Stability of a Galactic Disk with a Rotation Law Specified by an External Field

The stability of a galactic disk in the gravitational field of a massive body is studied. The mutual friction of the gaseous and dust components is taken into account. Criteria for dynamical and secular instability are found. Asymptotic expressions are obtained for the growth rates in the case of a low density of dust. The cosmogonic significance of these results is discussed.     

Electromagnetic instability of partially overlapping flows in the interstellar medium

The stability of the charged particle velocity distributions resulting from intermixing of neighboring flows of interstellar medium are examined, in particular, within the confines of spiral galaxies. It is shown for a specific example that the customary electrostatic instability shows up when the difference in the systematic velocities of both flows is sufficiently large compared to the thermal speed. On the other hand, the electromagnetic instability occurs, in principle, for an arbitrarily small difference in the flow velocities. The characteristic time for this instability to develop is much longer than for the electrostatic instability, but is still short compared to the ordinary evolution time for the interstellar medium. __________ Translated from Astrofizika, Vol. 49, No. 4, pp.637–649 (November 2006).     

The effect of spatial variations in viscosity on the structure of mantle flows

According to an opinion widespread in the literature, high viscosity regions (HVRs) in the mantle always affect the structure of mantle flows, changing it in both the HVR itself and the entire mantle. Moreover, a simplified relation is often adopted according to which the flow velocity in the HVR decreases in inverse proportion to viscosity. Therefore, in order to treat a smoother value, some authors introduce a new variable equal to the product of the flow velocity and the viscosity value in a given place. On the basis of numerical modeling, this paper shows that HVRs of two types should be distinguished in the mantle. If an HVR is immobile, mantle flows actually do not penetrate it. If the viscosity increase is more than five orders, the HVR behaves as a solid and flow velocities within it almost vanish. However, if an HVR is free, it moves together with the mantle flow. Then, the general structure of flows changes weakly and flow velocities within the HVR become approximately equal to the average velocity of flows in the absence of the HVR. Horizontal layers and vertical columns differing in viscosity from the mantle behave as regions of the first type, whose flow velocities can differ by a few orders. However, even such large-scale regions as the continental lithosphere, whose viscosity is four to five orders higher than in the surrounding mantle, float together with continents at velocities comparable to mantle flows, i.e., behave as regions of the second type.     

Influence of low viscosity asthenosphere on mantle flows

Numerical simulation in recent years has revealed that the cold lithosphere, whose viscosity is three to four orders of magnitude higher than that of the underlying mantle, behaves during mantle convection as a stagnant lid. On the basis of model calculations, this paper shows how convection changes over to this regime with increasing viscosity. Spatially fixed high viscosity inclusions and those moving with the convective flow have fundamentally different effects on the structure of convective flows. The model calculations indicate that anomalously low viscosity asthenospheric regions also lead to a specific regime of convection. With a decrease in the viscosity by more than three orders of magnitude, a further reduction in the viscosity of the region ceases to influence the structure of convection in the outer region. The boundary with this region behaves as a freely permeable boundary. In the low viscosity asthenospheric region itself, autonomous convection can arise under certain conditions.     

Exact analytical solutions of the stokes equation for testing the equations of mantle convection with a variable viscosity

Presently, the study of the mantle flow structure is mainly based on numerical modeling. The most important stage of the development of a computer program is its testing. For this purpose, results of various test models of convection flows with a given set of parameters are compared. The solution of the Stokes equation, involving the derivative of viscous stresses, is most difficult. Exact analytical solutions of the Stokes equation are obtained in this work for various cases of special loads. These solutions can be used as benchmarks for testing programs of numerical calculation of viscous flows in both geophysics and engineering. The advantage of this testing technique is the exceptional simplicity of the solution form, the admissibility of any spatial viscosity variations, and the fact that solutions can be compared not for a narrow set of the solution parameters but for any distributions of velocities, viscous stresses, and pressures at all points of the space.     

A model for interpenetrating differential streams and the problem of their stability

The role of the stability of differential streams in a self-gravitating medium is studied. A simple model with two two-dimensional streams in a thin layer that interact only gravitationally is considered. Instability can develop if the stream-shear parameters have opposite signs; however this condition is not sufficient, and, for some combinations of parameters, the Jeans instability can disappear due to the drift of the perturbations when shear is introduced. The opposite situation is also possible: the system as a whole can be unstable even if both subsystems are stable. Under certain conditions, perturbations do not grow in time but waves are continuously emitted. Criteria are presented for the instability of the system as the whole, depending on the region where the parameters of the subsystem are localized. Common drawbacks of stability analyses in stellar dynamics are briefly discussed in this context.     

Seismic facies and specific character of the bottom simulating reflector on the western margin of Paramushir Island,Sea of Okhotsk

Seismic profiles from a venting area on the western margin of Paramushir Island (Sea of Okhotsk) reveal a local complex structure and an interesting, unusual pattern of the bottom simulating reflector (BSR). The BSR is gradual rising towards the venting area. The geothermal gradient and the bottom temperature confirmed the methane hydrate. The temperature appears to be the most important factor controlling the hydrate stability. A locally higher heat flow caused the upward migration of the hydrate stability field and the subsequent degradation of the hydrated sediments, causing gas vent formation and the flux of methane gas into the water column.