共查询到20条相似文献，搜索用时 296 毫秒
1.
J. B. Singh 《Astrophysics and Space Science》1983,96(1):153-158
Similarity solutions, for one-dimensional unsteady of a perfect gas behind a spherical shock wave produced on account of a sudden explosion or driven out by an expanding piston including the effects of radiative cooling and an idealised azimuthal magnetic field, are studied. The shock is assumed to be strong and it is propagating into a transparent medium at rest with varying density. The magnetic field is proportional tor ?1. The total energy of the wave is time dependent obeying a power law. 相似文献
2.
The model of the solar wind interaction with interstellar medium suggested by Baranovet al. (1970) is developed. In this model (TSM) the presence of two shock waves is assumed, through which the solar wind and interstellar gas pass, the latter moving relative to the Sun at supersonic speed (20 km s–1).The distance between shocks was considered earlier (Baranovet al., 1970; Baranov and Krasnobaev, 1971) to be small compared with their distance from the Sun, due to the hypersonic character of the flow. The structure of the subsonic flow portion may not be taken into account.In the present paper the distribution of the gas parameters in the region between shocks is calculated which, in particular, allows us to estimate the possibility of its experimental detection, observing radio-scintillation on interstellar irregularities (Baranovet al., 1975).The possible influence on the model of galactic hydrogen neutral atoms penetrating into interplanetary medium is estimated. 相似文献
3.
The initially supersonic flow of the solar wind passes through a magnetic shock front where its velocity is supposed to be reduced to subsonic values. The location of this shock front is primarily determined by the energy density of the external interstellar magnetic field and the momentum density of the solar wind plasma. Interstellar hydrogen penetrating into the heliosphere undergoes charge exchange processes with the solar wind protons and ionization processes by the solar EUV radiation. This results in an extraction of momentum from the solar wind plasma. Changes of the geometry and the location of the shock front due to this interaction are studied in detail and it is shown that the distance of the magnetic shock front from the Sun decreases from 200 to 80 AU for an increase of the interstellar hydrogen density from 0.1 to 1.0 cm−3. The geometry of the shock front is essentially spherical with a pronounced embayment in the direction opposite to the approach of interstellar matter which depends very much on the temperature of the interstellar gas. Due to the energy loss by the interaction with neutral matter the solar wind plasma reduces its velocity with increasing distance from the Sun. This modifies Parker's solution of a constant solar wind velocity. 相似文献
4.
The problem of explosion along a line in a gas cloud in the presence of transverse magnetic field has been considered. Similarity solutions of the adiabatic motion of a gas behind an infinitely strong cylindrical shock wave propagating into an infinitely conducting medium at rest is obtained. Shock radius varies exponentially with time and density is inversely proportional to fourth power of shock radius just ahead of the shock front. 相似文献
5.
A transient flow system containing several streams and shocks associated with the Bastille Day 2000 solar event was observed
by the WIND and ACE spacecraft at 1 AU. Voyager 2 (V2) at 63 AU observed this flow system after it moved through the interplanetary medium and into the distant heliosphere,
where the interstellar pickup protons strongly influence the MHD structures and flow dynamics. We discuss the Voyager 2 magnetic and plasma observations of this event. Increases in the magnetic field strength B, density N, temperature T and
speed V were observed at the front of a stream at V2, consistent with presence of a shock related to the Bastille Day shock
at 1 AU. However, the jumps occurred in a 16.9-hour data gap, so that the shock was not observed directly, and the properties
of the candidate shock cannot be determined precisely. The candidate shock was followed by a merged interaction region (MIR)
that moved past V2 for at least 10 days. The first part of this MIR contains a structure that might be a magnetic cloud. Just
ahead of the shock there was an abrupt increase in density associated with a decrease in temperature such that the solar wind
thermal pressure was constant across it. Just behind the shock there was an abrupt decrease in density associated with a net
increase in magnetic field strength. This appears to be a pressure balanced structure in which the interstellar pickup protons
make a significant contribution. 相似文献
6.
We consider the self-similar problem of a supernova explosion in a radially inhomogeneous medium by taking into account the generation of accelerated relativistic particles. The initial density of the medium is assumed to decrease with distance from the explosion center as a power law, ρ 0 = A/r θ. We use a two-fluid approach in which the total pressure in the medium is the sum of the circumstellar gas pressure and the relativistic particle pressure. The relativistic particle pressure at the shock front is specified as an external parameter. This approach is applicable in the case where the diffusion coefficient of accelerated particles is small and the thickness of the shock front is much smaller than its radius. We have numerically solved a system of ordinary differential equations for the dimensionless quantities that describe the velocity and density behind the shock front as well as the nonrelativistic gas and relativistic particle pressures for various parameters of the inhomogeneity of the medium and various compression ratios of the medium at the shock front. We have established that the shock acceleration of cosmic rays affects most strongly the formation of a supernova shell (making it thinner) in a homogeneous circumstellar medium. A decrease in the circumstellar matter density with distance from the explosion center causes the effect of shock-accelerated relativistic particles on the supernova shell formation to weaken considerably. Inhomogeneity of the medium makes the shell thicker and less dense, while an increase in the compression ratio of the medium at the shock front causes the shell to become thinner and denser. As the relativistic particle density increases, the effect of circumstellar matter inhomogeneity on the shell formation becomes weaker. 相似文献
7.
We present a model which describes the evolution of the energy spectrum of relativistic electrons in supernova remnants, with radiation losses of electrons taken into account. The model can be used to calculate the synchrotron X-ray emission from supernova remnants in the uniform interstellar medium and in the uniform interstellar magnetic field. The importance of various factors in the variations of spatial distributions of nonthermal electrons and their synchrotron emissive capacity is demonstrated. We analyze the errors which arise in the magnetic field strength when it is estimated with the use of the models which ignore the detailed pattern of the evolution of the magnetic field and the electron spectrum behind the shock front in the remnant. The evolution of synchrotron emission spectrum and the ratio between the synchrotron radio and X-ray fluxes from supernova remnants are calculated. 相似文献
8.
K. Ferrire 《Astronomische Nachrichten》2010,331(1):27-33
We review the current observational knowledge of the interstellar magnetic field within ∼150 pc ofthe Galactic center. We also discuss the various theoretical scenarios that have been put forward to explain the existing observations. Our critical overview leads to two important conclusions: (1) The interstellar magnetic field near the GC is approximately poloidal on average in the diffuse intercloud medium and approximately horizontal in dense interstellar clouds. (2) In the general intercloud medium, the field is relatively weak and probably close to equipartition with cosmic rays (B ∼ (6–20) μ G), but there exist a number of localized filaments where the field is much stronger (some filaments could possibly have B ≳ 1 mG). In dense interstellar clouds, the field is probably rather strong, with typical values ranging between a few 0.1 mG and a few mG (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
9.
E.A. Dorfi 《Astrophysics and Space Science》2000,272(1-3):227-238
Supernova Remnants (SNRs) are the most likely sources of the galactic cosmic rays up to energies of about 1015 eV/nuc. The large scale shock waves of SNRs are almost ideal sites to accelerate particles up to these highly non-thermal
energies by a first order Fermi mechanism which operates through scattering of the particles at magnetic irregularities. In
order to get an estimate on the total amount of the explosion energy E
SNconverted into high energy particles the evolution of a SNR has to be followed up to the final merging with the interstellar
medium. This can only be done by numerical simulations since the non-linear modifications of the shock wave due to particle
acceleration as well as radiative cooling processes at later SNR stages have to be considered in such investigations. Based
on a large sample of numerical evolution calculations performed for different ambient densities n
ext, SN explosion energies, magnetic fields etc. we discuss the final ‘yields’ of cosmic rays at the final SNR stage where the
Mach number of the shock waves drops below 2. At these times the cosmic rays start to diffuse out of the remnant. In the range
of external densities of10-2 ≤ n
ext/[cm-3] ≤ 30 we find a the total acceleration efficiency of about 0.15 E
SN with an increase up to 0.24 E
SN at maximum for an external density of n
ext = 10 cm-3. Since for the larger ambient densities radiative cooling can reduce significantly the total thermal energy content of the
remnant dissipation of Alfvén waves can provide an important heating mechanism for the gas at these later stages. From the
collisions of the cosmic rays with the thermal plasma neutral pions are generated which decay subsequently into observable
γ-rays above 100 MeV. Hence, we calculate these γ-ray luminosities of SNRs and compare them with current upper limits of ground
based γ-raytelescopes. The development of dense shells due to cooling of the thermal plasma increases the γ-ray luminosities
and e.g. an external density of n
ext = 10 cm-3 with E
SN = 1051 erg can lead to a γ-ray flux above 10-6 ph cm-2 s-1 for a remnant located at a distance of 1 kpc.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
V. P. Dokuchaev 《Astronomy Letters》2003,29(3):170-174
Radially pulsating stars are shown to radiate fast and slow magnetoacoustic waves into the interstellar gas. No Alfvén waves are excited, because the oscillations are radially symmetric. Calculations were performed for the following two limiting cases: hot, weakly magnetized interstellar plasma and cold plasma with a strong magnetic field. In these limiting cases, pulsating stars excite mostly fast magnetoacoustic waves, while the excitation of slow magnetoacoustic waves is weak. Magnetogasdynamic fields of density, velocity, and magnetic-field perturbations in the interstellar medium were found. Relations were derived to calculate the radiated power and its estimates are given for various conditions in the medium. It is shown that radially stratified wave structures with wavelengths from 1 AU to several tenths of a parsec must exist in the vicinity of pulsating stars. 相似文献
11.
The role of the magnetic field in the confinenment or compression of interstellar gas clouds is reconsidered. The virial theorem for an isolated magnetized cloud in the presence of distant magnetic sources is reformulated in terms of moments of the internal and external currents, and an equilibrium condition is derived. This condition is applied to the interaction between isolated clouds for the simple- and artificial-case in which the field of each cloud is a dipole. With the simplest of statistical assumptions, the probability of any given cloud being compressed is calculated as 10%, the magnetic field acting as a medium which transmits the kinetic pressure between clouds. Even when compression occurs the magnetic pressure 1/2B
2 may decrease on leaving the cloud surface.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988. 相似文献
12.
Vladislav V. Izmodenov 《Astrophysics and Space Science》2000,274(1-2):55-69
During 30 years, a big theoretical effort to understand the physical processes in the heliospheric interface has followed
the pioneer papers by Parker (1961) and Baranov et al. (1971). The heliospheric interface is a shell formed by the solar wind interaction with the ionized component of the circumsolar
local interstellar medium (LISM). For fully ionized supersonic interstellar plasma two-shocks (the termination shock and the
bow shock) and a contact discontinuity (the heliopause) are formed in the solar wind/LISM interaction. However, LISM consists
of at least of three components additional to plasma: H-atoms, galactic cosmic rays and magnetic field. The interstellar atoms
that penetrate into the solar wind, are ionized there and form pickup ions. A part of the pickup ions is accelerated to high
energies of anomalous cosmic rays (ACRs). ACRs may modify the plasma flow upstream the termination shock and in the heliosheath.
In this short review I summarize current understanding of the physical and gasdynamical processes in the heliospheric interface,
outline unresolved problems and future perspectives.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
J. P. Vallée 《Astrophysics and Space Science》1993,207(1):109-117
Large ( > 100 pc) interstellar magnetic bubbles are necessary in the cosmic-ray-driven fast galactic dynamo, as pioneered by Parker in 1992. In a first part, a look is made at the available data on nearby (< 1000 pc) large interstellar magnetic bubbles. Here the magnetic field strengthB in a large shell of densityn around an OB association is found to be a few times greater than that outside in the general interstellar medium, varying typically likeB ~n, as expected for a shocked medium. In a second part, some tests are made of the predictions about interstellar magnetic bubbles made by the theory of a cosmic-ray driven fast galactic dynamo. The bubble tests generally support the idea of a cosmic-ray-driven fast galactic dynamo for the Milky Way. 相似文献
14.
Gerrit L. Verschuur 《Astrophysics and Space Science》1995,227(1-2):187-198
Observed properties of interstellar neutral hydrogen filaments suggest the presence of the Bennett pinch as described by the Carlqvist relationship with rotation around the filament axes included. A brief summary is first given of three ways in which a filament model for interstellar cloud structure was tested. Preliminary results from highresolution HI mapping of gas and dust in an apparent HI cloud indicate that the neutral gas and dust within and around its boundary is itself highly filamentary. An attempt to detect magnetic fields in this and similar features using the Zeeman effect technique at the 21-cm wavelength of interstellar neutral hydrogen set upper limits of a fewµG. In contrast, the strength of the toroidal magnetic field expected from the examination of the Carlqvist relationship is of order 5µG, which would be produced by a current of 1.4 · 1013 A. Zeeman effect technology is at present not able to detect toroidal magnetic fields of this order at the edge of barely resolved HI filaments. Nevertheless, currently available high-resolution HI data suggest that interstellar filament physics should take into account the role of currents and pinches for creating and stabilizing the structures. 相似文献
15.
In this paper we obtain similarity solutions for the propagation of plane relativistic shock waves in the presence of a transverse magnetic field for the medium, where the nucleon number density obeys a power law of distance from the plane of explosion. The shock surface moves with constant velocity and the total energy of the disturbance is dependent on time. The solutions are applicable only to an isothermal medium or a cold gas. 相似文献
16.
We numerically analyze a magnetohydrodynamic, steady-state model for the interaction of a spherically symmetric solar wind with a three-component local interstellar medium (LISM), which is composed of plasma, hydrogen atoms, and a magnetic field. The magnetic field is assumed to be parallel to the velocity in the LISM. In this case, the model is axisymmetric. We study the effects of magnetic field on the plasma-flow geometry and on the distribution of hydrogen-atom parameters. In particular, we show that the presence of hydrogen atoms does not affect the qualitative change in the shape of the bow shock, the heliopause, and the solar-wind shock with increasing strength of the interstellar magnetic field. The presence of a magnetic field in the LISM can strongly affect the parameters of the energetic hydrogen atoms originated in the solar wind, although its effect on the “hydrogen wall” observed with the GHRS instrument onboard the HST spacecraft (Linsky and Wood 1996) is marginal. 相似文献
17.
A. K. Dwivedi 《Astrophysics and Space Science》1993,202(1):185-200
Similarity solutions of an isothermal flow of a perfect gas behind spherical shock waves are studied in the presence of magnetic field. The flow is caused by a propelling contact surface (or expanding piston) and its total energy increases with time. The shock is propagating in a medium at rest with uniform and non-uniform density. The two important models have been also considered here in which the magnetic field is proportional to
–1 and
–3/2. 相似文献
18.
A. S. Baranov 《Astrophysics》2007,50(4):525-532
A uniform collisionless interstellar or intergalactic medium with a constant magnetic field is considered. The reaction of
this medium to a slowly varying electromagnetic field from external sources or generated in the medium itself is determined
using an improved method which is more compact than the existing methods. Convenient formulas are obtained for determining
the reaction of an interstellar or intergalactic medium in various wave number and frequency ranges and the characteristics
of magnetoacoustic waves are discussed.
__________
Translated from Astrofizika, Vol. 50, No. 4, pp. 633–640 (November 2007). 相似文献
19.
We investigate the dynamics of magnetic fields in spiral galaxies by performing 3D magnetohydrodynamics simulations of galactic discs subject to a spiral potential using cold gas, warm gas and a two-phase mixture of both. Recent hydrodynamic simulations have demonstrated the formation of interarm spurs as well as spiral arm molecular clouds, provided the interstellar medium model includes a cold H i phase. We find that the main effect of adding a magnetic field to these calculations is to inhibit the formation of structure in the disc. However, provided a cold phase is included, spurs and spiral arm clumps are still present if β≳ 0.1 in the cold gas. A caveat to the two-phase calculations though is that by assuming a uniform initial distribution, β≳ 10 in the warm gas, emphasizing that models with more consistent initial conditions and thermodynamics are required. Our simulations with only warm gas do not show such structure, irrespective of the magnetic field strength.
Furthermore, we find that the introduction of a cold H i phase naturally produces the observed degree of disorder in the magnetic field, which is again absent from simulations using only warm gas. Whilst the global magnetic field follows the large-scale gas flow, the magnetic field also contains a substantial random component that is produced by the velocity dispersion induced in the cold gas during the passage through a spiral shock. Without any cold gas, the magnetic field in the warm phase remains relatively well ordered apart from becoming compressed in the spiral shocks. Our results provide a natural explanation for the observed high proportions of disordered magnetic field in spiral galaxies and we thus predict that the relative strengths of the random and ordered components of the magnetic field observed in spiral galaxies will depend on the dynamics of spiral shocks. 相似文献
Furthermore, we find that the introduction of a cold H i phase naturally produces the observed degree of disorder in the magnetic field, which is again absent from simulations using only warm gas. Whilst the global magnetic field follows the large-scale gas flow, the magnetic field also contains a substantial random component that is produced by the velocity dispersion induced in the cold gas during the passage through a spiral shock. Without any cold gas, the magnetic field in the warm phase remains relatively well ordered apart from becoming compressed in the spiral shocks. Our results provide a natural explanation for the observed high proportions of disordered magnetic field in spiral galaxies and we thus predict that the relative strengths of the random and ordered components of the magnetic field observed in spiral galaxies will depend on the dynamics of spiral shocks. 相似文献
20.
Based on the magnetorotational model of a supernova explosion with core collapse, we investigate the significant processes of neutrino heating of the supernova shock. These processes should be taken into account in self-consistent modeling, since the neutrino heating mechanism is capable of increasing the explosion efficiency. We show that, even in the presence of a strong magnetic field (B ～ 1015 G) in the shock formation region, the heating rate is determined with good accuracy by the absorption and emission of neutrinos in direct URCA processes. Moreover, the influence on them of a magnetic field is reduced to insignificant corrections. 相似文献