共查询到20条相似文献，搜索用时 31 毫秒
1.
Avery E. Broderick Abraham Loeb 《Monthly notices of the Royal Astronomical Society》2005,363(2):353-362
Solar active regions are distinguished by their strong magnetic fields. Modern local helioseismology seeks to probe them by observing waves which emerge at the solar surface having passed through their interiors. We address the question of how an acoustic wave from below is partially converted to magnetic waves as it passes through a vertical magnetic field layer where the sound and Alfvén speeds coincide (the equipartition level), and find that (i) there is no associated reflection at this depth, either acoustic or magnetic, only transmission and conversion to an ongoing magnetic wave; and (ii) conversion in active regions is likely to be strong, though not total, at frequencies typically used in local helioseismology, with lower frequencies less strongly converted. A simple analytical formula is presented for the acoustic-to-magnetic conversion coefficient. 相似文献
2.
Arvind Bhatnagar 《Journal of Astrophysics and Astronomy》2006,27(2-3):59-78
In this presentation we briefly describe the Sun through large number of illustrations and pictures of the Sun taken from
early times to the present day space missions. The importance of the study of the Sun is emphasized as it is the nearest star
which presents unparallelled views of surface details and numerous phenomena. Our Sun offers a unique celestial laboratory
where a large variety of phenomena take place, ranging in temporal domain from a few milliseconds to several decades, in spatial
domain from a few hundred kilometers to thousands of kilometers, and in the temperature domain from a few thousand degrees
to several million degrees. Its mass motion ranges from thousandths to thousands of kilometers per second. Such an object
provides us with a unique laboratory to study the state of matter in the Universe.
The existing solar ground-based and space missions have already revealed several mysteries of the outer environment of our
Sun and much more is going to come in the near future from planned new sophisticated ground-based solar telescopes and Space
missions.
The new technique of helioseismology has unravelled many secrets of the solar interior and has put the Standard Solar Model
(SSM) on firm footing. The long-standing problem of solar neutrinos has been recently sorted out, and even the ‘back side’
view of the Sun can be seen using the technique of holographic helioseismology. 相似文献
3.
Time–distance helioseismology is a set of powerful tools to study localized features below the Sun’s surface. Inverse methods are needed to robustly interpret time–distance measurements, with many examples in the literature. However, techniques that utilize a more statistical approach to inferences, and that are broadly used in the astronomical community, are less-commonly found in helioseismology. This article aims to introduce a potentially powerful inversion scheme based on Bayesian probability theory and Monte Carlo sampling that is suitable for local helioseismology. We first describe the probabilistic method and how it is conceptually different from standard inversions used in local helioseismology. Several example calculations are carried out to compare and contrast the setup of the problems and the results that are obtained. The examples focus on two important phenomena that are currently outstanding issues in helioseismology: meridional circulation and supergranulation. Numerical models are used to compute synthetic observations, providing the added benefit of knowing the solution against which the results can be tested. For demonstration purposes, the problems are formulated in two and three dimensions, using both ray- and Born-theoretical approaches. The results seem to indicate that the probabilistic inversions not only find a better solution with much more realistic estimation of the uncertainties, but they also provide a broader view of the range of solutions possible for any given model, making the interpretation of the inversion more quantitative in nature. The probabilistic inversions are also easy to set up for a broad range of problems, and they can take advantage of software that is publicly available. Unlike the progress being made in fundamental measurement schemes in local helioseismology that image the far side of the Sun, or have detected signatures of global Rossby waves, among many others, inversions of those measurements have had significantly less success. Such statistical methods may help overcome some of these barriers to move the field forward.
相似文献4.
Duvall T. L. Scherrer P. H. Bogart R. S. Bush R. I. De forest C. Hoeksema J. T. Schou J. Saba J. L. R. Tarbell T. D. Title A. M. Wolfson C. J. Milford P. N. 《Solar physics》1997,170(1):63-73
In time-distance helioseismology, the travel time of acoustic waves is measured between various points on the solar surface. To some approximation, the waves can be considered to follow ray paths that depend only on a mean solar model, with the curvature of the ray paths being caused by the increasing sound speed with depth below the surface. The travel time is affected by various inhomogeneities along the ray path, including flows, temperature inhomogeneities, and magnetic fields. By measuring a large number of times between different locations and using an inversion method, it is possible to construct 3-dimensional maps of the subsurface inhomogeneities. The SOI/MDI experiment on SOHO has several unique capabilities for time-distance helioseismology. The great stability of the images observed without benefit of an intervening atmosphere is quite striking. It has made it possible for us to detect the travel time for separations of points as small as 2.4 Mm in the high-resolution mode of MDI (0.6 arc sec pixel-1). This has enabled the detection of the supergranulation flow. Coupled with the inversion technique, we can now study the 3-dimensional evolution of the flows near the solar surface. 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.
Comparison of Travel-Time and Amplitude Measurements for Deep-Focusing Time–Distance Helioseismology
The purpose of deep-focusing time–distance helioseismology is to construct seismic measurements that have a high sensitivity to the physical conditions at a desired target point in the solar interior. With this technique, pairs of points on the solar surface are chosen such that acoustic ray paths intersect at this target (focus) point. Considering acoustic waves in a homogeneous medium, we compare travel-time and amplitude measurements extracted from the deep-focusing cross-covariance functions. Using a single-scattering approximation, we find that the spatial sensitivity of deep-focusing travel times to sound-speed perturbations is zero at the target location and maximum in a surrounding shell. This is unlike the deep-focusing amplitude measurements, which have maximum sensitivity at the target point. We compare the signal-to-noise ratio for travel-time and amplitude measurements for different types of sound-speed perturbations, under the assumption that noise is solely due to the random excitation of the waves. We find that, for highly localized perturbations in sound speed, the signal-to-noise ratio is higher for amplitude measurements than for travel-time measurements. We conclude that amplitude measurements are a useful complement to travel-time measurements in time–distance helioseismology. 相似文献
7.
M. Lazrek A. Pantel E. Fossat B. Gelly F. X. Schmider D. Fierry-Fraillon G. Grec S. Loudagh S. Ehgamberdiev I. Khamitov J. T. Hoeksema P. L. Pallé C. Régulo 《Solar physics》1996,166(1):1-16
The Sun is not a rigid body and it is well known that its surface rotation is differential, the polar regions rotating substantially slower than the equator. This differential rotation has been demonstrated by helioseismology to continue down to the base of the convective zone, below which it becomes closer to a rigid body rotation. Far deeper, inside the energy generating core, the rotation has generally been assumed to be much faster, keeping memory of the presumably high speed of the young Sun. However, several recent results of helioseismology have decreased this likelihood more and more, so that the core rotation could be suspected to be only marginally, or even not at all faster than the envelope. Certain results would even imply a core rotation slower than the envelope, an interesting but unlikely possibility. We present here a complete analysis of the rotational splitting of the low degree modes measured in three different time series obtained in 1990, 1991, and 1992 by the IRIS full-disk network. With a time of integration slightly longer than 4 months, the splitting has been measured by 4 different global methods on 42 doublets of l
= 1, 35 triplets of l = 2, and 30 quadruplets of l = 3. With a high level of confidence, our result is consistent with a rigid solar core rotation. 相似文献
8.
B. Pintér 《Solar physics》2008,251(1-2):329-340
Helioseismic global modes change in time, in particular on time scales of the solar cycle. These changes show, in fact, strong correlation with the magnetic activity cycle of the Sun, indicating that a most likely cause of the variation of the mode characteristics, such as frequency, is the magnetic field. In the present paper I attempt to find out in what ways and to what degree the magnetic atmosphere of the Sun can influence the f and p modes of helioseismology. Frequency shifts of the order of a microhertz, line widths of the order of a nanohertz, and penetration depths of the order of a megameter are obtained. 相似文献
9.
We suggest a solution to an important problem in observational helioseismology of the separation of lines of solar acoustic (p) modes of low angular degree in oscillation power spectra by constructing optimal masks for Doppler images of the Sun. Accurate measurements of oscillation frequencies of low-degree modes are essential for the determination of the structure and rotation of the solar core. However, these measurements for a particular mode are often affected by leakage of other p-modes arising when the Doppler images are projected on to spherical harmonic masks. The leakage results in overlapping peaks corresponding to different oscillation modes in the power spectra. In this Letter, we present a method for calculating optimal masks for a given (target) mode by minimizing the signals of other modes appearing in its vicinity. We apply this method to time series of 2 yr obtained from the Michelson Doppler Imager instrument on board the Solar and Heliospheric Observatory space mission and demonstrate its ability to reduce efficiently the mode leakage. 相似文献
10.
Sensitivity Kernels for Time-Distance Inversion 总被引：1，自引：0，他引：1
Jensen Jesper Munk Jacobsen Bo Holm Christensen-Dalsgaard Jørgen 《Solar physics》2000,192(1-2):231-239
Inversion of local-area helioseismic time-distance data has so far only been done in the ray approximation (Kosovichev, 1996). Since this is a high-frequency approximation its applicability can be questioned for the solar case. Bogdan (1997) showed that for a simple solar model the localized wave packets do follow, but are not confined to, the ray path. We use an approximation based on the first Fresnel zone that has been developed in geophysics by Snieder and Lomax (1996) to go beyond the ray approximation in the inversions. We have calculated sensitivity kernels using both approximations. To test them we use a finite-difference forward modeling of the whole wave field in an acoustic medium reminiscent of the Sun. We use the finite-difference modeling to calculate sensitivity kernels for the full wave field and compare this with the other kernels. The results show that the Fresnel-zone-based kernels are in good agreement with the sensitivity obtained from the modeling. Thus these new kernels represent a significant step forward in the inversion of time-distance data. 相似文献
11.
Local helioseismology is providing new views of subphotospheric flows from supergranulation to global-scale meridional circulation and for studying structures and dynamics in the quiet Sun and active regions. In this short review we focus on recent developments, and in particular on a number of current issues, including the sensitivity of different measures of travel time and testing the forward modelling used in local helioseismology. We discuss observational and theoretical concerns regarding the adequacy of current analyses of waves in sunspots and active regions, and we report on recent progress in the use of numerical simulations to test local helioseismic methods. 相似文献
12.
Inferences of subsurface flow velocities using local domain ring-diagram helioseismology depend on measuring the frequency shifts of oscillation modes seen in acoustic power spectra. Current methods for making these measurements use maximum-likelihood fitting techniques to match a model of modal power to the spectra. The model typically describes a single oscillation mode, and each mode in a given power spectrum is fit independently. We present a new method that produces measurements with higher reliability and accuracy by fitting multiple modes simultaneously. We demonstrate that this method permits measuring sub-surface flows deeper into the Sun while providing higher uniformity in data coverage and velocity response closer to the limb of the solar disk. While the previous fitting method performs better for some measurements of low phase-speed modes, we find this new method to be particularly useful for high phase-speed modes and small spatial areas. 相似文献
13.
Two competing fundamental hypotheses are usually postulated in the solar coronal heating problem: heating by nanoflares and heating by waves. In the latter it is assumed that acoustic and magnetohydrodynamic disturbances whose amplitude grows as they propagate in a medium with a decreasing density come from the convection zone. The shock waves forming in the process heat up the corona. In this paper we draw attention to yet another very efficient shock wave generation process that can be realized under certain conditions typical for quiet regions on the Sun. In the approximation of stationary dissipative hydrodynamics we show that a shock wave can be generated in the quiet solar chromosphere–corona transition region by the fall of plasma from the corona into the chromosphere. This shock wave is directed upward, and its dissipation in the corona returns part of the kinetic energy of the falling plasma to the thermal energy of the corona. We discuss the prospects for developing a quantitative nonstationary model of the phenomenon. 相似文献
14.
We derive, following the standard first Born approximation approach used in the geophysics literature, an expression for the travel time perturbation caused by a perturbation to sound speed. In our simple model we employ a point source at one point and calculate the time taken for a wave packet created at the source to move to a second point. In the first Born approximation the travel time delay caused by a perturbation to the background model can be expressed as the integral over the whole sun of some function, called the travel time sensitivity kernel, multiplied by the perturbation. The sensitivity kernels are zero along the geometrical ray connecting the two points and have maximum weight in a tube around the ray; they are the solar equivalent of `the banana-doughnut' kernels discussed in the geophysics literature. Calculating sensitivity kernels that are more accurate than those derived from ray theory is important for the future of inversions done with time-distance helioseismology data as they will allow greater confidence in the results as well as increased resolution. 相似文献
15.
Bifurcations of nonlinear electron acoustic solitary waves and periodic waves in an unmagnetized quantum plasma with cold and hot electrons and ions has been investigated. The one dimensional quantum hydrodynamic model is used to study electron acoustic waves (EAWs) in quantum plasma. Applying the well known reductive perturbation technique (RPT), we have derived a Korteweg-de Vries (KdV) equation for EAWs in an unmagnetized quantum plasma. By using the bifurcation theory and methods of planar dynamical systems to this KdV equation, we have presented the existence of two types of traveling wave solutions which are solitary wave solutions and periodic traveling wave solutions. Under different parametric conditions, some exact explicit solutions of the above waves are obtained. 相似文献
16.
The current interpretations of the travel-time measurements in quiet and active regions on the Sun are discussed. These interpretations are based on various approximations to the 3-D wave equation such as the Fermat principle for acoustic rays and the Born approximation. The ray approximation and its modifications have provided the first view of the 3-D structures and flows in the solar interior. However, more accurate and computationally efficient approximations describing the relation between the wave travel times and the internal properties are required to study the structures and flows in detail. Inversion of the large three-dimensional datasets is efficiently carried out by regularized iterative methods. Some results of time-distance inversions for emerging active regions, sunspots, meridional flows and supergranulation are presented. An active region which emerged on the solar disk in January 1998, was studied from SOHO/MDI for eight days, both before and after its emergence at the surface. The results show a complicated structure of the emerging region in the interior, and suggest that the emerging flux ropes travel very quickly through the depth range of our observations. The estimated speed of emergence is about 1.3 km s–1. Tomographic images of a large sunspot reveal sunspot `fingers' - long narrow structures at a depth of about 4 Mm, which connect the sunspot with surrounding pores of the same polarity. 相似文献
17.
M. Stix 《Solar physics》2000,196(1):19-27
Amplitude and phase of an acoustic oscillation in the solar convection zone vary in response to the local variation of the speed of sound and the convection velocity. Such wave modulation is considered by means of a two-dimensional periodic model, with alternating vertical channels of hot rising and cool sinking gas. According to this model, vertically propagating waves show only amplitude modulation. For low wave frequencies the amplitude is larger in the upflow channels, for high frequencies it is larger in the downflow channels. The transition occurs at a frequency for which the vertical wavelength is approximately equal to the horizontal period of the model. Waves with an inclined propagation direction show a similar amplitude modulation but, in addition, a modulation of their phase. The present results are compared with recent observational studies. There is evidence that wave modulation indeed occurs on the Sun, on the granular as well as on the mesogranular scale, in addition to the episodic amplitude enhancement that has been interpreted in terms of local acoustic events. 相似文献
18.
J. Jackiewicz A. C. Birch L. Gizon S. M. Hanasoge T. Hohage J.-B. Ruffio M. ?vanda 《Solar physics》2012,276(1-2):19-33
Inversions for local helioseismology are an important and necessary step for obtaining three-dimensional maps of various physical quantities in the solar interior. Frequently, the full inverse problems that one would like to solve prove intractable because of computational constraints. Due to the enormous seismic data sets that already exist and those forthcoming, this is a problem that needs to be addressed. To this end, we present a very efficient linear inversion algorithm for local helioseismology. It is based on a subtractive optimally localized averaging (SOLA) scheme in the Fourier domain, utilizing the horizontal-translation invariance of the sensitivity kernels. In Fourier space the problem decouples into many small problems, one for each horizontal wave vector. This multichannel SOLA method is demonstrated for an example problem in time–distance helioseismology that is small enough to be solved both in real and Fourier space. We find that both approaches are successful in solving the inverse problem. However, the multichannel SOLA algorithm is much faster and can easily be parallelized. 相似文献
19.
The upward propagation of linear acoustic waves in a gravitationally stratified solar atmosphere is studied. The wave motion is governed by the Klein?–?Gordon equation, which contains a cutoff frequency introduced by stratification. The acoustic cutoff may act as a potential barrier when the temperature decreases with height. It is shown that waves trapped below the barrier could be subject to a resonance that extends into the entire unbounded atmosphere of the Sun. The parameter space characterizing the resonance is explored. 相似文献
20.
The development of solar acoustic holography has opened a major new diagnostic avenue in local helioseismology. It has revealed `acoustic moats' surrounding sunspots, `acoustic glories' surrounding complex active regions, and `acoustic condensations' suggesting the existence of significant seismic anomalies up to 20 Mm beneath active-region photospheres. Phase-sensitive seismic holography is now yielding high-resolution maps of sound travel-time anomalies caused by magnetic forces in the immediate subphotosphere, apparent thermal enhancements in acoustic moats, and Doppler signatures of subsurface flows. It has given us the first seismic images of a solar flare, and has uncovered a remarkable anomaly in the statistical distribution of seismic emission from acoustic glories. Seismic holography will probably give us the means for early detection of large active regions on the far-surface of the Sun, and possibly of deep subsurface activity as well. This powerful diagnostic now promises a new insight into the hydromechanical and thermal environments of the solar interior in the local perspective. 相似文献