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1.
Impulsively generated waves in solar coronal loops are numerically simulated in the frame-work of cold magnetohydrodynamics. Coronal inhomogeneities are approximated by gas density slabs embedded in a uniform magnetic field. The simulations show that an initially excited pulse results in the propagation of wave packets which correspond to both trapped and leaky waves. Whereas the leaky waves propagate outside the slab, the trapped waves occur as a result of a total reflection from the slab walls. Time signatures of these waves are made by a detection of the trapped waves at a fixed spatial location. For waves excited within the slab, time signatures exhibit periodic, quasi-periodic and decay phases. The time signatures for waves excited outside the slab, or for a multi-series of variously shaped impulses generated at different places and times, can possess extended quasi-periodic phases. The case of parallel slabs, when the presence of a second slab influences the character of wave propagation in the first slab, exhibits complex time signatures as a result of solitary waves interaction. 相似文献
2.
Internal gravity waves excited by overshoot at the bottom of the convection zone can be influenced by rotation and by the
strong toroidal magnetic field that is likely to be present in the solar tachocline. Using a simple Cartesian model, we show
how waves with a vertical component of propagation can be reflected when traveling through a layer containing a horizontal
magnetic field with a strength that varies with depth. This interaction can prevent a portion of the downward traveling wave
energy flux from reaching the deep solar interior. If a highly reflecting magnetized layer is located some distance below
the convection zone base, a duct or wave guide can be set up, wherein vertical propagation is restricted by successive reflections
at the upper and lower boundaries. The presence of both upward and downward traveling disturbances inside the duct leads to
the existence of a set of horizontally propagating modes that have significantly enhanced amplitudes. We point out that the
helical structure of these waves makes them capable of generating an α-effect, and briefly consider the possibility that propagation in a shear of sufficient strength could lead to instability,
the result of wave growth due to over-reflection. 相似文献
3.
Oscillations of magnetic flux tubes are of great importance as they contain information about the geometry and fine structure
of the flux tubes. Here we derive and analytically solve in terms of Kummer’s functions the linear governing equations of
wave propagation for sausage surface and body modes (m=0) of a magnetically twisted compressible flux tube embedded in a compressible uniformly magnetized plasma environment in cylindrical geometry. A general dispersion relation is obtained for such flux
tubes. Numerical solutions for the phase velocity are obtained for a wide range of wavenumbers and for varying magnetic twist.
The effect of magnetic twist on the period of oscillations of sausage surface modes for different values of the wavenumber
and vertical magnetic field strength is calculated for representative photospheric and coronal conditions. These results generalize
and extend previous studies of MHD waves obtained for incompressible or for compressible but nontwisted flux tubes. It is
found that magnetic twist may change the period of sausage surface waves of the order of a few percent when compared to counterparts
in straight nontwisted flux tubes. This information will be most relevant when high-resolution observations are used for diagnostic
exploration of MHD wave guides in analogy to solar-interior studies by means of global eigenoscillations in helioseismology. 相似文献
4.
With modern imaging and spectral instruments observing in the visible, EUV, X-ray, and radio wavelengths, the detection of
oscillations in the solar outer atmosphere has become a routine event. These oscillations are considered to be the signatures
of a wave phenomenon and are generally interpreted in terms of magnetohydrodynamic (MHD) waves. With multiwavelength observations
from ground- and space-based instruments, it has been possible to detect waves in a number of different wavelengths simultaneously
and, consequently, to study their propagation properties. Observed MHD waves propagating from the lower solar atmosphere into
the higher regions of the magnetized corona have the potential to provide excellent insight into the physical processes at
work at the coupling point between these different regions of the Sun. High-resolution wave observations combined with forward
MHD modeling can give an unprecedented insight into the connectivity of the magnetized solar atmosphere, which further provides
us with a realistic chance to reconstruct the structure of the magnetic field in the solar atmosphere. This type of solar
exploration has been termed atmospheric magnetoseismology. In this review we will summarize some new trends in the observational
study of waves and oscillations, discussing their origin and their propagation through the atmosphere. In particular, we will
focus on waves and oscillations in open magnetic structures (e.g., solar plumes) and closed magnetic structures (e.g., loops and prominences), where there have been a number of observational highlights in the past few years. Furthermore, we
will address observations of waves in filament fibrils allied with a better characterization of their propagating and damping
properties, the detection of prominence oscillations in UV lines, and the renewed interest in large-amplitude, quickly attenuated,
prominence oscillations, caused by flare or explosive phenomena. 相似文献
5.
The propagation of linear and non-linear magnetohydrodynamic (MHD) waves in a straight homogeneous cylindrical magnetic flux tube embedded in a homogeneous magnetic environment is investigated. Both the tube and its environment are in steady state. Steady flows break the symmetry of forward (field-aligned) and backward (anti-parallel to magnetic field) propagating MHD wave modes because of the induced Doppler shifts. It is shown that strong enough flows change the sense of propagation of MHD waves. The flow also induces shifts in cut-off values and phase-speeds of the waves. Under photospheric conditions, if the flow is strong enough, the slow surface modes may disappear and the fast body modes may become present. The crossing of modes is also observed due to the presence of flows. The effect of steady-state background has to be considered particularly carefully when evaluating observation signatures of MHD waves for diagnostics in the solar atmosphere. 相似文献
6.
Although the inhomogeneous nature of solar magnetic fields is now well established, most theoretical analyses of hydromagnetic wave propagation assume infinite homogeneous fields. Here we reformulate the hydromagnetic wave problem for magnetic fields which vary in one direction perpendicular to the field. The permitted modes of small amplitude hydromagnetic oscillations are considered, first in the case of a single interface between semi-infinite magnetic and non-magnetic compressible regions, and secondly for a magnetic flux sheath of given thickness imbedded in a nonmagnetic region. It is shown that, for small values of R (the ratio of the Alfvén to the sound speed), an acoustic or p-mode wave front passes through the flux sheath with only minor deformation. However, for large R, the transmitted acoustic wave is attenuated and, depending upon the thickness of the flux sheath and the angle of incidence, a hydromagnetic wave may be effectively trapped and guided along the flux sheath. It is also shown that, for the symmetric vibration of the flux sheath in the absence of incident acoustic waves, only slow mode type waves are permitted. Thus, in compressible regions for which R > 1 the Alfvénic-type fast mode is not a permitted mode of free vibration of a flux sheath. 相似文献
7.
V. E. Abramov-Maximov G. B. Gelfreikh N. I. Kobanov K. Shibasaki S. A. Chupin 《Solar physics》2011,270(1):175-189
The nature of the three-minute and five-minute oscillations observed in sunspots is considered to be an effect of propagation
of magnetohydrodynamic (MHD) waves from the photosphere to the solar corona. However, the real modes of these waves and the
nature of the filters that result in rather narrow frequency bands of these modes are still far from being generally accepted,
in spite of a large amount of observational material obtained in a wide range of wave bands. The significance of this field
of research is based on the hope that local seismology can be used to find the structure of the solar atmosphere in magnetic
tubes of sunspots. We expect that substantial progress can be achieved by simultaneous observations of the sunspot oscillations
in different layers of the solar atmosphere in order to gain information on propagating waves. In this study we used a new
method that combines the results of an oscillation study made in optical and radio observations. The optical spectral measurements
in photospheric and chromospheric lines of the line-of-sight velocity were carried out at the Sayan Solar Observatory. The
radio maps of the Sun were obtained with the Nobeyama Radioheliograph at 1.76 cm. Radio sources associated with the sunspots
were analyzed to study the oscillation processes in the chromosphere – corona transition region in the layer with magnetic
field B=2000 G. A high level of instability of the oscillations in the optical and radio data was found. We used a wavelet analysis
for the spectra. The best similarities of the spectra of oscillations obtained by the two methods were detected in the three-minute
oscillations inside the sunspot umbra for the dates when the active regions were situated near the center of the solar disk.
A comparison of the wavelet spectra for optical and radio observations showed a time delay of about 50 seconds of the radio
results with respect to the optical ones. This implies an MHD wave traveling upward inside the umbral magnetic tube of the
sunspot. For the five-minute oscillations the similarity in spectral details could be found only for optical oscillations
at the chromospheric level in the umbral region or very close to it. The time delays seem to be similar. Besides three-minute
and five-minute ones, oscillations with longer periods (8 and 15 minutes) were detected in optical and radio records. Their
nature still requires further observational and theoretical study for even a preliminary discussion. 相似文献
8.
B. Roberts 《Solar physics》1983,87(1):77-93
The nature of non-adiabatic wave propagation in a slender magnetic flux tube is explored. The results of the theory are compared with the observations of Giovanelli et al. (1978), and found to be in general agreement. Those observations, of tubes in the photosphere-chromosphere, show outwardly propagating waves, with periods of 300 s, which take some 19 s to propagate from one level of line formation to another level higher in the atmosphere. In sharp contrast to this, is the time of 7 s for a similar disturbance outside the tube to propagate between the same two levels of line formation, estimated to be some 600 km apart in the field-free atmosphere. It is argued that the sharply contrasting propagation times for the tube and its environment is principally due to the elasticity of the tube and its subsequent propensity for propagation. A non-adiabatic disturbance may be essentially propagating within the tube but essentially non-propagating outside, with considerably slower phase speeds thus arising inside the tube. The theory suggests that the observed disturbances are non-adiabatic, acoustic-gravity waves channelled along a magnetic flux tube and modulated by external pressure variations. 相似文献
9.
Phase-speed diagrams, showing the allowable spectrum of surface and body waves in a magnetically structured atmosphere, are constructed for the interface and the slab. The diagrams (illustrated for photospheric flux tubes, photosphere-chromosphere magnetic canopy, and coronal conditions) classify disturbances for both the normal modes of a structure and incident wave propagation on a structure, allowing a simple application once sufficiently detailed observations of waves become available. 相似文献
10.
Wolfgang Kalkofen 《Journal of Astrophysics and Astronomy》2008,29(1-2):163-166
The nonmagnetic interior of supergranulation cells has been thought since the 1940s to be heated by the dissipation of acoustic waves. But all attempts to measure the acoustic flux have failed to show sufficient energy for chromospheric heating. Recent space observations with TRACE, for example, have found 10% or less of the necessary flux. To explain the missing energy it has been speculated that the nonmagnetic chromosphere is heated mainly by waves related to the magnetic field. If that were correct, the whole chromosphere, magnetic as well as nonmagnetic, would be heated mainly by waves related to the magnetic field. But contrary to expectation, the radiation emerging from the nonmagnetic chromosphere shows none of the signatures of magnetic waves, only those of acoustic waves. Nearly all the heating of the nonmagnetic chromosphere must therefore be due to acoustic waves. In the magnetic network on the boundary of supergranulation cells, on the other hand, the small filling factor of the magnetic field in the photosphere implies that only a small fraction of the wave flux that travels upward to heat the chromosphere can be channeled by the magnetic field. Hence, while some of the energy that is dissipated in the magnetic network is in the form of magnetic waves, most of it must be in the form of acoustic waves. Thus, the quiet solar chromosphere, instead of being heated mainly by magnetic waves throughout, must be heated mainly by acoustic waves throughout. The full wave flux heating the quiet chromosphere must travel through the photosphere. In the nonmagnetic medium, this flux is essentially all in the form of acoustic waves; TRACE registers at most 10% of it, perhaps because of limited spatial resolution. 相似文献
11.
The Io-Jupiter S-bursts are series of quasi-periodic impulsive decameter radio emissions from the magnetic flux tube connecting Jupiter to its closest galilean satellite Io. This paper discusses the possibility, suggested by previous works by Hess et al., that the S-bursts are triggered by upgoing electrons accelerated (downward) by trapped Alfvén waves, that have mirrored above the Jupiter ionosphere. According to this theory, the S-bursts would correspond to wave modes that propagate at oblique angles with respect to the magnetic field. Oblique propagation is also inferred for the more slowly varying components of Io-Jupiter radio emissions. Previous works, mainly based on observations of the terrestrial AKR, whose generation process is closely related to those of S-bursts, showed that these waves are emitted on perpendicular wave modes. This discrepancy between the Jovian and Terrestrial cases has led to a controversy about the credibility of the S-bursts model by Hess et al. In the present paper, we show that indeed, the most unstable wave modes for Earth AKR, and Io-Jupiter S-bursts, as they are seen from ground based radio-telescopes, are not the same. Several causes are evaluated: observational bias, the different degree of plasma magnetization above Earth and Jupiter, the role of a cold plasma component and of plasma auroral cavities. Furthermore, we make predictions about what kind of radiation modes a probe crossing the low altitude Io-Jupiter flux tube will see. 相似文献
12.
S. Vargas Domínguez D. MacTaggart L. Green L. van Driel-Gesztelyi A. W. Hood 《Solar physics》2012,278(1):33-45
Recent studies of NOAA active region 10953, by Okamoto et al. (Astrophys. J. Lett.
673, 215, 2008; Astrophys. J.
697, 913, 2009), have interpreted photospheric observations of changing widths of the polarities and reversal of the horizontal magnetic
field component as signatures of the emergence of a twisted flux tube within the active region and along its internal polarity
inversion line (PIL). A filament is observed along the PIL and the active region is assumed to have an arcade structure. To
investigate this scenario, MacTaggart and Hood (Astrophys. J. Lett.
716, 219, 2010) constructed a dynamic flux emergence model of a twisted cylinder emerging into an overlying arcade. The photospheric signatures
observed by Okamoto et al. (2008, 2009) are present in the model although their underlying physical mechanisms differ. The model also produces two additional signatures
that can be verified by the observations. The first is an increase in the unsigned magnetic flux in the photosphere at either
side of the PIL. The second is the behaviour of characteristic photospheric flow profiles associated with twisted flux tube
emergence. We look for these two signatures in AR 10953 and find negative results for the emergence of a twisted flux tube
along the PIL. Instead, we interpret the photospheric behaviour along the PIL to be indicative of photospheric magnetic cancellation
driven by flows from the dominant sunspot. Although we argue against flux emergence within this particular region, the work
demonstrates the important relationship between theory and observations for the successful discovery and interpretation of
signatures of flux emergence. 相似文献
13.
M. R. Bate G. I. Ogilvie S. H. Lubow J. E. Pringle 《Monthly notices of the Royal Astronomical Society》2002,332(3):575-600
We analyse the non-linear propagation and dissipation of axisymmetric waves in accretion discs using the ZEUS-2D hydrodynamics code. The waves are numerically resolved in the vertical and radial directions. Both vertically isothermal and thermally stratified accretion discs are considered. The waves are generated by means of resonant forcing, and several forms of forcing are considered. Compressional motions are taken to be locally adiabatic ( γ =5/3) . Prior to non-linear dissipation, the numerical results are in excellent agreement with the linear theory of wave channelling in predicting the types of modes that are excited, the energy flux by carried by each mode, and the vertical wave energy distribution as a function of radius. In all cases, waves are excited that propagate on both sides of the resonance (inwards and outwards). For vertically isothermal discs, non-linear dissipation occurs primarily through shocks that result from the classical steepening of acoustic waves. For discs that are substantially thermally stratified, wave channelling is the primary mechanism for shock generation. Wave channelling boosts the Mach number of the wave by vertically confining the wave to a small cool region at the base of the disc atmosphere. In general, outwardly propagating waves with Mach numbers near resonance ℳr ≳0.01 undergo shocks within a distance of order the resonance radius. 相似文献
14.
The propagation of waves in a magnetic slab embedded in a magnetic environment is investigated. The possible modes of propagation are examined from the general dispersion relation, both analytically and numerically, for disturbances which are evanescent in the environment. Approximate dispersion relations governing propagation in a slender slab of field are derived both from the general dispersion relation and from an application of the slender flux tube approximation.Several different situations, representative of both photospheric and coronal conditions, are considered. In general, the structures are found to support both fast and slow, body and surface, waves. Under coronal conditions, for two dimensional propagation, disturbances propagate as fast and slow body waves. The fast body waves are analogous to the ducted shear waves of seismology (Love waves). 相似文献
15.
To investigate the seismic waves generated at the surface of the convection zone by a sunquake, the solar convection zone is modeled as an incompressible fluid layer of finite depth which is excited by a pressure pulse just above the solar surface. Solutions for the surface displacement ζ as a function of time are obtained by solving the linearized Euler equations for wave propagation in an inviscid, incompressible fluid. Approximate solutions are derived using the method of stationary phase and formulas are obtained for the position of the wave crests versus time and the decay of the wave amplitude versus distance. Despite the very simple nature of the model, the resulting time–distance relation is found to exhibit the correct order of magnitude when compared to the observations of the flare initiated sunquake of 9 July 1996. However, the water wave model cannot fully explain the observations because, for one thing, the distance in between successive wave crests is greater than that seen in the observations. One may conclude that the sunquake is probably composed primarily of acoustic waves, that is, p modes and not f modes. 相似文献
16.
Sanjay Tyagi 《Astrophysics and Space Science》1993,209(2):229-235
The problem of surface wave propagation on a magnetised cylindrical plasma is investigated allowing for different axial magnetic fields inside and outside the flux tube. Properties of surface waves for symmetric and asymmetric modes of perturbations are investigated idealising the material inside and outside the cylinder to be insulator or infinitely conducting both in compressible and incompressible approximations for the tube material. 相似文献
17.
Robert J. Stéfant 《Astrophysics and Space Science》1981,76(2):301-328
The linear coupling between the different kinds of waves propagating in a warm plasma inhomogeneous along thex direction is investigated in order to locate the regions (,k) space where two of the roots of the characteristic equation coalesce. Firstly, using the approximation of geometrical optics the differential equation is derived and wave propagation at fixed wave numberk
z
is studied in these special cases for which the characteristic equation reduces to a biquadratic. When the density gradient is parallel to the magnetic field, a detailed analysis of the different properties of the waves shows that the mechanism proposed by Gurnett and others to explain the characteristics of the proton whistler is unlikely to operate, even if a wave coupling occurs at the so called cross over frequency for small incidence angles. The only relevant process occurs when the density gradient is perpendicular to the magnetic field for waves propagating at small incidence angles. It is shown that, close to a coalescence point, but within the limit of the geometrical optics approximation, one of the WKB solutions is a mixed (transverse-longitudinal) mode which becomes purely longitudinal in the limit of large wave numbers. Consequently, as this wave has E almost parallel tok, coalescence implies that the waves are nearly longitudinal at the singular point, in agreement with other results. Next, application of the theory is made to some relevant space observations. It is shown that the proton whistler could be the result of a linear coupling between the extraordinary and the slow ion cyclotron waves close to the Buchsbaum resonance in ionospheric regions above 300 to 400 km where the H+ density begins to grow. Transformation conditions are given which favour the coupling mechanism in regions of strong latitudinal gradients. Finally, a comparison is made with experiment which confirms the principal features of the present theory. 相似文献
18.
A possible mechanism for the formation and heating of coronal loops through the propagation and damping of fast mode waves is proposed and studied in detail. Loop-like field structures are represented by a dipole field with the point dipole at a given distance below the solar surface. The density of the medium is determined by hydrostatic equilibrium along the field lines in an isothermal atmosphere. The fast mode waves propagating outward from the coronal base are refracted into regions with a low Alfvén speed and suffer collisionless damping when the gas pressure becomes comparable to the magnetic pressure. The propagation and damping of these waves are studied for three different cases: a uniform density at the coronal base, a density depletion within a given flux tube, and a density enhancement within a given flux tube. The fast mode waves are found to be important in the formation and heating of the loops if the wave energy flux density is of the order 105 ergs cm-2 s-1 at the coronal base.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
19.
Inhomogeneities in wave propagation conditions near and below the solar surface have been detected by means of time-distance helioseismology. Here we calculate the effect of temperature inhomogeneities on the travel times of sound waves. A temperature increase, e.g., in active regions, not only increases the sound speed but also lengthens the path along which the wave travels because the expansion of the heated layers shifts the upper turning of the waves upward. Using a ray-tracing approximation we find that in many cases the net effect of a temperature enhancement is an increase of the travel times. We argue that the reduced travel times that are observed are caused by a combination of magnetic fields in the active region and reduced subsurface temperatures. Such a reduction may be related to the increased radiative energy loss from small magnetic flux tubes. 相似文献
20.
Robert L. McPherron 《Planetary and Space Science》1984,32(11):1343-1359
Many types of ULF pulsations observed at geosynchronous orbit exhibit properties of standing shear Alfvén waves. Observation of the harmonic mode, polarization state and azimuthal wave number is crucial for determining the source of energy responsible for excitation of these waves. In recent years it has become possible to identify the harmonic mode of standing waves from dynamic spectral analysis, as well as simultaneous observations of electric and magnetic fields of the waves or a comparison between plasma mass density estimated from the frequency of the waves and that observed by direct measurement. It is then more reasonable to classify pulsations according to their physical properties, including the harmonic mode, polarization state, azimuthal wave number, and localization in occurrence, than according to the conventional scheme based on the wave form and period range. From analysis of magnetic pulsations observed at geosynchronous orbit, at least two distinctively different types of waves have been identified. One is azimuthally polarized waves simultaneously excited at the fundamental and several harmonics of a standing Alfvén wave which are observed throughout the day side. They have relatively small azimuthal numbers (less than 10) and propagate tailward. They are likely to be excited by the interaction of the solar wind with the magnetopause or bow shock. Another type is radially polarized waves most strongly excited at the second harmonic. They are observed mainly on the afternoon side. Bounce resonance of a few keV ions has been suggested as the mechanism for excitation of the radially polarized waves. 相似文献