共查询到20条相似文献，搜索用时 660 毫秒
1.
H. Alfvén 《Astrophysics and Space Science》1978,54(2):279-292
The methodology of cosmic plasma physics is discussed. It is very hazardous to try to describe plasma phenomena by theories which have not been carefully tested experimentally. One present approach is to rely on laboratory measurements andin situ measurements in the magnetosphere and heliosphere, and to approach galactic phenomena by scaling up the wellknown phenomena to galactic dimensions. A summary is given of laboratory investigations of electric double layers, a phenomenon which is known to be very important in laboratory discharges. A summary is also given of thein situ measurements in the magnetosphere by which the importance of electric double layers in the Earth's surrounding is established. The scaling laws between laboratory and magnetospheric double layers are studied. The successful scaling between laboratory and magnetospheric phenomena encourages an extrapolation to heliospheric phenomena. A further extrapolation to galactic phenomena leads to a theory of double radio sources.In analogy with the Sun which, acting as a homopolar inductor, energizes the heliospheric current system, a rotating magnetized galaxy should produce a similar current system. From analogy with laboratory and magnetospheric current systems it is argued that the galactic current might produce double layers where a large energy dissipation takes place. This leads to a theory of the double radio sources which, within the necessary wide limits of uncertainty, is quantitatively reconcilable with observations. 相似文献
2.
3.
Lennart Lindberg 《Astrophysics and Space Science》1988,144(1-2):3-13
Observations of current disruptions and strong electric fields along the magnetic field in a high-density (2×1019 m–3), highly-ionized, moving, and expanding plasma column are reported. The electric field is interpreted in terms of propagating, strong electric double layers (3–5 kV).An initial plasma column is formed in an axial magnetic field (0.1 T) by means of a conical theta-pinch plasma gun. When an axial current (max 5 kA, 3–5 kV) is drawn through the column spontaneous disruptions and double-layer formation occur within a few microseconds.Floating, secondary emitting Langmuir probes are used. They often indicate very high positive potential peaks (1–2 kV above the anode potential during a few s) in the plasma on the positive side of the double layer. Short, intense bursts of HF radiation are detected at the disruptions.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988. 相似文献
4.
Kent R. Davey 《Astrophysics and Space Science》1983,95(1):47-63
After examining some of the background work relevant to double layers, the role constant current driven systems might play in the double layer formation is examined from two perspectives. First, a fluids analysis is considered via the method of characteristics. A density perturbation growth is predicted. Two model deficiencies, zero electron mass and lack of particle kinematics, suggest an improvement in the model via the Vlasov equation. This second model using the Vlasov equation still maintains a non-quasineutral posture. In addition to predicting the expected two-stream instability, the model also predicts a double layer-type electric field. 相似文献
5.
Jan Martin Winters 《Astrophysics and Space Science》1997,255(1-2):257-266
The problem of dust formation in the circumstellar envelopes of Asymptotic Giant Branch stars is addressed. We summarize the
basic thermodynamic prerequisites necessary to enable the formation and growth of solid particles from the gas phase and draw
some conclusions on the evolution of the emergent dust component. In a circumstellar environment the dust grains interact
with the stellar radiation field, which leads to a strong coupling among the local thermodynamic conditions and the dust formation
process itself. By a consistent treatment of the physics describing the dust forming circumstellar shells of evolved stars
we demonstrate, that the non-linear interaction among the dust formation process and the hydrodynamic and thermodynamic conditions
of the dust forming system leads to a complex dynamical structure of these shells. Some observable consequences resulting
from corresponding model calculations are given.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
6.
Using a one-dimensional electrostatic particle code, we examine processes associated with current interruption in a collisionless plasma when a density depression is present along the current channel. Current interruption due to double layers was suggested by Alfvén and Carlqvist (1967) as a cause of solar flares. At a local density depression, plasma instabilities caused by an electron current flow are accentuated, leading to current disruption. Our simulation study encompasses a wide range of the parameters in such a way that under appropriate conditions, both the Alfvén and Carlqvist (1967) regime and the Smith and Priest (1972) regime take place. In the latter regime the density depression decays into a stationary structure (ion-acoustic layer) which spawns a series of ion-acoustic solitons and ion phase space holes travelling upstream. A large inductance of the current circuit tends to enhance the plasma instabilities. 相似文献
7.
P. Carlqvist 《Astrophysics and Space Science》1982,87(1-2):21-39
A model of a strong, time-independent, and relativistic double layer is studied. Besides double layers having the electric field parallel to the current the model also describes a certain type of oblique double layers. The Langmuir condition (ratio of ion current density to electron current density) as well as an expression for the potential drop of the double layer are derived. Furthermore, the distributions of charged particles, electric field, and potential within the double layer are clarified and discussed. It is found that the properties of relativistic double layers differ substantially from the properties of corresponding non-relativistic double layers. 相似文献
8.
The nonlinear excitation of fast magnetosonic waves by phase mixing Alfvén waves in a cold plasma with a smooth inhomogeneity of density across a uniform magnetic field is considered. If initially fast waves are absent from the system, then nonlinearity leads to their excitation by transversal gradients in the Alfvén wave. The efficiency of the nonlinear Alfvén–fast magnetosonic wave coupling is strongly increased by the inhomogeneity of the medium. The fast waves, permanently generated by Alfvén wave phase mixing, are refracted from the region with transversal gradients of the Alfvén speed. This nonlinear process suggests a mechanism of indirect plasma heating by phase mixing through the excitation of obliquely propagating fast waves. 相似文献
9.
Eugen Willerding 《Planetary and Space Science》1998,46(11-12)
In this paper we investigate both the global and the local hydrodynamics of axisymmetric accretion disks around young stellar objects under the simultaneous action of viscosity, self-gravity and pressure forces. For simplicity, we take for the global model a polytropic equation of state, make the infinitely thin disk approximation and characterize the surface density and temperature profiles in the disk as power laws in the radial distance r from the protostar. We solve the problem of the general density profile of a Keplerian disk showing that self-gravity could not be an important factor for the fast formation of the rocky cores of giant gaseous planets in our solar system. Under the hypothesis that the unperturbed rotation curve of the disk is nearly Keplerian throughout the radial extent, we can estimate with our polytropic model a lower limit for the resulting masses Md(r) of stable disks up to 100 AU. These masses are in the range of the so-called minimum mass solar nebular (d/Ms ≈ 0.01–0.02).By adopting a simplified viscosity model, where the height-integrated turbulent dynamical viscosity ν is a function of the surface density σ like η ∝ σΓ, we derive in the local shearing sheet model linearized evolution equations for small density perturbations describing both a diffusion process and the propagation of acoustic density waves. We solve a special initial value problem and calculate the appropriate Green's function. The analytical solutions so obtained describe in the case Γ < 0 the successive formation of quasi-stationary ring-shaped density structures in a disk with a definite mode of maximum instability, whereas in the case Γ > Γc the density wave equation describes the propagation of an “overstable” ring-shaped acoustic density wavelet to the outer ranges of the accretion disk. Whereas the group velocity of the wave packet is subsonic, the phase velocities of individual wave crests in the wave packet are supersonic. The mode of maximum instability, the growth rate and the number of growing waves in the wavelet are controlled by Γ and α. Our present knowledge concerning turbulent viscosity in protoplanetary disks is not sufficient to decide whether or not the case Γ > Γc is realized.The suggested structuring processes in the linear theory should initiate in the non-linear regime the formation of narrow ring-shaped density shock waves moving through the protoplanetary disk. These non-linear waves could produce extremely spatially and temporally heterogeneous temperature regions in the disk. We speculate that ring-shaped density waves, excited by inner boundary conditions and which have dominated the disk's evolution at early times, are responsible both for the fast growth of dust to planetesimals and at least for the rapid accretion of the rocky cores of giant gaseous planets in the protoplanetary accretion disk (shock wave trigger hypothesis). We derive provisional scaling rules for planetary systems regarding the spacing of orbits as a function of the mass ratio of the protoplanetary disk to the protostar. However, further analytical work and linear as well as nonlinear numerical simulations of density waves excited by inner boundary conditions are needed to consolidate the results and speculations of our linear wave mechanics in the future. 相似文献
10.
The localized ion density depletion region observed at comet Halley can be understood in terms of a thermal instability of the cometary plasma due to the excitation of rotational and vibrational levels of water molecules. The electron energy losses due to these processes peak near 4000 K and at higher temperatures a localized cooling leads to the thermal instability due to the increased radiation loss. The resulting increase in recombination leads to the ion density depletion and the estimates for this depletion at comet Halley agree with the observations. The transport effects at the density depletion region due to the diffusion and thermal conductivity are found have time scales larger than the instability time scale and does not affect the formation of the depletion region. 相似文献
11.
It is shown by numerical simulations that enhanced current density can generate double layers, even when the electron drift speed is significantly below the electron thermal speed. The double layer potential is spontaneously produced by the space charge self-consistently developed inside the simulation domain. The particle influxes from the low-potential boundary of our simulation domain are independent of the outfluxes. The potential difference φ0 is shown increase with increasing number density of the injection current. Strong double layers with potential energy eφ0 ? kT0 (the electron thermal energy) are stably formed when the injection electron current much exceeds the thermal current of ambient electrons. The backscattered and mirrored electrons are found to have stabilizing effects on the current-driven double layers. 相似文献
12.
Coherent structures entailing the existence of double layers have been studied in magnetised plasma contaminated with dust
charging fluctuations. It has been shown that the dust charging in magnetic plasma leads to complexity in the derivation of
the Sagdeev wave equation, but under way new procedure enable one to study the nature of double layers showing the effective
role of the constituents of the plasma. A parametric analysis is a subject of interest in laboratory and space plasmas, and
it has been explained with the input of various typical plasma numerics. The proposed mathematical mechanism has shown the
success to yield plasma acoustic modes in a dusty plasma which, in turn, has been solved convincingly for double layers. Observations
have been evaluated in an appropriate model with a view to agree with the observations in astrophysical problems dealing with
present new findings. 相似文献
13.
14.
B. Vršnak 《Solar physics》1989,120(1):79-92
The properties and development of a high-temperature current sheet characterized by increasing merging velocity are studied and related to the early phases of solar flares. It is shown that the system can be described by the Petschek-type geometry for a wide range of merging velocities. In the diffusion region and the standing MHD shocks a certain low-frequency plasma microturbulence is generated from the very beginning of the reconnection process. We present qualitative solutions for the case of ion-acoustic turbulence in marginally stable state, which provide a comparison with observations. The increasing merging velocity leads to the appearance of the soft X-ray precursor. The precursor temperature maximum should appear during the current sheet formation, before the Petschek regime is established. In the Petschek regime the temperature of the hot plasma decreases due to the decrease of the magnetic field strength at the diffusion region boundary, while the soft X-ray radiation still increases, reaching precursor maximum for merging velocities about 1% of the external Alfvén velocity. The precursor phase ends when the value of the merging velocity surpasses the upper limit for the Petschek regime and the system enters into the pile-up regime, causing a new increase of plasma temperature and soft X-ray radiation.It is shown that Alfvén velocities in the range 800–1200 km s –1 are sufficient to explain typical soft X-ray precursors. Cases of low merging velocities and low Alfvén velocities are discussed and can be applied to describe the properties of spotless flares. 相似文献
15.
H. Alinejad 《Astrophysics and Space Science》2012,339(2):249-254
The effects of dust polarity and superthermal electrons are incorporated in the study of dust ion-acoustic (DIA) solitary
waves (SWs) as well double layers (DLs) in a dusty plasma containing warm adiabatic ions, superthermal electrons, and arbitrarily
(positively or negatively) charged immobile dust. Based on the energy-like integral equation, a new relationship between the
localized electrostatic disturbances and dust polarity is derived. It is shown that there exists rarefactive SWs and DLs with
qualitatively different structures in a way that depends on the population of superthermal electrons. As the electrons evolve
their thermodynamic equilibrium, the localized structures are found with larger amplitude. It is also found that their amplitude
increases (decreases) with the increase in the negative (positive) dust number density. 相似文献
16.
T. Penz N.V. Erkaev H. Lammer H. Gunell S. Barabash A. Milillo 《Planetary and Space Science》2004,52(13):1157-1167
Mars Global Surveyor detected cold electrons above the Martian ionopause, which can be interpreted as detached ionospheric plasma clouds. Similar observations by the Pioneer Venus Orbiter electron temperature probe showed also extreme spatial irregularities of electrons in the form of plasma clouds on Venus, which were explained by the occurrence of the Kelvin-Helmholtz instability. Therefore, we suggest that the Kelvin-Helmholtz instability may also detach ionospheric plasma clouds on Mars. We investigate the instability growth rate at the Martian ionopause resulting from the flow of the solar wind for the case where the interplanetary magnetic field is oriented normal to the flow direction. Since the velocity shear near the subsolar point is very small, this area is stable with respect to the Kelvin-Helmholtz instability. We found that the highest flow velocities are reached at the equatorial flanks near the terminator plane, while the maximum plasma density in the terminator plane appears at the polar areas. By comparing the instability growth rate with the magnetic barrier formation time, we found that the instability can evolve into a non-linear stage at the whole terminator plane but preferably at the equatorial flanks. Escape rates of O+ ions due to detached plasma clouds in the order of about 2×1023-3×1024 s-1 are found. Thus, atmospheric loss caused by the Kelvin-Helmholtz instability should be comparable with other non-thermal loss processes. Further, we discuss our results in view of the expected observations of heavy ion loss rates by ASPERA-3 on board of Mars Express. 相似文献
17.
Walter J. Heikkila 《Solar physics》1983,88(1-2):329-336
It has been proposed that magnetospheric substorms and solar flares are a result of the same mechanism. In our view this mechanism is connected with the escape, or attempted escape, of energized plasma from a region of closed magnetic field lines bounded by a magnetic bottle. In the case of the Earth, it must be plasma that is able to maintain a discrete auroral arc, and we propose that the cross-tail current connected to the arc is filamentary in nature to provide the field-aligned current sheet above the arc. A localized meander of such an intense current filament could be caused by a tearing instability in the neutral sheet. Such a meander will cause an inductive electric field opposing the current change everywhere. In trying to reduce the component of the induction electric field parallel to the magnetic field lines, the plasma must enhance the transverse or cross-tail component; this action leads to eruptive behavior, in agreement with tearing theories. This enhanced induction electric field will cause a discharge along the magnetic neutral line at the apex of the magnetic arches, constituting an impulsive acceleration of all charged particles originally near the neutral line. The products of this phase then undergo betatron acceleration for a second phase. This discharge eventually reduces the electric field along the neutral line, and thereafter the enclosed magnetic flux through the neutral line remains nearly constant. The result is a plasmoid that has definite identity; its buoyancy leads to its escape. The auroral breakup (and solar flare) is the complex plasma response to the changing electromagnetic field. 相似文献
18.
An analytical model for oscillating pair creation above the pulsar polar cap is presented in which the parallel electric field is treated as a large amplitude, superluminal, electrostatic wave. An exact formalism for such wave is derived in one dimension and applied to both the low-density regime in which the pair plasma density is much lower than the corotating charge density and the high-density regime in which the pair plasma density is much higher than the corotating charge density. In the low-density regime, which is relevant during the phase leading to a pair cascade, a parallel electric field develops resulting in a rapid acceleration of particles. The rapid acceleration leads to bursts of pair production and the system switches to the oscillatory phase, corresponding to the high-density regime, in which pairs oscillate with net drift motion in the direction of wave propagation. Oscillating pairs lead to a current that oscillates with large amplitude about the Goldreich–Julian current. The drift motion can be highly relativistic if the phase speed of large amplitude waves is moderately higher than the speed of light. Thus, the model predicts a relativistic outflow of pairs, a feature that is required for avoiding overheating of the pulsar polar cap and is also needed for the pulsar wind. 相似文献
19.
Yasunori Nejoh 《Astrophysics and Space Science》1996,235(2):245-253
The stationary ion-acoustic double layer is investigated in a plasma with an electron beam. The condition of the existence sensitively depends on the parameters such as the electron beam temperature, the ion temperature, the beam density and the effect of the trapped electrons. The properties of the double layer are also depicted. It turns out that the electron beam velocity is relatively small. This investigation predicts new findings of the ion-acoustic double layers in a plasma with an electron beam. 相似文献
20.
V. I. Shematovich 《Solar System Research》2004,38(1):28-38
The hot planetary and satellite coronas are populated by the suprathermal particles produced in the transition region between the collision-dominated and free-molecule atmospheric layers under the external effects of electromagnetic and corpuscular solar radiation and magnetospheric plasma. We construct a numerical stochastic model to investigate both the local formation and kinetics of suprathermal particles and their transport to exospheric heights from underlying atmospheric layers. In contrast to other commonly used approaches, the suggested numerical model is suitable for studying the flows of atmospheric gas weakly and strongly perturbed by suprathermal particles, i.e., for studying the formation of hot planetary and satellite coronas proper. Highly efficient Monte-Carlo algorithms with weighted particles underlie the numerical implementation of the model. This numerical model is used to investigate the following: (i) the hot oxygen corona of Europa, a Jovian satellite, which is an example of a highly nonequilibrium near-surface atmosphere; and (ii) the nonthermal losses of nitrogen from Titan, a Saturnian satellite, when suprathermal atoms and molecules of nitrogen are only a small admixture to the surrounding thermal molecular nitrogen—the main atmospheric component of Titan. 相似文献