaging can be expressed as an integral of the product of the relative sound-speed perturbation and a
kernel. Forward computations show that the vertical resolution of phase travel time perturbation is poor even in the ray approximation.
In this study, we discuss the inversion of phase travel time perturbations to estimate the relative sound-speed perturbation
with a regularized least-squares inversion method. The tests with model perturbations of sound speed show that the inversion
reasonably recovers the distribution of the model perturbation. We also apply the inversion method to the measured phase travel
time perturbation of active region NOAA 7981. 相似文献
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