共查询到20条相似文献，搜索用时 546 毫秒
1.
Using a Curtis-matrix model of 15 μm CO2 radiative cooling rates for the martian atmosphere, we have computed vertical scale-dependent IR radiative damping rates from 0 to 200 km altitude over a broad band of vertical wavenumbers ∣m∣ = 2π(1-500 km)−1 for representative meteorological conditions at 40°N and average levels of solar activity and dust loading. In the middle atmosphere, infrared (IR) radiative damping rates increase with decreasing vertical scale and peak in excess of 30 days−1 at ∼50-80 km altitude, before gradually transitioning to scale-independent rates above ∼100 km due to breakdown of local thermodynamic equilibrium. We incorporate these computed IR radiative damping rates into a linear anelastic gravity-wave model to assess the impact of IR radiative damping, relative to wave breaking and molecular viscosity, in the dissipation of gravity-wave momentum flux. The model results indicate that IR radiative damping is the dominant process in dissipating gravity-wave momentum fluxes at ∼0-50 km altitude, and is the dominant process at all altitudes for gravity waves with vertical wavelengths ?10-15 km. Wave breaking becomes dominant at higher altitudes only for “fast” waves of short horizontal and long vertical wavelengths. Molecular viscosity plays a negligible role in overall momentum flux deposition. Our results provide compelling evidence that IR radiative damping is a major, and often dominant physical process controlling the dissipation of gravity-wave momentum fluxes on Mars, and therefore should be incorporated into future parameterizations of gravity-wave drag within Mars GCMs. Lookup tables for doing so, based on the current computations, are provided. 相似文献
2.
K. F. Tapping 《Solar physics》1978,59(1):145-158
During the May 21, 1972, noise storm, flux density measurements were made, at a frequency of 140 MHz, of 14 pulsating bursts. These comprised trains of up to 20 pulses, having pulse repetition rates of up to 15 s–1.A model for the burst mechanism is described, based upon the hypothesis that the pulsations are generated by the modulation of a thermally damped plasma wave source by periodic trains of whistler packets originating in unstable regions deep in the corona and passing through the meter wavelength source. These whistler packets further enhance the emissions by increasing the efficiency of the conversion of the plasma waves into electromagnetic waves. 相似文献
3.
Observations show that small-amplitude prominence oscillations are usually damped after a few periods. This phenomenon has been theoretically investigated in terms of non-ideal magnetoacoustic waves, non-adiabatic effects being the best candidates to explain the damping in the case of slow modes. We study the attenuation of non-adiabatic magnetoacoustic waves in a slab prominence embedded in the coronal medium. We assume an equilibrium configuration with a transverse magnetic field to the slab axis and investigate wave damping by thermal conduction and radiative losses. The magnetohydrodynamic equations are considered in their linearised form and terms representing thermal conduction, radiation and heating are included in the energy equation. The differential equations that govern linear slow and fast modes are numerically solved to obtain the complex oscillatory frequency and the corresponding eigenfunctions. We find that coronal thermal conduction and radiative losses from the prominence plasma reveal as the most relevant damping mechanisms. Both mechanisms govern together the attenuation of hybrid modes, whereas prominence radiation is responsible for the damping of internal modes and coronal conduction essentially dominates the attenuation of external modes. In addition, the energy transfer between the prominence and the corona caused by thermal conduction has a noticeable effect on the wave stability, radiative losses from the prominence plasma being of paramount importance for the thermal stability of fast modes. We conclude that slow modes are efficiently damped, with damping times compatible with observations. On the contrary, fast modes are less attenuated by non-adiabatic effects and their damping times are several orders of magnitude larger than those observed. The presence of the corona causes a decrease of the damping times with respect to those of an isolated prominence slab, but its effect is still insufficient to obtain damping times of the order of the period in the case of fast modes. 相似文献
4.
K. A. P. Singh 《Journal of Astrophysics and Astronomy》2006,27(2-3):321-326
We study the spatial damping of magnetoacoustic waves in an unbounded quiescent prominence invoking the technique of MHD seismology.
We consider Newtonian radiation in the energy equation and derive a fourth order general dispersion relation in terms of wavenumberk. Numerical solution of dispersion relation suggests that slow mode is more affected by radiation. The high frequency waves
have been found to be highly damped. The uncertainty in the radiative relaxation time, however, does not allow us to conclude
if the radiation is a dominant damping mechanism in quiescent prominence. 相似文献
5.
We use linear analysis to simulate the evolution of a coronal loop in response to a localized impulsive event. The disturbance is modeled by injecting a narrow Gaussian velocity pulse near one footpoint of a loop in equilibrium. Three different damping mechanisms, namely viscosity, thermal conduction, and optically thin radiation, are included in the loop calculations. We consider homogeneous and gravitationally stratified, isothermal loops of varying length (50≤L≤400 Mm) and temperature (2≤T≤10 MK). We find that a localized pulse can effectively excite slow magnetoacoustic waves that propagate up along the loop. The amplitudes of the oscillations increase with decreasing loop temperature and increasing loop length and size of the pulse width. At T≥4 MK, the waves are dissipated by the combined effects of viscosity and thermal conduction, whereas at temperatures of 2 MK, or lower, wave dissipation is governed by radiative cooling. We predict periods in the range of 4.6?–?41.6 minutes. The wave periods remain unaltered by variations of the pulse size, decrease with the loop temperature, and increase almost linearly with the loop length. In addition, gravitational stratification results in a small reduction of the periods and amplification of the waves as they propagate up along the loop. 相似文献
6.
The radiative damping of trapped gravity waves in an optically thin atmosphere is studied for a stratified Boussinesq fluid. The character of the atmospheric eigenmodes depends on the distribution of the Brunt-Väisälä frequency N and the radiative relaxation time . The calculations for simple layer models show that if N is large over some finite fraction of the trapping region, then modes of long lifetime can exist. In order to suppress gravity waves entirely, it is necessary that N < 1 over the entire trapping region. Qualitative application of the results to the solar atmosphere leads to the conclusion that gravity wave eigenmodes of the solar atmosphere, although damped, are by no means eliminated by radiative effects. 相似文献
7.
E. Schatzman 《Astrophysics and Space Science》1999,265(1-4):97-97
The aim of this work is to present a transport process which is likely to have a great importance for the internal constitution
of the stars. In order to set the problem, we first give a short presentation of the physical properties of the Sun and stars,
described usually under the names of `Standard Solar Model' or `Standard Stellar Models' (SSM). Next we show that an important question about SSM is that they do not explain the age dependance of lithium deficiency
of stars of known age: stars of galactic clusters and the Sun. It has been suggested a long time ago to assume the presence
of a macrosocpic diffusion process in the radiative zone, below the surface convective zone of solar like stars. It is then
possible for the lithium present in the convective zone to be carried to the thermonuclear burning level below the convective
zone. The first assumption was that differential rotation generates turbulence and therefore that a turbulent diffusion process
takes place. However, this model predicts a lithium abundance which is strongly rotation dependant, contrary to the observations.
Furthermore, the diffusion coefficient being large all over the radiative zone, it prevents the possibility of gravitational
separation by diffusion and consequently leads to an impossibility of explaining the difference of helium abundance between
the surface and the center of the Sun. The consequence is obviously that we need to take into account another physical process.
Stars having a mass M < 1.3 M
⊙ have a convective zone which begins close to the stellar surface and extends down to a depth which is an appreciable fraction
of stellar radius. In the convective zone, strong stochastic motions take care, at least partially, of heat transfer. These
motions do not vanish at the lower boundary and generate internal waves into the radiative zone. These random internal waves
are at the origin of a diffusion process which can be considered as responsible of the diffusive transport of lithium down
to the lithium burning level. This is certainly not the only physical process responsible of lithium deficiency in main sequence
stars, but its properties open the way to a completely consistent analysis of lithium deficiency. The model of generation
of gravity waves is based on a model of heat transport in the convective zone by diving plumes. The horizontal component of
the turbulent motion at the boundary of the convective zone is supposed to generate the horizontal motion of internal waves.
The result is a large horizontal component of the diffusion coefficient, which produces in a short time an horizontal uniform
chemical composition. It is known that gravity waves, in the absence of any dissipative process, cannot generate vertical
mixing. Therefore, the vertical component of the diffusion coefficient is entirely dependant of radiative damping. It decreases
quickly in the radiative zone, but is large enough to be responsible of lithium burning. Due to the radial dependance of velocity
amplitude, the diffusion coeficient increases when approaching the stellar center. However, very close to the center, non-linear
dissipative and radiative damping of internal waves become large and the diffusion coefficient vanishes at the very center.
The development of this abstract can be found in E. Schatzman (1996, J. Fluid Mech.
322, 355).
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
We investigate the influence of the finite Alfvén velocity on the evolution of an active region filament. In general, variations of a current result in variations of the magnetic fields which spread around with the Alfvén velocity. As a consequence of the fact that a magnetic field can only change with the Alfvén velocity, a filament will experience the photospheric boundary conditions as these were at an Alfvén travel time back in time. The inclusion of this retardation effect in the momentum equation of a filament leads effectively to an extra force term. This force contribution acts in the direction in which the filament moves and has therefore a destabilizing effect on the filament. Because a moving filament acts as an antenna of Alfvén waves, the filament loses energy by the emission process. This leads to a radiative damping term in the equation of motion of the filament. In general, the radiative damping will be sufficiently strong to counteract the retardation instability. Numerical simulations show that during the energy build-up phase a filament follows the van Tend-Kuperus equilibrium curve. After the van Tend-Kuperus equilibrium has disappeared the filament goes through a transient phase moving with a sub-Alfvénic velocity upward. At greater heights the repulsive Lorentz force of the photospheric surface current magnetic field is balanced by the radiative damping, resulting in a decreasing filament velocity.Parts of this paper were presented at the 4th CESRA Workshop in Ouranopolis (Greece) in 1991. 相似文献
9.
The effect of radiation losses on the dispersion and damping of magnetohydrodynamic waves in the solar corona is studied. The conditions are determined under which radiation losses are most appreciable. A damping of kink modes of coronal loops with plasma temperatures within 106–106.3 K and 106.3–107 K are compared. It is concluded that the radiation damping dominates in the temperature range 106–106.3 K, which can cause the observed fast damping of kink oscillations of coronal loops. Radiation losses should be taken into account in full magnetohydrodynamic equations with radiative transfer. 相似文献
10.
The degree of convective instability as expressed by the growth rate of linear modes, is calculated for a plane parallel polytropic atmosphere in the presence of radiative damping, without using Boussinesq approximation. A comparison with the results based on the Boussinesq approximation reveals that the use of the Boussinesq approximation leads to an overestimation of the radiative damping. The computation of as a function of the horizontal wave number yields a wavelength of maximal instability under a variety of conditions. For reasonable choices of physical parameters appropriate to the solar atmosphere, the fastest growing wavelengths turn out to be in the range 600–1200 km, and their e-folding times are in the range 200–2000 s.NAS-NRC Senior Research Associate on leave of absence from the Tate Institute of Fundamental Research, Bombay. 相似文献
11.
The effect of temperature inhomogeneity on the periods, their ratios (fundamental versus first overtone), and the damping times of the standing slow modes in gravitationally stratified solar coronal loops are studied. The effects of optically thin radiation, compressive viscosity, and thermal conduction are considered. The linearized one-dimensional magnetohydrodynamic (MHD) equations (under low-?? condition) were reduced to a fourth-order ordinary differential equation for the perturbed velocity. The numerical results indicate that the periods of nonisothermal loops (i.e., temperature increases from the loop base to apex) are smaller compared to those of isothermal loops. In the presence of radiation, viscosity, and thermal conduction, an increase in the temperature gradient is followed by a monotonic decrease in the periods (compared with the isothermal case), while the period ratio turns out to be a sensitive function of the temperature gradient and the loop lengths. We verify that radiative dissipation is not a main cooling mechanism in both isothermal and nonisothermal hot coronal loops and has a small effect on the periods. Thermal conduction and compressive viscosity are primary mechanisms in the damping of slow modes of the hot coronal loops. The periods and damping times in the presence of compressive viscosity and/or thermal conduction dissipation are consistent with the observed data in specific cases. By tuning the dissipation parameters, the periods and the damping times could be made consistent with the observations in more general cases. 相似文献
12.
The damping of standing slow waves in hot (T>6 MK) coronal loops of semicircular shape is revisited in both the linear and nonlinear regimes. Dissipation by thermal conduction,
compressive viscosity, radiative cooling, and heating are examined for nonstratified and stratified loops. We find that for
typical conditions of hot SUMER loops, thermal conduction increases the period of damped oscillations over the sound-crossing
time, whereas the decay times are mostly shaped by compressive viscosity. Damping from optically thin radiation is negligible.
We also find that thermal conduction alone results in slower damping of the density and velocity waves compared to the observations.
Only when compressive viscosity is added do these waves damp out at the same rate as the observed rapidly decaying modes of
hot SUMER loop oscillations, in contrast to most current work, which has pointed to thermal conduction as the dominant mechanism.
We compare the linear predictions with numerical hydrodynamic calculations. Under the effects of gravity, nonlinear viscous
dissipation leads to a reduction of the decay time compared to the homogeneous case. In contrast, the linear results predict
that the damping rates are barely affected by gravity. 相似文献
13.
We analyse a sample of bright long bursts and find that the pulse durations have a lognormal distribution while the intervals between pulses have an excess of long intervals (relative to lognormal distribution). This excess can be explained by the existence of quiescent times , long periods with no signal above the background level. The lognormal distribution of the intervals (excluding the quiescent times ) is similar to the distribution of the pulse widths. This result suggests that the quiescent times are made by a different mechanism than the rest of the intervals. It also suggests that the intervals (excluding the quiescent times ) and the pulse width are connected to the same parameters of the source. We find that there is a correlation between a pulse width and the duration of the interval preceding it. There is a weaker, but still a significant, correlation between a pulse width and the interval following it. The significance of the correlation drops substantially when the intervals considered are not adjacent to the pulse. 相似文献
14.
We examine the influence of nonadiabatic effects on the modes of an isothermal stratified magnetic atmosphere. The present investigation is a continuation of earlier work by Hasan and Christensen-Dalsgaard (1992) and Banerjee, Hasan, and Christensen-Dalsgaard (1995, 1996), where the interaction of various elementary modes in a stratified magnetized atmosphere was studied in the purely adiabatic limit. The inclusion of radiative dissipation based on Newton's law of cooling demonstrates the importance of this effect in the study of magnetoatmospheric waves. We analyze the physical nature of magnetoacoustic gravity (or MAG) oscillations in the presence of Newtonian cooling and find that the eigenfrequency curves in the diagnostic diagram, as in the previous analysis, undergo avoided crossings. However, the qualitative nature of the mode interaction is strongly influenced by radiative dissipation, which leads to strong mode damping in the avoided-crossing regions. We demonstrate this effect for the interaction between the Lamb mode and a magnetic mode. Our results could be important in the analysis of waves in flux tubes on the Sun. 相似文献
15.
Henrik N. Latter Gordon I. Ogilvie 《Monthly notices of the Royal Astronomical Society》2006,372(4):1829-1839
We investigate the growth or decay rate of the fundamental mode of even symmetry in a viscous accretion disc. This mode occurs in eccentric discs and is known to be potentially overstable. We determine the vertical structure of the disc and its modes, treating radiative energy transport in the diffusion approximation. In the limit of very long radial wavelength, an analytical criterion for viscous overstability is obtained, which involves the effective shear and bulk viscosity, the adiabatic exponent, and the opacity law of the disc. This differs from the prediction of a two-dimensional model. On shorter wavelengths (a few times the disc thickness), the criterion for overstability is more difficult to satisfy because of the different vertical structure of the mode. In a low-viscosity disc a third regime of intermediate wavelengths appears, in which the overstability is suppressed as the horizontal velocity perturbations develop significant vertical shear. We suggest that this effect determines the damping rate of eccentricity in protoplanetary discs, for which the long-wavelength analysis is inapplicable and overstability is unlikely to occur on any scale. In thinner accretion discs and in decretion discs around Be stars overstability may occur only on the longest wavelengths, leading to the preferential excitation of global eccentric modes. 相似文献
16.
A. M. Uralov 《Astronomy Letters》2003,29(7):486-493
We suggest a way of self-consistently solving the problem of the excitation and rapid damping of coronal loop oscillations observed from the TRACE (Transition Region and Coronal Explorer) satellite. Oscillations are excited on the dispersion branch of fast magnetoacoustic waves, which propagate mainly across the magnetic field. The rapid damping of the observed oscillations is governed by the dispersion spreading of the pulse of these waves that was produced, for example, by a solar flare. The fundamental oscillation period is close to the period of the fundamental mode. Dissipative processes attributable to the nonideality of the plasma and the coronal-loop footpoints play no fundamental role. 相似文献
17.
Using a theoretical model describing pulse shapes, we have clarified the relations between the observed pulses and their corresponding timescales, such as the angular spreading time, the dynamic time as well as the cooling time. We find that the angular spreading timescale caused by curvature effect of fireball surface only contributes to the falling part of the observed pulses, while the dynamic one in the co‐moving frame of the shell merely contributes to the rising portion of pulses provided the radiative time is negligible. In addition, the pulses resulted from the pure radiative cooling time of relativistic electrons exhibit properties of fast rise and slow decay (a quasi‐FRED) profile together with smooth peaks. Besides, we interpret the phenomena of wider pulses tending to be more asymmetric to be a consequence of the difference in emission regions. Meanwhile, we find the intrinsic emission time is decided by the ratios of lorentz factors and radii of the shells between short and long bursts. Based on the analysis of asymmetry, our results suggest that the long GRB pulses may occur in the regions with larger radius, while the short bursts could locate at the smaller distance from central engine. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
18.
M. Godart A. Noels M.-A. Dupret Y. Lebreton 《Monthly notices of the Royal Astronomical Society》2009,396(4):1833-1841
Thanks to their past history on the main-sequence phase, supergiant massive stars develop a convective shell around the helium core. This intermediate convective zone (ICZ) plays an essential role in governing which g-modes are excited. Indeed, a strong radiative damping occurs in the high-density radiative core but the ICZ acts as a barrier preventing the propagation of some g-modes into the core. These g-modes can thus be excited in supergiant stars by the κ-mechanism in the superficial layers due to the opacity bump of iron, at log T = 5.2 . However, massive stars are submitted to various complex phenomena such as rotation, magnetic fields, semiconvection, mass loss, overshooting. Each of these phenomena exerts a significant effect on the evolution and some of them could prevent the onset of the convective zone. We develop a numerical method which allows us to select the reflected, thus the potentially excited, modes only. We study different cases in order to show that mass loss and overshooting, in a large enough amount, reduce the extent of the ICZ and are unfavourable to the excitation of g-modes. 相似文献
19.
We investigate the damping of longitudinal (i.e., slow or acoustic) waves in nonisothermal, hot (T≥ 5.0 MK), gravitationally stratified coronal loops. Motivated by SOHO/SUMER and Yohkoh/SXT observations, and by taking into account a range of dissipative mechanisms such as thermal conduction, compressive viscosity, radiative cooling, and heating, the nonlinear governing equations of one-dimensional hydrodynamics are solved numerically for standing-wave oscillations along a magnetic field line. A semicircular shape is chosen to represent the geometry of the coronal loop. It was found that the decay time of standing waves decreases with the increase of the initial temperature, and the periods of oscillations are affected by the different initial footpoint temperatures and loop lengths studied by the numerical experiments. In general, the period of oscillation of standing waves increases and the damping time decreases when the parameter that characterises the temperature at the apex of the loop increases for a fixed footpoint temperature and loop length. A relatively simple second-order scaling polynomial between the damping time and the parameter determining the apex temperature is found. This scaling relation is proposed to be tested observationally. Because of the lack of a larger, statistically relevant number of observational studies of the damping of longitudinal (slow) standing oscillations, it can only be concluded that the numerically predicted decay times are well within the range of values inferred from Doppler shifts observed by SUMER in hot coronal loops. 相似文献
20.
P. Nicolaï C. Stenz V. Tikhonchuk X. Ribeyre A. Kasperczuk T. Pisarczyk L. Juha E. Krousky K. Masek M. Pfeifer K. Rohlena J. Skala J. Ullschmied M. Kalal D. Klir J. Kravarik P. Kubes P. Pisarczyk 《Astrophysics and Space Science》2009,322(1-4):11-17
The interaction of supersonic plasma jets with dense gases and plasmas has been studied experimentally and theoretically. Collimated plasma jets were generated from the laser pulse interaction with solid targets. The jet propagates with the velocity exceeding 400 km/s and transports the energy of a few kJ/cm2. The interaction of such a jet with an Ar and He gases at various pressures has been studied by using optical and X-ray diagnostics. Qualitative estimates and numerical simulations with a radiative hydrodynamic code explain a sequence of physical processes during the interaction. Experimental and numerical results show that, by changing ambient material, the working surface structure changes from an adiabatic outflow to a radiative cooling jet. The applications of this phenomenon to astrophysical conditions and the inertial confinement fusion are discussed. 相似文献