Instabilities of spiral shocks – I. Onset of wiggle instability and its mechanism     

Keiichi Wada  Jin Koda † 《Monthly notices of the Royal Astronomical Society》2004,349(1):270-280

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Analytical Study of Supernova Remnant Non-Stationary Expansions     

X. Ribeyre  V. T. Tikhonchuk  S. Bouquet 《Astrophysics and Space Science》2005,298(1-2):75-80

We study analytically the Rayleigh–Taylor instability in expanding supernova gas shell. The instability appears at the inner shell surface accelerated by blowing pulsar wind. The most dangerous perturbations correspond to wavelengths comparable to the shell thickness. We analyze the fragility of the supernova remnant shell in function of the initial perturbation amplitude and the shell thickness.  相似文献   

A model for the coherent emission of solar radio moving type IV bursts     

Wang De-yu   《Chinese Astronomy and Astrophysics》2004,28(4):404-411

A theoretical model is proposed for interpreting the coherent emissionmechanism of solar radio moving type IV bursts. Energetic electrons produced in flares captured by an expanding and rising magnetic flux tube exhibit a beam-like distribution of velocities on the top of the flux tube. These excite beaming plasma instability and directly amplifies O-mode electromagnetic waves. The instability growth rate sensitively depends on the coronal plasma parameter, ƒ_pe/ƒ_ce and the beam-temperature T_b. This can qualitatively explain the high brightness temperature and high degree of polarization as well as the broad spectrum observed in this type of solar radio bursts.  相似文献   

The generation of ionization-shock front oscillations by a variable radiation flux     

K. V. Krasnobaev 《Astronomy Letters》2002,28(9):594-600

The effect of a time-varying radiation flux incident on an ionization front on the generation of ionization-shock front oscillations in the interstellar medium is analyzed analytically and numerically. We take into account both variations in the flux of ionizing radiation directly from the source that produces the ionization front and the absorption of energetic photons by the post-front plasma. Based on our calculations, we show that the time dependence of the radiation flux can be an additional factor (apart from small inhomogeneities in the interstellar medium) that contributes to the amplification of oscillations and to the kinetic energy input to the observed turbulent motions in H II regions.  相似文献   

A model of rotating hotspots for the 3 : 2 frequency ratio of high-frequency quasi-periodic oscillations in black hole X-ray binaries     

Ding-Xiong Wang  Yong-Chun Ye  Guo-Zheng Yao  Ren-Yi Ma 《Monthly notices of the Royal Astronomical Society》2005,359(1):36-42

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Weibel Turbulence in Laboratory Experiments and GRB/SN Shocks     

Mikhail V. Medvedev 《Astrophysics and Space Science》2007,307(1-3):245-250

It has recently been realized that the Weibel instability plays a major role in the formation and dynamics of astrophysical shocks of gamma-ray bursts and supernovae. Thanks to technological advances in the recent years, experimental studies of the Weibel instability are now possible in laser-plasma interaction devices. We, thus, have a unique opportunity to model and study astrophysical conditions in laboratory experiments – a key goal of the Laboratory Astrophysics program. Here we briefly review the theory of strong non-magnetized collisionless GRB and SN shocks, emphasizing the crucial role of the Weibel instability and discuss the properties of radiation emitted by (isotropic) electrons moving through the Weibel-generated magnetic fields, which is referred to as the jitter radiation. We demonstrate that the jitter radiation field is anisotropic with respect to the direction of the Weibel current filaments and that its spectral and polarization characteristics are determined by microphysical plasma parameters. We stress that the spectral analysis can be used for accurate diagnostics of the plasma conditions in laboratory experiments and in astrophysical GRB and SN shocks.  相似文献   

Self-regulated gravitational accretion in protostellar discs     

E. I. Vorobyov  Shantanu Basu 《Monthly notices of the Royal Astronomical Society》2007,381(3):1009-1017

We present a numerical model for the evolution of a protostellar disc that has formed self-consistently from the collapse of a molecular cloud core. The global evolution of the disc is followed for several million years after its formation. The capture of a wide range of spatial and temporal scales is made possible by use of the thin-disc approximation. We focus on the role of gravitational torques in transporting mass inward and angular momentum outward during different evolutionary phases of a protostellar disc with disc-to-star mass ratio of order 0.1. In the early phase, when the infall of matter from the surrounding envelope is substantial, mass is transported inward by the gravitational torques from spiral arms that are a manifestation of the envelope-induced gravitational instability in the disc. In the late phase, when the gas reservoir of the envelope is depleted, the distinct spiral structure is replaced by ongoing irregular non-axisymmetric density perturbations. The amplitude of these density perturbations decreases with time, though this process is moderated by swing amplification aided by the existence of the disc's sharp outer edge. Our global modelling of the protostellar disc reveals that there is typically a residual non-zero gravitational torque from these density perturbations, i.e. their effects do not exactly cancel out in each region. In particular, the net gravitational torque in the inner disc tends to be negative during first several million years of the evolution, while the outer disc has a net positive gravitational torque. Our global model of a self-consistently formed disc shows that it is also self-regulated in the late phase, so that it is near the Toomre stability limit, with a near-uniform Toomre parameter Q ≈ 1.5–2.0. Since the disc also has near-Keplerian rotation, and comparatively weak temperature variation, it maintains a near-power-law surface density profile proportional to r ^−3/2.  相似文献   

Coronal expansion and the transfer of solar angular momentum to the interplanetary space     

Li Zhong-yuan  Chen Zong-yi  Mo Guo-biao 《Chinese Astronomy and Astrophysics》1992,16(4):450-456

We discuss the question of loss of angular momentum through coronal expansion. From a large volume of data on Type-1 cometary tails we have confirmed the presence of a tangential component in the coronal expansion, which has not only a stochastic component but also a constant component of 9.8 km/s. Through coronal expansion the Sun has lost 80% of its angular momentum since it evolved on to the main sequence and the angular velocity of the Sun is decreasing exponentially. This result should have a large effect on the dynamical evolution of the Sun.  相似文献   

Propagation of Extremely Low Frequency Waves Through the Ionosphere     

Singh  Ashok K.  Narayan  D.  Singh  R. P. 《Earth, Moon, and Planets》2002,91(3):161-179

The propagation features of extremely low frequency electromagnetic waves through the multicomponent ionospheric plasma are studied. It is shown that at relatively lower frequencies refractive index for right hand mode is higher than the left-hand mode, which is reversed at higher frequencies. The thermal temperature of plasma particle causes decrease in phase and group velocities of both right and left-hand modes. The crossover frequencies for different plasma models are computed and variation with ion concentration and thermal velocity is studied. Explicit expression for group velocity and travel time has been derived and studied numerically. Finally, we have presented simulation of the ion whistler spectrograms for Hydrogen, Helium and Oxygen ions present in the ionospheric plasma. The results are compared with the experimentally detected hydrogen and helium ion whistlers. The importance of the present study in the exploration of ionospheric plasma is illustrated.  相似文献   

Interfacial instabilities driven by self-gravity: first numerical results     

R.M. Hueckstaedt  J.H. HunterJr 《Monthly notices of the Royal Astronomical Society》2001,327(4):1097-1102

The evolution of the interstellar medium (ISM) is driven by a variety of phenomena, including turbulence, shearing flows, magnetic fields and the thermal properties of the gas. Among the most important forces at work is self-gravity, which ultimately drives protostellar collapse. As part of an ongoing study of instabilities in the ISM, Hunter, Whitaker & Lovelace have discovered another process driven by self-gravity: the instability of an interface of discontinuous density. Theory predicts that this self-gravity driven interfacial instability persists in the static limit and in the absence of a constant background acceleration. Disturbances to a density interface are found to grow on a time-scale of the order of the free-fall time, even when the perturbation wavelength is much less than the Jeans length. Here we present the first numerical simulations of this instability. The theoretical growth rate is confirmed and the non-linear morphology displayed. The self-gravity interfacial instability is shown to be fundamentally different from the Rayleigh–Taylor instability, although both exhibit similar morphologies under the condition of a high density contrast, such as is commonly found in the ISM. Such instabilities are a possible mechanism by which observed features, such as the pillars of gas seen near the boundaries of interstellar clouds, are formed.  相似文献