共查询到20条相似文献，搜索用时 812 毫秒
1.
D. B. Melrose 《Solar physics》1989,119(2):385-398
A suggested application of the theory of wave localization to type III solar radio events in the solar wind is discussed critically. A classical wave theory that enables one to relate wave localization to the observed spectrum of density fluctuations is summarized. Localization (in one dimension) is due to backscattering and depends on the density spectrum at a wavenumber equal to twice that of the scattered wave. The localization length is estimated for the Langmuir waves, for which the appropriate density fluctuations require ion sound waves, and for transverse waves, for which (at least for the fundamental) the spectrum of the appropriate density fluctuations has been measured in situ. In all cases the localization length is much shorter than the size of a type III event. For fundamental radiation the localization length can be even shorter than the observed sizes of clumps of Langmuir waves.It is concluded that although wave localization may be significant in type III events, most of its consequences have already been recognized in models that invoke multiple scattering. A notable exception is localization of fundamental transverse waves to the clumps of Langmuir waves, which provides a natural explanation for the observed brightness temperatures and for the initial predominance of fundamental over harmonic emission. 相似文献
2.
P. A. Robinson 《Solar physics》1992,139(1):147-163
A model is developed for the clumpy Langmuir waves observed in type III source regions. In this model the waves are generated by instability of a beam which propagates outward from the Sun in a state close to marginal stability. Ambient density perturbations cause fluctuations about the marginally stable state, leading to nonuniformities in both beam and waves and, hence, to spatially inhomogeneous growth. High damping rates and high wave levels are strongly anti-correlated, leading to suppression of the net damping. Below saturation stochastic growth causes the waves to follow a random walk in the logarithm of their energy density and the resulting probability of observing a field of magnitude E is approximately proportional to E
-1. Comparison with observations shows that this model can account for the levels and clumpiness of the Langmuir waves, the small net dissipation required for the beams to propagate to 1 AU, the characteristic decay time of type III electromagnetic emission, and the negative mean growth rate observed in situ in type III sources. At 1 AU only the very highest fields approach the threshold for nonlinear wave collapse, but this threshold may be more commonly exceeded closer to the Sun. 相似文献
3.
The exact nonlinear cylindrical solution for incompressible Hall – magnetohydrodynamic (HMHD) waves, including dissipation,
essentially from electron – neutral collisions, is obtained in a uniformly rotating, weakly ionized plasma such as exists
in photospheric flux tubes. The ω – k relation of the waves, called here Hall – MHD waves, demonstrates the dispersive nature of the waves, introduced by the Hall
effect, at large axial and radial wavenumbers. The Hall – MHD waves are in general elliptically polarized. The partially ionized
plasma supports lower frequency modes, lowered by the factor δ≡ratio of the ion mass density to the neutral particle mass density, as compared to the fully ionized plasma (δ=1). The relation between the velocity and the magnetic field fluctuations departs significantly from the equipartition found
in Alfvén waves. These short-wavelength and arbitrarily large amplitude waves could contribute toward the heating of the solar
atmosphere. 相似文献
4.
D. B. Melrose 《Solar physics》1989,120(2):369-381
There is a characteristic maximum brightness temperature T
B 1015K for type III solar radio bursts in the solar wind. The suggestion is explored that the maximum observed values of T
Bmay be attributed to saturation of the processes involved in the plasma emission. The processes leading to fundamental and second harmonic emission saturate when T
Bis approximately equal to the effective temperature T
Lof the Langmuir waves. The expected maximum value of T
Bis estimated for this saturation model in two ways: from the growth rate for the beam instability, and from the maximum amplitude of the observed Langmuir turbulence. The agreement with the observed values is satisfactory in view of the uncertainties in the estimates (a) of the intrinsic brightness temperature from the observed brightness temperature, (b) of the actual growth rate of the beam instability, which must be driven by local, transient features (that are unobservable using available instruments) in the electron distribution, and (c) in the k-space volume filled by the Langmuir waves, and this is consistent with the observational data on two well-studied events at the orbit of the Earth and with statistical data for events over a range of radial distances from the Sun. 相似文献
5.
P. A. Robinson 《Solar physics》1996,168(2):357-374
Energy-balance arguments are combined with the stochastic-growth theory of type III radio sources to determine the properties of the source in average dynamical equilibrium with the beam, and the beam's long-term evolution. Purely linear stochastic-growth theory has previously emphasized that the beam evolves to a state close to marginal stability. Small mean residual deviations from marginal stability are present at dynamical equilibrium and these lead to residual energy flows that feed the waves observed in situ and by remote receivers; consequently the beam energy is depleted. Here, dynamical equilibrium beam and wave levels are estimated for the first time and it is found that the main sink of beam-driven Langmuir waves is either via electrostatic decay into product Langmuir and ion-sound waves or via scattering by short-wavelength density fluctuations, depending on the conditions. Improved estimates of energy branching ratios imply that, at 1 AU from the Sun, typically 20% of the beam energy is converted to Langmuir waves that are scattered off low-frequency density fluctuations and then dissipated, with almost all the remaining waves undergoing electrostatic decay, although as little as one-third of the Langmuir waves may decay in atypical circumstances. Of order 10–3 of the beam energy is converted into sound waves, which are mostly dissipated, and of order 10–5 is converted into potentially observable electromagnetic waves. The mean lifetime of the Langmuir waves at 1 AU is 1–40 s, while that of the beam is of order 1000 s. The beam density decreases relative to that of the background as the beam propagates. For most parameters, analysis of energy losses from the beam to the waves shows that the beam velocity decreases at roughly the same rate as the thermal velocity of the background plasma. It is argued from these considerations, and from in situ observations at 1 AU, that these trends imply that only the densest and fastest type III beams will be able to penetrate much past 1 AU from the Sun. This implies a low-frequency cutoff to type III emission at roughly 10 kHz, in good agreement with recent Ulysses remote observations, showing their consistency with in situ measurements. 相似文献
6.
Peter Goldreich 《Astrophysics and Space Science》2001,278(1-2):17-23
The inertial range of incompressible MHD turbulence is most conveniently described in terms of counter propagating waves.
Shear Alfvén waves control the cascade dynamics. Slow waves play a passive role and adopt the spectrum set by the shear Alfvén
waves. Cascades composed entirely of shear Alfvén waves do not generate a significant measure of slow waves. MHD turbulence
is anisotropic with energy cascading more rapidly along k
⊥ than along k
∥. Anisotropy increases with k
⊥ such that the excited modes are confined inside a cone bounded by k
∥∝ k
perp
2/3. The opening angle of the cone, θ(k
⊥)∝ k
⊥
-1/3, defines the scale dependent anisotropy. MHD turbulence is generically strong in the sense that the waves which comprise
it are critically damped. Nevertheless, deep inside the inertial range, turbulent fluctuations are small. Their energy density
is less than that of the background field by a factor θ2(k
⊥)≪. MHD cascades are best understood geometrically. Wave packets suffer distortions as they move along magnetic field lines
perturbed by counter propagating wave packets. Field lines perturbed by unidirectional waves map planes perpendicular to the
local field into each other. Shear Alfvén waves are responsible for the mapping's shear and slow waves for its dilatation.
The former exceeds the latter by θ-1(k
⊥)≫ 1 which accounts for dominance of the shear Alfvén waves in controlling the cascade dynamics.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
David Tsiklauri 《Solar physics》2010,267(2):393-410
1.5D Vlasov – Maxwell simulations are employed to model electromagnetic emission generation in a fully self-consistent plasma
kinetic model for the first time in the context of solar physics. The simulations mimic the plasma emission mechanism and
Larmor-drift instability in a plasma thread that connects the Sun to Earth with the spatial scales compressed appropriately.
The effects of spatial density gradients on the generation of electromagnetic radiation are investigated. It is shown that
a 1.5D inhomogeneous plasma with a uniform background magnetic field directed transverse to the density gradient is aperiodically
unstable to the Larmor-drift instability. The latter results in a novel effect of generation of electromagnetic emission at
plasma frequency. The generated perturbations consist of two parts: i) non-escaping (trapped) Langmuir type oscillations, which are localised in the regions of density inhomogeneity, and are
highly filamentary, with the period of appearance of the filaments close to electron plasma frequency in the dense regions;
and ii) escaping electromagnetic radiation with phase speeds close to the speed of light. When the density gradient is removed (i.e. when plasma becomes stable to the Larmor-drift instability) and a low density super-thermal, hot beam is injected along the domain, in the direction perpendicular to the magnetic field, the plasma emission
mechanism generates non-escaping Langmuir type oscillations, which in turn generate escaping electromagnetic radiation. It
is found that in the spatial location where the beam is injected, standing waves, oscillating at the plasma frequency, are
excited. These can be used to interpret the horizontal strips (the narrow-band line emission) observed in some dynamical spectra.
Predictions of quasilinear theory are: i) the electron free streaming and ii) the long relaxation time of the beam, in accord with the analytic expressions. These are corroborated via direct, fully-kinetic
simulation. Finally, the interplay of the Larmor-drift instability and plasma emission mechanism is studied by considering
a dense electron beam in the Larmor-drift unstable (inhomogeneous) plasma. The latter case enables one to study the deviations from
the quasilinear theory. 相似文献
8.
We perform the correlation and spectral analysis of phase-space density and potential fluctuations in a model of an open star cluster for various values of the smoothing parameter ? of the force functions in the equations of motion of cluster stars, and compute the mutual correlation functions for the fluctuations of potential U and phase-space density f of the cluster model at different clustercentric distances. We use the Fourier transform of the mutual correlation functions to compute the power spectra and dispersion curves of the potential and phase-space density fluctuations. The spectrum of potential fluctuations proves to be less complex than that of phase-space density fluctuations. The most powerful potential fluctuations are associated with phase-space density fluctuations, and their spectrum lies in the domain of low frequencies ν < 3/τ v.r.; at intermediate and high frequencies (ν > 3/τ v.r.), the contribution of potential fluctuations to those of the phase-space density is small or equal to zero (here τ v.r. is the violent relaxation time scale of the cluster). We find a number of unstable potential fluctuations in the core of the cluster model (up to 30 pairs of fluctuations with different complex conjugate frequencies). We also find and analyze the dependences of the spectra and dispersion curves of phase-space density and potential fluctuations on ?. We find a “repeatability” (significant correlation) of the spectra at some values of parameter ?. The form of the dispersion curve is unstable against small variations of ?. We discuss the astrophysical applications of our results: the break-up in the cluster core of the phase-space density wave running from the cluster periphery toward its center into several waves with frequencies commensurable to that of the external (tidal) influence; emission and reflection of phase-space and potential waves near the cluster core boundary; possible wavelength and phase discretization of the phase-space and potential waves in the cluster model. 相似文献
9.
S. Grzędzielski 《Solar physics》1971,21(1):225-236
The propagation of non-radial, small amplitude perturbations superposed on a zero-order, stationary, non-magnetic, polytropic, rotating stellar wind is studied in the limit of the local theory, i.e. for k r 1, k being the module of the wave vector and r the characteristic scale of the zero-order flow. The resulting dispersion equation is of the 3rd order in (complex) frequency and the possible modes correspond to two acoustic type waves, and to a gravity-shear wave with strongly anisotropic propagation properties, due to coupling between the internal gravity waves and shear motion. The gravity-shear mode allows velocity differences in the medium to exist with no corresponding density fluctuations and hence with no shock wave formation. It is suggested that this mode corresponds to some of the fast-slow velocity streams observed in the interplanetary medium and may provide means for wave energy being transported outwards with the zero-order flow, with little dissipation in the inner region of the solar wind. 相似文献
10.
It was found recently that fast electrons travel through the plasma of the solar corona in the form of beam-plasma structure
(BPS), which consists of electrons and Langmuir waves. In this paper the influence of scattering BPS Langmuir waves off plasma
ions (l+i=l+i) on BPS velocity is studied. We show that the maximum BPS velocity equals 0.35c, which is close to the velocity of Type III bursts sources. 相似文献
11.
Nonlinear resonant interactions of different kinds of fast magnetosonic (FMS) waves trapped in the inhomogeneity of a low- plasma density, stretched along a magnetic field (as, for example, in coronal loops) are investigated. A set of equations describing the amplitudes of interactive modes is derived for an arbitrary density profile. The quantitative characteristics of such interactions are found. The decay instability of the wave with highest frequency is possible in the system. If amplitudes of interactive modes have close values, the long-period temporal and spatial oscillations are in the system.For a quantitative illustration, the parabolic approximation of the transverse density profile has been chosen. Dispersion relations of FMS waves trapped in a low- plasma slab with a parabolic transverse density profile are found. The transverse structure of the waves in this case can be expressed through Hermitian polynomials. The interaction of kink and sausage waves is investigated. The sausage wave, with a sufficiently large amplitude, may be unstable with respect to the decay into two kink waves, in particular. The spatial scale of a standing wave structure and the time spectrum of radiation are formed due to the nonlinear interactions of loop modes which contain information about the parameters of the plasma slab. 相似文献
12.
The adiabatic theory of interaction between high and low frequency waves has been studied for the case of electron plasma oscillations and ion acoustic waves and the results are applied to the solar wind. The modified dispersion relation for ion acoustic waves has been derived, taking a Gaussian distribution for plasmons. Two limiting cases of the spectrum are studied. For a broad spectrum, the plasma turbulence has a destabilising effect by introducing a growth rate denoted by turbulence, which is positive for k
0 > (m
e/
m
i
)1/2
De
–1
, k
0 being the central wave numger of the spectrum,
De the electron Debye length. Also, even for v
d(drift velocity between electrons and ions) < c
s, we arrive at unstable ion acoustic modes. For narrow spectrum, the plasma turbulence has a stabilising effect. 相似文献
13.
Recent models of the heating and acceleration of the fast solar wind invoke the dissipation of relatively high-frequency waves by ion-cyclotron absorption. It is shown that modulations in the heating rate induced by compressive perturbations in the flow may drive the system unstable in the supersonic region. Therefore it is possible that density fluctuations generated by this instability and maintained at a level determined by the marginal stability condition are manifest in the wind. 相似文献
14.
The dynamical evolution of six open star cluster models is analyzed using the correlation and spectral analysis of phase-space density fluctuations. The two-time and mutual correlation functions are computed for the fluctuations of the phase-space density of cluster models. The data for two-time and two-particle correlations are used to determine the correlation time for phase-space density fluctuations ((0.1–1) τ v.r., where τ v.r. is the violent relaxation time of the model) and the average phase velocities of the propagation of such fluctuations in cluster models. These velocities are 2–20 times smaller than the root mean square velocities of the stars in the cluster core. The power spectra and dispersion curves of phase-space density fluctuations are computed using the Fourier transform of mutual correlation functions. The results confirm the presence of known unstable phase-space density fluctuations due to homologous fluctuations of the cluster cores. The models are found to exhibit a number of new unstable phase-space density fluctuations (up to 32–41 pairs of fluctuations with different complex conjugate frequencies in each model; the e-folding time of the amplitude growth of such fluctuations is (0.4–10) τ v.r. and their phases are distributed rather uniformly). Astrophysical applications of the obtained results (irregular structure of open star clusters, formation and decay of quasi-stationary states in such clusters) are discussed. 相似文献
15.
We study the fundamental modes of radiation hydrodynamic waves arising from one-dimensional small-amplitude initial fluctuations
with wave number k in a radiating and scattering grey medium using the Eddington approximation. The dispersion relation analyzed is the same
as that of Paper I (Kaneko et al., 2000), but is solved as a quintic in angular frequency ω while a quadratic in k
2 in Paper I. Numerical results reveal that wave patterns of five solutions are distinguished into three types of the radiation-dominated
and type 1 and type 2 matter-dominated cases. The following wave modes appear in our problem: radiation wave, conservative
radiation wave, entropy wave, Newtonian-cooling wave, opacity-damped and cooling-damped waves, constant-volume and constant-pressure
diffusion modes, adiabatic sound wave, cooling-damped and drag–force-damped isothermal sound waves, isentropic radiation-acoustic
wave, and gap mode. The radiation-dominated case is characterized by the gap between the isothermal sound and isentropic radiation-acoustic
speeds within which there is not any acoustic wave propagating with real phase speed. One of the differences between type
1 and type 2 matter-dominated cases is the connectivity of the constant-volume diffusion mode, which originates from the radiative
mode in the former case, while from the Newtonian-cooling wave in the latter case. Analytic solutions are derived for all
wave modes to discuss their physical significance. The criterion, which distinguishes between radiation-dominated and type
1 matter-dominated cases, is given by Γ0 = 9, where Γ0 = C
p
(tot)/C
V
(tot) is the ratio of total specific heats at constant pressure and constant volume. Waves in a scattering grey medium are also
analyzed, which provides us some hints for the effects of energy and momentum exchange between matter and radiation. 相似文献
16.
We study the fundamental modes of radiation hydrodynamic linear waves that arise from one-dimensional small-amplitude initial fluctuations with wave number k in a radiating and scattering grey medium by taking into account the gravitational effects. The equation of radiative acoustics is derived from three hydrodynamic equations, Poisson’s equation, and two moment equations of radiation, by assuming a spherical symmetry for the matter and radiation and by using the Eddington approximation. We solve the dispersion relation as a quintic function of angular frequency ω, the wave number k being a real parameter. Numerical results reveal that wave patterns of five solutions are distinguished into three types: the radiation-dominated, type 1, and type 2 matter-dominated cases. In the case of no gravitaional effects (Kaneko et al., 2005), the following wave modes appear: radiation wave, conservative radiation wave, entropy wave, Newtonian-cooling wave, opacity-damped and cooling-damped waves, constant-volume and constant-pressure diffusions, adiabatic sound wave, cooling-damped and drag-force-damped isothermal sound waves, isentropic radiation-acoustic wave, and gap mode. Meanwhile, the gravitaional effects being taken into account, the growing gravo-diffusion mode newly arises from the constant-pressure diffusion at the point that k agrees with Jeans’ wave number specified by the isothermal sound speed. This mode changes to the growing radiation-acoustic gravity mode near the point that k becomes Jeans’ wave number specified by the isentropic radiation-acoustic speed. In step with a transition between them, the isentropic radiation-acoustic wave splits into the damping radiation-acoustic gravity mode and constant-volume diffusion. The constant-volume diffusion emerges twice if the gravitational effects are taken into account. Since analytic solutions are derived for all wave modes, we discuss their physical significance. The critical conditions are given which distinguish between radiation-dominated and type 1 matter-dominated cases, and between type 1 and type 2 matter-dominated cases. Waves in a self-gravitating scattering grey medium are also analyzed, which provides us some hints for the effects of energy and momentum exchange between matter and radiation. 相似文献
17.
B. Buti 《Astrophysics and Space Science》1978,55(1):85-92
A non-linear Schrödinger equation which characterizes the non-linear electrostatic waves in collisionless turbulent plasma is derived. Detailed analysis of this equation for the non-linear Langmuir waves is presented to show how the ion dynamics affects the envelope behaviour of these waves. Necessary condition for the existence of Langmuir envelope solitons is found to bek
2
D
2
(m/M);k being the characteristic wave number, D the electron Debye length andm andM the electron and the proton mass.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May, 1978. 相似文献
18.
The influence of turbulence on the frequencies of free acoustic modes in convection zones is considered. The frequencies are modified via the speed of sound by the turbulence-induced alterations of the effective pressure: (i) by the correlated fluctuations of temperature and density and (ii) the pressure part of the Reynolds stress. The two effects shift the frequency of low l p-modes in opposite directions. In addition, the correlation of the density fluctuations with the random velocity — the eddy-mass flow — is also relevant. It is, in a steady state, balanced by a vertical mean velocity. The balance results in a rather small net effect completely disappearing for highly nonradial oscillations. Both effects of the density fluctuations produce a redshift of the low l p-mode frequencies. The Reynolds stress, however, makes a blueshift of the frequencies relative to that computed for a laminar gas. This effect dominates for subsonic turbulences. The applied second-order correlation-approximation, however, only holds for the lowest frequencies, where the KORONAS (solar minimum) data are indicating a blueshift. Of particular importance for the present concept is the expected cycle-variations of the lineshifts, i.e. the consideration of the magnetic modification of the various contributions. Observations may show whether the suggested modifications of the solar oscillation theory are correct. 相似文献
19.
G. Thejappa 《Solar physics》1991,132(1):173-193
A self-consistent theoretical model for storm continuum and bursts is presented. We propose that the Langmuir waves are emitted spontaneously by an anisotropic loss-cone distribution of electrons trapped in the magnetic field above active regions. These high-frequency electrostatic waves are assumed to coalesce with lower-hybrid waves excited either by the trapped protons or by weak shocks, making the observed brightness temperature equal to the effective temperature of the Langmuir waves.It is shown that whenever the collisional damping (
c
) is more than the negative damping (-
A
) due to the anisotropic distribution, there is a steady emission of Langmuir waves responsible for the storm continuum. The type I bursts are generated randomly whenever the collisional damping (
c
) is balanced by the negative damping (-
A
) at the threshold density of the trapped particles, since it causes the effective temperature of Langmuir waves to rise steeply. The number density of the particles responsible for the storm radiation is estimated. The randomness of type I bursts, brightness temperature, bandwidth and transition from type I to type III storm are self-consistently explained.On leave from Indian Institute of Astrophysics, Bangalore 560034, India. 相似文献
20.
We develop a theory for radar signal scattering by anisotropic Langmuir turbulence in the solar corona due to a t+l ⇄ t process. Langmuir turbulence is assumed to be generated within a cone by a narrow type III burst electron beam. Using wave-kinetic
theory we obtain expressions for the frequency shift, scattering cross-section of the turbulence, coefficient of absorption
(due to scattering) and optical depth. On the basis of those expressions we give some estimates for an echo spectrum. We show
that the minimum radar echo frequency shift is determined by the minimal phase velocity of the Langmuir waves, the maximum
shift is determined by the electron beam velocity, but in any case it can not exceed −wt/2 (decay) and wt (coalescence), where
wt is the frequency of a radar signal. The angular characteristics of the scattered signal differ dramatically for the cases
of coalescence and decay. The signal is scattered into a narrow cone high above the specular reflection point (wp ≪ wt), but
in the vicinity of wp ∼ wt/2 the red-shifted echo is scattered isotropically, while the blue-shifted echo is scattered into
a even narrower cone. We show that absorption (due to scattering) increases with increasing radar frequency. The dependence
of the absorption on the local plasma frequency is strongly determined by the Langmuir turbulence spectrum. Our theory shows
that the role of the nonlinear scattering process t+l ⇄ t is essential and that such process can be used for radar studies of the spectral energy density of anisotropic Langmuir turbulence. 相似文献