共查询到20条相似文献，搜索用时 46 毫秒
1.
Max K. Wallis 《Celestial Mechanics and Dynamical Astronomy》1976,13(1):65-74
Gas streaming through the solar system experiences both destructive and scattering processes, the latter primarily in collisional interactions with the solar wind protons. The scattering interactions can be important in filling the downstream wake. They may effectively increase the velocity dispersion and also cause discrete orbit changes.The downstream intensity moment is here evaluated analytically for particles suffering a single, discrete collision, and compared with the moment from a thermal velocity dispersion (both in the absence of a central force field). The elastic scattering collisions of protons in H-gas lead to a contribution to theL backscatter from the wake equivalent to an initial thermal velocity of about 1 km s–1, giving an intensity for cool gas of the order of 10R. This exceeds the contribution due to focussing in the solar gravitational field if the radiation pressure is not less than 0.8 of the gravitational attraction. 相似文献
2.
Vladislav V. Izmodenov Yury G. Malama Alexandr P. Kalinin Mike Gruntman Rosine Lallement Irina P. Rodionova 《Astrophysics and Space Science》2000,274(1-2):71-76
Williams et al. (1997) have suggested that a population of hot hydrogen atoms is created in the heliosphere through elastic H-H collisions
between energetic `solar' atoms (neutralized solar wind) and interstellar atoms. They used a BGK-like approximation (Bhatnagar
et al., 1954) for the Boltzmann collision term and the collision cross sections suggested by Dalgarno (1960). We show that both
assumptions result in a significant overestimation of the the H-H collision effect. On the basis of calculated momentum transfer
cross-sections for elastic H-H collisions, we argue that elastic H-H and H-p collisions cannot produce hot H atoms in the
heliosphere.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
J. H. Piddington 《Solar physics》1972,27(2):402-419
The solar atmosphere may be divided into a number of isolated active components and a quiet residue. On the largest scale the latter is dominated by a general dipole magnetic field of strength 1–2 G; its observable components are flux concentrations in supergranule boundary regions (SBRs), spicules, mottles and polar plumes. The velocity field in the SBRs is discussed. There are continuous gas streaming motions up and down between the photosphere and the corona; spicules may be mainly downward moving gas.A unifying model is developed of these various components, as well as the heating mechanism of the whole quiet atmosphere. Highly ordered velocity fields of the cell, together with a gravitational wave, cause a vertical magnetic force tube to collapse below a critical level; the result is an upward eruption of a vortex ring at the Alfvén velocity. The complex mass velocity pattern may explain spicules, mottles and plumes, as well as unobservable streaming motions.The quiet atmosphere is divided into regions above SBRs and those above the inner parts of the cells. Hydromagnetic eruptions from the former may account for the entire heat requirement of the atmosphere. The model atmosphere has a chromosphere-corona transition layer which bulges upwards above the SBRs and so conforms with EUV data. The energy and mass balances in this solar atmosphere are considered, and it is also shown to be consistent with the radio data. 相似文献
4.
Kashif Arshad Fizza Siddique Arshad M. Mirza Aman-ur-Rehman 《Astrophysics and Space Science》2014,350(1):169-174
Employing Vlasov-Poisson model for nonthermal distributed permeating plasma consisting of electron-positron-ion plasma of our earth’s magnetosphere and the solar wind plasma with some fixed streaming velocity, can drive ion-acoustic waves unstable. The growth rates are computed with respect to the variation in spectral index of the kappa or generalized Lorentzian distribution and streaming velocity of the solar wind. It is found that the growth rate increases with the decrease of spectral index and increases with the streaming velocity of the solar wind. The numerical results are also presented by choosing some suitable parameters. 相似文献
5.
We studied fragmentation process of the interstellar molecular cloud which is predominated by supersonic turbulence with special regard to collisions of turbulent gas elements and formation of a shock-compressed layer by receding shock waves. The propagation of the shock waves and the evolution of the compressed layer are followed by one-dimensional gas dynamical simulation until self-gravity becomes significant, taking account of the effects of thermal properties of the molecular gas and magnetic fields. It is shown that the efficient cooling by CO molecules and its sensitive dependence on gas density make the shock-compressed layer so cold and dense that the layer becomes gravitationally unstable and breaks into fragments even if the gas elements are gravitationally stable prior to the collision. The mass of the unstable fragment is estimated to be about two solar masses or less, irrespective of the presence of the magnetic field. The stars formed by collisions of supersonic turbulent gas elements accelerate the surrounding gas in T Tauri stage and replenish the turbulent energy to maintain the mechanical equilibrium of the molecular cloud. 相似文献
6.
Walter Dehnen Jonathan Bland-Hawthorn Andreas Quirrenbach Gerald N. Cecil 《Astrophysics and Space Science》1997,248(1-2):33-42
We present the kinematics of the ionized gas over the inner 140″ (10 kpc) from observations with the HIFI Fabry-Perot interferometer.
There is clear evidence for density wave streaming and bar-driven streaming across the field, with bi-symmetric arms that
penetrate to within 200 pc of the nucleus. CO maps show linear structures along (although slightly offset from) the bar consistent
with a strong shock. Along the spiral arms which encircle the bar, the H II regions lie downstream of the CO gas in the rest
frame of the bar, as do the dust lanes, only if the gas outruns the stellar bar. As a first step towards understanding the
details of the gas kinematics, and attempting to determine the mass inflow rate towards the nucleus, we build a mass model
for the central disk constrained by near-infrared images. We plan to use this model as gravitational background potential
for hydrodynamical simulations of the gas response to the bar. Comparing these with the data presented should enable us to
constrain various quantities such as pattern speed, stellar mass-to-light ratio, central mass concentration, and gas fueling
rate.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
Exact solutions have been found to the Fokker-Planck equations, incorporating stochastic velocity changes and modelling particles moving in an inverse square central force field under an inverse square collision frequency. The solutions for the velocity distribution contain a combination of collisional and dynamical (reversible) heating. At a general position, there are two populations each with three distinct temperatures, one normal to the orbital plane and the others closely parallel and perpendicular to the mean orbit. Collisional heating is strong and most readily detected in the secondary component of gas which reaches upstream directions along indirect orbits (attractive central force). For interplanetary helium gas reaching 1 a.u., the collisional heating ranges from effective transverse increase of 200 K and radial increase of 1500 K in the downstream wake, to several thousand K increase in radial temperature of the secondary component transverse to the initial gas stream. In interpreting 584 Å sky background radiation observations, the dynamical changes in the velocity spread have to be taken into account for helium gas that is initially hot, when Doppler shifts relative to the solar emission line are significant; the present solutions being the thermal approximations to the distribution function reveal the appropriate radial temperature as a function of space. 相似文献
8.
The heating of inflowing interstellar gas by the solar wind is calculated. The experimental differential cross sections have been used for calculating electron-H(He) and proton-H(He) elastic scattering rate coefficients. The solar wind is assumed to be a two-component (protons and electrons), steady, spherically symmetric stream moving radially outward, with the inflowing gas following Keplerian trajectories. The spatial distributions of effective temperature increase within interplanetary space have been obtained. 相似文献
9.
A formula is obtained for the rate coefficient for the production of a particle with a specific energy in an elastic collision with the atoms of a thermal gas. Accurate calculations are carried out of the energy transfer in elastic collisions of oxygen and hydrogen atoms. The fractions of hydrogen atoms with sufficient energy to escape from Venus produced in collisions of oxygen atoms with energies of 2.6 eV in a gas of atomic hydrogen at 100, 200 and 300 K are respectively 5.1, 6.9 and 8.5%.The mutual diffusion coefficient D of oxygen and hydrogen atoms is obtained. It can be represented by Dn = 7.25 × 1017T0.71cm?1s?1 where n is the total atom density and T is the temperature. 相似文献
10.
V. I. Shematovich 《Solar System Research》2017,51(4):249-257
This is a study of the kinetics and transport of hot oxygen atoms in the transition region (from the thermosphere to the exosphere) of the Martian upper atmosphere. It is assumed that the source of the hot oxygen atoms is the transfer of momentum and energy in elastic collisions between thermal atmospheric oxygen atoms and the high-energy protons and hydrogen atoms precipitating onto the Martian upper atmosphere from the solar-wind plasma. The distribution functions of suprathermal oxygen atoms by the kinetic energy are calculated. It is shown that the exosphere is populated by a large number of suprathermal oxygen atoms with kinetic energies up to the escape energy 2 eV; i.e., a hot oxygen corona is formed around Mars. The transfer of energy from the precipitating solar-wind plasma protons and hydrogen atoms to the thermal oxygen atoms leads to the formation of an additional nonthermal escape flux of atomic oxygen from the Martian atmosphere. The precipitation-induced escape flux of hot oxygen atoms may become dominant under the conditions of extreme solar events, such as solar flares and coronal mass ejections, as shown by recent observations onboard NASA’s MAVEN spacecraft (Jakosky et al., 2015). 相似文献
11.
The sequence of evolution of the protoplanetary gas-and-dust disk around the parent star includes, according to modern concepts, its compression in the central plane and decay into separate dust condensations (clusters) due to the occurrence of various types of instabilities. The interaction of dust clusters of a fractal structure during their collisions is considered as a key mechanism for the formation and growth of primary solids, which serve as the basis for the subsequent formation of planetesimals and embryos of planets. Among the mechanisms contributing to the formation of planetesimals, an important place belongs, along with gravitational instability, hydrodynamic instabilities, in particular, the socalled streaming instability of the two-phase gas-dust layer due to its ability to concentrate dispersed particles in dense clots. In contrast to a number of existing models of streaming instability, in which dust particles are considered structurally compact and monodisperse, this paper proposes a more realistic model of polydisperse particles of fractal nature, forming dust clusters as a result of coagulation. The instability of the dust layer in the central plane of the protoplanetary disk under linear axisymmetric perturbations of its parameters is considered. A preliminary conclusion can be drawn that the proposed model of dust fractal aggregates of different scales increases the efficiency of linear growth of hydrodynamic instabilities, including the streaming instabilities associated with the difference between the velocities of the dust and gas phases.
相似文献12.
Using Boltzmann-Vlasov kinetic model for nonthermal distributed electron-positron-ion plasma of our Earth’s magnetosphere and the solar wind streaming plasma can drive ion-acoustic waves unstable. It is found that the growth rate increases with the decrease of spectral index and increases with the streaming velocity of the solar wind. The numerical results are also presented by choosing some suitable parameters of magnetospheric plasma. 相似文献
13.
The effect of ion–neutral collisions on the propagation of MHD waves and surface waves at a single magnetic interface is investigated. The dispersion equations for MHD waves in a partially ionized medium are derived. There are three damped propagating modes in a uniform unbounded medium: an Alfvén mode, and fast and slow modes. The damping of waves depends on both the collisional frequency and the ionization fraction. Wave damping increases as ionization fraction decreases. Surface waves are discussed in three cases: (a) the incompressible limit, (b) the low plasma, and (c) for parallel propagation. The incompressible limit leads to Alfvén surface waves in a partially ionized medium and the dispersion characteristics are similar to those obtained by Uberoi and Datta. In the low plasma of the Earth's auroral F region there are two damped propagating magnetoacoustic surface waves for =/3. There is only one damped surface mode for =/2, but no surface wave is able to propagate for =0°. For the case of parallel propagation (=0°) the results obtained in the absence of ion-neutral collisions are consistent with the results of Jain and Roberts. It is found that a three-mode structure of damped propagating waves occurs owing to ion–neutral collisions for a comparatively high ionization fraction. For the case of the solar photosphere, where the ionization fraction is low, two weakly damped surface waves are found, though the damping is almost negligible. The pattern of propagation is similar to that found in the case discussed by Jain and Roberts, but the wave speeds are lower due to ion–neutral collisions. The strong collisions tie the ion–neutral species together and reduce the damping. 相似文献
14.
The effect of an interplanetary atomic hydrogen gas on solar wind proton, electron and α-particle temperatures beyond 1 AU
is considered. It is shown that the proton temperature (and probably also the α-particle temperature) reaches a minimum between
2 AU and 4 AU, depending on values chosen for solar wind and interstellar gas parameters. Heating of the electron gas depends
primarily on the thermal coupling of the protons and electrons. For strong coupling (whenT
p
≳T
e
), the electron temperature reaches a minimum between 4 AU and 8 AU, but for weak coupling (Coulomb collisions only), the
electron temperature continues to decrease throughout the inner solar system. A spacecraft travelling to Jupiter should be
able to observe the heating effect of the solar wind-interplanetary hydrogen interaction, and from such observations it may
be possible of infer some properties of the interstellar neutral gas.
Currently a National Research Council Resident Research Associate. 相似文献
15.
E. V. Mishin 《Astrophysics and Space Science》1974,27(2):367-382
Heat transport is considered both for quiet and disturbed solar winds. It is shown that heat may be transferred during solar flares by sharp fronted thermal wave pulses. Energy dissipation in the wave front arises from the firehose instability excitation. The effects of ionosonic turbulence on heat transport in a quiet solar wind are also investigated. A quasi-steady state, in which there is a balance between wave-particle interations and particle collisions is found. It is shown that the effect of wave-particle ‘collisions’ is to produce a significant decrease of the electron heat flow and electron temperature, and increase of the ion temperature relative to calculations which take into account particle particle collisions only. 相似文献
16.
The clustering of fine particles by mutual thermal collisions is investigated experimentally. Fine particles are prepared in an argon gas atmosphere by the gas evaporation technique. Mass distributions of clusters of the particles are obtained from micrographs of specimen grids placed at different heights above the evaporating source. The cluster growth is clearly seen in the change of mass distribution with height. A comparison of the experimental results with a theoretical model indicates that the cluster of fine particles does not grow in the spherical manner usually assumed, but in a planar manner. As an important consequence of the conclusion to the primordial solar nebula, the sedimentation time of the grains sinking towards the equatorial plane of the solar disk becomes longer than the value previously adopted because of the large ratio of surface to volume of a planar cluster. This longer time should alter the scenario of the evolution of the solar system after sedimentation.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978. 相似文献
17.
As planetary embryos grow, gravitational stirring of planetesimals by embryos strongly enhances random velocities of planetesimals and makes collisions between planetesimals destructive. The resulting fragments are ground down by successive collisions. Eventually the smallest fragments are removed by the inward drift due to gas drag. Therefore, the collisional disruption depletes the planetesimal disk and inhibits embryo growth. We provide analytical formulae for the final masses of planetary embryos, taking into account planetesimal depletion due to collisional disruption. Furthermore, we perform the statistical simulations for embryo growth (which excellently reproduce results of direct N-body simulations if disruption is neglected). These analytical formulae are consistent with the outcome of our statistical simulations. Our results indicate that the final embryo mass at several AU in the minimum-mass solar nebula can reach about ∼0.1 Earth mass within 107 years. This brings another difficulty in formation of gas giant planets, which requires cores with ∼10 Earth masses for gas accretion. However, if the nebular disk is 10 times more massive than the minimum-mass solar nebula and the initial planetesimal size is larger than 100 km, as suggested by some models of planetesimal formation, the final embryo mass reaches about 10 Earth masses at 3-4 AU. The enhancement of embryos’ collisional cross sections by their atmosphere could further increase their final mass to form gas giant planets at 5-10 AU in the Solar System. 相似文献
18.
We consider the conditions required for a cluster core to shrink, by adiabatic accretion of gas from the surrounding cluster, to densities such that stellar collisions are a likely outcome. We show that the maximum densities attained, and hence the viability of collisions, depend on the balance between core shrinkage (driven by accretion) and core puffing up (driven by relaxation effects). The expected number of collisions scales as , where N core is the number of stars in the cluster core and is the free-fall velocity of the parent cluster (gas reservoir). Thus, whereas collisions are very unlikely in a relatively low-mass, low-internal-velocity system such as the Orion Nebula Cluster, they become considerably more important at the mass and velocity scales characteristic of globular clusters. Thus, stellar collisions in response to accretion-induced core shrinkage remain a viable prospect in more massive clusters, and may contribute to the production of intermediate-mass black holes in these systems. 相似文献
19.
The equations for the viscous motion of a mixture of gas and dust in a gravitational field are derived from the statistics of particle orbits and radiative processes in a general form which gives the Navier-Stokes equation as a special case. Diffusion, partially elastic collisions and — for larger bodies — the gravitational encounters are included. The results are applied to the evolution of circumstellar discs. 相似文献
20.
The problem of particle acceleration in collapsing magnetic traps in the solar corona has been solved by taking into account the particle scattering and braking in the high-temperature plasma of solar flares. The Coulomb collisions are shown to be weak in traps with lifetimes t l < 10 s and strong for t l > 100 s. In the approximation of strong collisions, collapsing magnetic traps are capable of confining up to 20% of the injected particles in the corona for a long time. In the collisionless approximation, this value exceeds 90%. The question about the observational manifestations of collisions is examined. For collision times comparable to t l , the electron spectrumat energies above 10 keV is shown to be a double-power-law one. Such spectra were found by the RHESSI satellite in flares. 相似文献