共查询到20条相似文献，搜索用时 31 毫秒
1.
B. Lehnert 《Astrophysics and Space Science》1976,40(1):225-229
A one-dimensional model is being considered where a fully ionized plasma is separated from a neutral gas by a homogeneous magnetic field directed along the plasma boundary. The plasma and the neutral gas consist of two different types of ions and neutral particles. In a stationary state the outflux of plasma by diffusion across the magnetic field is compensated by an influx of neutrals which are ionized in a partially ionized boundary region. It is found that the ratio between the ion densities in the fully ionized region will in general differ from the density ratio of the two types of neutrals being present in the gas region. This provides a separation mechanism with applications both to cosmical and laboratory plasmas, such as in the following cases:
- The abundance anomalies in magnetic variable stars and in the solar wind.
- Separation processes of non-identical ions and neutral atoms in gas blanket systems.
2.
P. K. Bhatia 《Astrophysics and Space Science》1973,23(2):275-284
The hydromagnetic stability of a cosmical plasma interacting with neutral gas has been studied to include the effects of ion viscosity and the finiteness of the ion Larmor radius. It is first shown that the system is characterized by a variational principle. The explicit solution has then been obtained, by making use of the existence of the variational principle, for a semi-infinite plasma in which the density is stratified, exponentially, along the vertical. It is found that FLR, ion viscosity as well as neutral gas friction have all a stabilizing influence. 相似文献
3.
B. Lehnert 《Astrophysics and Space Science》1988,144(1-2):31-42
The problem is considered of a cloud of neutral dust moving into a cloud of static plasma which is confined in a magnetic field. Earlier experiments with rotating plasma devices and plasma guns on critical velocity limitation suggest that such limitation could also arise in the case of plasma-neutral dust interaction in cosmos.Nevertheless further analysis is required to provide a clear picture of the relations between the cosmical and laboratory conditions for plasma-neutral gas and plasma-neutral dust interaction. In particular this applies to the question how to relate the experiments, which are largely in the plasma-physical MHD range, to the cosmical interaction which appears to be mainly governed by kinetic effects.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988. 相似文献
4.
The model of a Local Hot Bubble has been widely accepted as providing a framework that can explain the ubiquitous presence
of the soft X-ray background diffuse emission. We summarize the current knowledge on this local interstellar region, paying
particular reference to observations that sample emission from the presumed local million degree K hot plasma. However, we
have listed numerous observations that are seemingly in conflict with the concept of a hot Local Bubble. In particular, the
discovery of solar wind charge exchange that can generate an appreciable soft X-ray background signal within the heliosphere,
has led to a re-assessment of the generally accepted model that requires a hot local plasma.
In order to explain the majority of observations of the local plasma, we forward two new speculative models that describe
the physical state of the local interstellar gas. One possible scenario is similar to the present widely accepted model of
the Local Hot Bubble, except that it accounts for only 50% of the soft X-ray emission currently detected in the galactic plane,
has a lower thermal pressure than previously thought, and its hot plasma is not as hot as previously believed. Although such
a model can solve several difficulties with the traditional hot Local Bubble model, a heating mechanism for the dimmer and
cooler gas remains to be found. The second possible explanation is that of the ‘Hot Top’ model, in which the Local Cavity
is an old supernova remnant in which no (or very little) million degree local plasma is presently required. Instead, the cavity
is now thought to be filled with partially ionized cloudlets of temperature ∼7000 K that are surrounded by lower density envelopes
of photo-ionized gas of temperature ∼20,000 K. Although this new scenario provides a natural explanation for many of the observations
that were in conflict with the Local Hot Bubble model, we cannot (as yet) provide a satisfactory explanation or the emission
levels observed in the B and Be ultra-soft X-ray bands. 相似文献
5.
As any comet nears the Sun, gas sublimes from the nucleus taking dust with it. Jupiter family comets are no exception. The neutral gas becomes ionized, and the interaction of a comet with the solar wind starts with ion pickup. This key process is also important in other solar system contexts wherever neutral particles become ionized and injected into a flowing plasma such as at Mars, Venus, Io, Titan and interstellar neutrals in the solar wind. At comets, ion pickup removes momentum and energy from the solar wind and puts it into cometary particles, which are then thermalised via plasma waves. Here we review what comets have shown us about how this process operates, and briefly look at how this can be applied in other contexts. We review the processes of pitch angle and energy scattering of the pickup ions, and the boundaries and regions in the comet-solar wind interaction. We use in-situ measurements from the four comets visited to date by spacecraft carrying plasma instrumentation: 21P/Giacobini-Zinner, 1P/Halley, 26P/Grigg-Skjellerup and 19P/Borrelly, to illustrate the process in action. While, of these, comet Halley is not a Jupiter class comet, it has told us the most about cometary plasma environments. The other comets, which are from the Jupiter family, give an interesting comparison as they have lower gas production rates and less-developed interactions. We examine the prospects for Rosetta at comet Churyumov-Gerasimenko, another Jupiter family comet where a wide range of gas production rates will be studied. 相似文献
6.
Assuming some hydrogen atoms are distributed in the magnetosphere of a pulsar, the gas we are dealing with is a mixture of plasma and hydrogen atoms. Because the induced electrical field in the plasma surrounding the pulsar is very strong, due to the rotation of the pulsar associated with a strong magnetic field, the electric polarization of an atom will include the nonlinear term of the electric field. We obtain the nonlinear dispersion relation for the mixed gas from the Maxwell equations and derive the nonlinear Schrödinger equation, which has solitons as its solution under a certain condition. The curvature radiation of solitons moving along the magnetic field lines is a plausible way to explain the strong radio emission which comes from a pulsar, particularly some field lines existing near the light cylinder with radii of curvature smaller than the radius of the pulsar. 相似文献
7.
C. D. Gregory J. Howe B. Loupias S. Myers M. M. Notley Y. Sakawa A. Oya R. Kodama M. Koenig E. Falize S. Bouquet C. Michaut N. C. Woolsey 《Astrophysics and Space Science》2009,322(1-4):37-41
We present the results of experiments in which jets are created through the collision of two laser-produced plasmas. These experiments use a simple ‘v-foil’ target design: two thin foils are placed at an angle of 140° to each other, and irradiated with a high-energy laser. The plasmas from the rear face of these foils collide and drive plasma jets moving with a velocity of ～300 km?s?1. By choosing the foil thickness and material to suit the laser conditions available, it has proven possible to create plasma jets for which the relevant scaling parameters show significant overlap with those of outflows associated with young stellar objects (YSOs). Preliminary results are also shown from experiments to study the effect of an ambient gas on jet propagation. Nominally identical experiments are conducted either in vacuum or in an ambient medium of 5 mbar of nitrogen gas. The gas is seen to increase the jet collimation, and to introduce shock structures at the head of the outflow. 相似文献
8.
Current-driven dust-acoustic instability in a collisional plasma 总被引：3，自引：0，他引：3
The excitation of the dust-acoustic instability in a collisional dusty plasma is investigated. For conditions similar to those of recent laboratory experiments with neutral gas pressures of 100mTorr, it is found that zero-order electric fields E0 10 Vm−1 are sufficient for the growth of perturbations with centimeter wavelengths. Much larger wavelengths generally require larger values of E0. Free electrons in the dusty plasma have a stabilizing effect, which can be very pronounced at the longest wavelengths. 相似文献
9.
A space charge sheath is found to be formed whenever a high-velocity magnetized plasma stream penetrates a gas cloud. The sheath is always located at the head of the plasma stream, and its thickness is very small compared to the length of the plasma stream. Soon after the sheath is formed it quickly slows down to the Alfvén critical velocity. The plasma behind the sheath continues to move at higher velocity until the whole plasma stream is retarded to the critical velocity. In the interaction at gas density 1019 m–3 the sheaths are observed to be accompanied by a single loop of current with current density of 105 Å m–2. Maximum potential in the sheath ranges between 50 and 200 V.Presently available models for the sheath may explain the initiation of the sheath formation. Physical processes like heating of the electrons and ionization of the gas cloud which come into play at a later stage of the interaction are not included in these models. These processes considerably alter the potential structure in the sheath region. A schematic model of the observed sheath is presented here.Experiments reveal a threshold value of the magnetic field for plasma retardation to occur. This seems to correspond to the threshold condition for excitation of the modified two-stream instability which can lead to the electron heating. The observed current are found sufficient to account for the plasma retardation at a gas density of 1017 m–3. 相似文献
10.
Charged particles emission in a magnetic field on the basis of Boltzman equation with source term is studied. The work develops the results of Narasimha (1962), where the neutral gas expansion into a vacuum is studied. The different specific cases of emission are given for moving sources. The results may be of use when we study phenomena which happen in plasma close to the Earth and in space, as well as for studies of phenomena which take place due to moving satellites in the ionosphere and in interplanetary or interstellar plasma. 相似文献
11.
Ingvar Axnäs 《Astrophysics and Space Science》1978,55(1):139-146
The critical ionization velocity which is of cosmogonic and astrophysical interest has hitherto mainly been investigated for pure gases. Since in space we always have gas mixtures, it is of interest also to study gas mixtures. The present report, which is a summary of a more detailed report (Axnäs, 1976), summarizes the results of systematic experiments on the critical ionization velocity as a function of the mixing ratio for binary gas mixtures of H2, He, N2, O2, Ne and Ar. The apparatus used is a coaxial plasma gun with an azimuthal magnetic field. The discharge parameters are chosen so that the plasma is weakly ionized. In some of the mixtures it is found that one of the components tends to dominate in the sense that only a small amount (regarding volume) of that component is needed for the discharge to adopt a limiting velocity close to that for the pure component. Thus in a mixture between a heavy and a light component having nearly equal ionization potentials, the heavy component dominates. Also, if there is a considerable difference in ionization potential between the components, the component with the lowest ionization potential tends to dominate.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May, 1978. 相似文献
12.
V. V. Prudskikh 《Astronomy Letters》2013,39(3):192-200
A criterion for the magnetorotational instability of a protostellar disk in which the dust particles are assumed to be well mixed with the gas over the entire disk volume has been obtained within the framework of Hall magnetohydrodynamics. It is shown that the dusty plasma component affects significantly the Hall current and, under certain conditions, can cause its direction to be reversed compared to the case of a weakly ionized electron-ion plasma. A significant expansion of the range of wave numbers for unstable magnetic fluctuations is a consequence of the Hall current reversal. The spatial localization of the regions of protostellar disks in which not only the long-wavelength Alfvén disturbances but also the short-wavelength ones are subject to the magnetorotational instability is investigated. Possible physical consequences of the presence of anomalously active zones in cold disks for their structure and evolution are pointed out. 相似文献
13.
Many observations indicate the occurrence of ionized gas in the distant haloes of galaxies (including our own). Since photoionization by stars (mainly O stars, young stars or evolved low-mass stars depending on the kind of galaxy) does not seem to be exclusively responsible for the ionization of the hydrogen filaments that should otherwise cool fast and recombine quickly, the question arises which extra energy source can produce the quasi-stationary ionization. We show that stationary localized magnetic reconnection in current filaments may contribute to the ionization of the extraplanar halo gas. In these filaments magnetic energy is dissipated. Consequently, the ionized as well as the neutral component is heated and re-ionized on a time-scale significantly shorter than the recombination time-scale. The amount of energy required for efficient re-ionization can in principle easily be provided by the free magnetic energy. We present quasi-static models that are characterized by plasma temperatures and densities that agree well with the observed values for the diffuse ionized gas component of the interstellar medium. Plasma–neutral gas fluid simulations are made to show that the recombination-induced dynamical reconnection process indeed works in a self-regulatory way. 相似文献
14.
The dynamic stability of a partially ionized, compressible Hall plasma of finite electrical conductivity has been investigated when the plasma is immersed in a uniform, horizontal magnetic field. Based on the variational principle, which is shown to characterize the problem, the solution has been obtained for a semi-infinite plasma confined between two planes and having an exponential density stratification along the vertical. It is found that the effect of neutral gas friction is stabilizing while magnetic resistivity, Hall currents and compressibility all have destabilizing influence.On leave of absence from Department of Mathematics, University of Jodhpur, Jodhpur, India. 相似文献
15.
The paper models the suprathermal plasma as the concatenation of two different gravitating fluids each having its own separate density and pressure. One of the fluids has isotropic pressure while the other has an anisotropic pressure. The system is subjected to a uniform magnetic field which is frozen with the fluids. The gravitational instability of the model for low frequency plane waves has been investigated in a linear framework for plasma parameters relevant to the spiral arms of the galaxy and the cosmic gas clouds. The analysis shows that the wavelength of the instability is strongly dependent upon the anisotropy of the non-thermal plasma. 相似文献
16.
The problem of influence of the ambiental intergalactic pressure on extended gas associated with normal field galaxies is briefly discussed. An empirically established characteristic radius of absorption can not be explained in the context of simple two-phase halo model as a consequence of pressure equilibrium of the hot halo with the smooth intergalactic medium. This conclusion is based upon the stringent constraints on the temperature and density of the intergalactic plasma obtained through the CMBR measurements. On the other hand, ram-pressure stripping caused by peculiar motions of galaxies does present a viable alternative for the hot halo truncation. It is shown that for a particular set of chosen parameters, a simple model is capable of producing the absolute upper limit on the extent of gas associated with the galaxy. The values obtained are compatible with the results of the recent QSO absorption-line studies, and are significantly higher than the radius at which the cooling timescale of the halo gas equates the dynamical timescale. 相似文献
17.
The propagation of MHD waves is studied when two ideal fluids, thermal and suprathermal gases, coupled by magnetic field are
moving with the steady flow velocity. The fluids move independently in a direction perpendicular to the magnetic field but
gets coupled along the field. Due to the presence of flow in suprathermal and thermal fluids there appears forward and backward
waves. All the forward and backward modes propagate in such a way that their rate of change of phase speed with the thermal
Mach number is same. It is also found that besides the usual hydromagnetic modes there appears a suprathermal mode which propagates
with faster speed. Surface waves are also examined on an interface formed with composite plasma (suprathermal and thermal
gases) on one side and the other is a non-magnetized plasma. In this case, the modes obtained are two or three depending on
whether the sound velocity in thermal gas is equal to or greater than the sound velocity in suprathermal gas. The results
lead to the conclusion that the interaction of thermal and suprathermal components may lead to the occurrence of an additional
mode called suprathermal mode whose phase velocity is higher than all the other modes. 相似文献
18.
Miguel A. de Avillez David L. Berry 《Monthly notices of the Royal Astronomical Society》2001,328(3):708-718
Three-dimensional modelling of the flow of gas and plasma in a section of the Galaxy has been carried out to study the evolution and formation of Galactic chimneys and worms. It is found that clustered supernovae located on either side of the Galactic plane are sources for the formation of well-collimated chimneys, having widths of ∼ . The thick gas disc may have a role in the collimation of chimneys. Channel maps of disc gas, obtained from the simulations, show the presence of sheet-like structures running perpendicular to the Galactic plane and resembling worms. Worms are believed to result from the break-up of the shells and supershells. However, the simulations show that although some worms correlate well with the debris of broken shells/supershells, others do not. They are cold gas that has been accelerated in the disc and rise on to the thick gas disc. 相似文献
19.
Charge‐transfer is the main process linking neutrals and charged particles in the interaction regions of neutral (or partly ionized) gas with a plasma. In this paper we illustrate the importance of charge‐transfer with respect to the dynamics and the structure of neutral gas‐plasma interfaces. We consider the following phenomena: (1) the heliospheric interface ‐ region where the solar wind plasma interacts with the partly‐ionized local interstellar medium (LISM) and (2) neutral interstellar clouds embedded in a hot, tenuous plasma such as the million degree gas that fills the so‐called “Local Bubble”. In (1), we discuss several effects in the outer heliosphere caused by charge exchange of interstellar neutral atoms and plasma protons. In (2) we describe the role of charge exchange in the formation of a transition region between the cloud and the surrounding plasma based on a two‐component model of the cloud‐plasma interaction. In the model the cloud consists of relatively cold and dense atomic hydrogen gas, surrounded by hot, low density, fully ionized plasma. We discuss the structure of the cloud‐plasma interface and the effect of charge exchange on the lifetime of interstellar clouds. Charge transfer between neutral atoms and minor ions in the plasma produces X‐ray emission. Assuming standard abundances of minor ions in the hot gas surrounding the cold interstellar cloud, we estimate the X‐ray emissivity consecutive to the charge transfer reactions. Our model shows that the charge‐transfer X‐ray emission from the neutral cloud‐plasma interface may be comparable to the diffuse thermal X‐ray emission from the million degree gas cavity itself (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
20.
David A. Buote Claude R. Canizares A. C. Fabian 《Monthly notices of the Royal Astronomical Society》1999,310(2):483-510
We examine the K shell emission lines produced by isothermal and simple multiphase models of the hot gas in elliptical galaxies and galaxy clusters to determine the most effective means for constraining the width of the differential emission measure, ( T ), in these systems which we characterize by a dimensionless parameter, . Comparison of line ratios of two-temperature ( <1) and cooling flow ( 1) models is presented in detail. We find that a two-temperature model can approximate very accurately a cooling flow spectrum over 0.510 keV.
We re-analyse the ASCA spectra of three of the brightest galaxy clusters to assess the evidence for multiphase gas in their cores: M87 (Virgo), the Centaurus cluster and the Perseus cluster. K emission-line blends of Si, S, Ar, Ca and Fe are detected in each system, as is significant Fe K emission. The Fe K /K ratios are consistent with optically thin plasma models and do not suggest resonance scattering in these systems. Consideration of both the ratios of H-like to He-like K lines and the local continuum temperatures clearly rules out isothermal gas in each case. To obtain more detailed constraints, we fitted plasma models over 1.69 keV where the emission is dominated by these K shell lines and by continuum. In each case the ASCA spectra cannot determine whether the gas emits at only two temperatures or over a continuous range of temperatures as expected for a cooling flow. The metal abundances are near-solar for all of the multiphase models. We discuss the implications of these results and examine the prospects for determining the temperature structure in these systems with upcoming X-ray missions. 相似文献
We re-analyse the ASCA spectra of three of the brightest galaxy clusters to assess the evidence for multiphase gas in their cores: M87 (Virgo), the Centaurus cluster and the Perseus cluster. K emission-line blends of Si, S, Ar, Ca and Fe are detected in each system, as is significant Fe K emission. The Fe K /K ratios are consistent with optically thin plasma models and do not suggest resonance scattering in these systems. Consideration of both the ratios of H-like to He-like K lines and the local continuum temperatures clearly rules out isothermal gas in each case. To obtain more detailed constraints, we fitted plasma models over 1.69 keV where the emission is dominated by these K shell lines and by continuum. In each case the ASCA spectra cannot determine whether the gas emits at only two temperatures or over a continuous range of temperatures as expected for a cooling flow. The metal abundances are near-solar for all of the multiphase models. We discuss the implications of these results and examine the prospects for determining the temperature structure in these systems with upcoming X-ray missions. 相似文献