Energy Source for Shingle-like Structures in Spiral-arms

The filaments or shingles observed in neighbouring spiral-arms were found in a previous paper to have a wavelength of ˜ 1.3 kpc. The life-time of the shingles was estimated at ˜ 10⁸ years. The energy source to regenerate the shingles is sought in terms of a JEANS -type instability modified by a two stream instability between population I and disk material. From the dispersion relation it is found that an instability of this type occurs at a critical-wavelength of 1.9 kpc.     

Effectivity of the modified two stream instability operating in the vicinity of Venus

This paper is devoted to the application of the modified two stream or cross current instability (MTSI) to the interaction of the solar wind and Venus. Two scenarios are presented providing favorable conditions for the excitation of the instability. For the first scenario, the free energy source of the MTSI is a significant gradient drift of the solar wind protons near the subsolar ionopause. The corresponding growth rates and frequencies of the MTSI are calculated within a full electromagnetic approach for a two-component plasma. The driving source of the second considered scenario consists in the relative drift velocity between solar wind and planetary particles. For modelling this situation, the dispersion equation for a four-component plasma is solved numerically.     

Simple tests for the ideal MHD stability of line-tied coronal magnetic fields

Methods for investigating the stability of line-tied, cylindrically-symmetric magnetic fields are presented. The energy method is used and the perturbed potential energy integral is manipulated to produce simple tests that predict either stability to general coronal disturbances or instability to localized modes, both satisfying photospheric line-tying. Using these tests the maximum amount of magnetic energy, that can be stored in the coronal magnetic field prior to an instability, can be estimated. The tests are applied to four different classes of equilibria and results are obtained for both arcade and loop geometries.     

On the propagation of the electron streams generating type III bursts

The suggestion is explored that the two-stream instability has little effect on the propagation of the electron streams which generate type III bursts because the time required (ti) for development of the instability is comparable with or greater than the time available (Δt) for growth of the waves. Inferred parameters for streams in the corona and measured parameters for streams at the orbit of the Earth are compatible with this suggestion. Quasi-linear relaxation, which should occur as the stream forms, ensures that equality ti = Δt is set up initially, and restricts the number of escaping electrons to N _s ≈ 10³¹. The minimum density requirement on the stream for the two-stream instability to occur is found to be much less restrictive than the requirement that there should be many streaming electrons per Debye sphere.     

Low T/|W| dynamical instability in differentially rotating stars: diagnosis with canonical angular momentum

We study the nature of non-axisymmetric dynamical instabilities in differentially rotating stars with both linear eigenmode analysis and hydrodynamic simulations in Newtonian gravity. We especially investigate the following three types of instability; the one-armed spiral instability, the low   T /| W |  bar instability, and the high   T /| W |  bar instability, where T is the rotational kinetic energy and W is the gravitational potential energy. The nature of the dynamical instabilities is clarified by using a canonical angular momentum as a diagnostic. We find that the one-armed spiral and the low   T /| W |  bar instabilities occur around the corotation radius, and they grow through the inflow of canonical angular momentum around the corotation radius. The result is a clear contrast to that of a classical dynamical bar instability in high   T /| W |  . We also discuss the feature of gravitational waves generated from these three types of instability.     

Modeling of type III bursts dynamic spectra

The one-dimensional process of spatially limited electron stream propagation in the solar corona is simulated. It is shown that the beam instability development results either in strong relaxation in velocity space and inhibition of spatial diffusion (high-stream density) or in velocity space relaxation decrease and simultaneous growth of spatial stream length (low-stream density). Assuming a profile of background plasma density to be exponential, dynamic spectra of type III bursts are modeled, which shows that the emission source velocity is constant, and a duration of the burst emission at a given frequency reduces for high-stream densities.     

Stability of thermally balanced gas flow

The linear analysis of hydrodynamic stability of the local thermal balance in a homogeneous moving gas is revisited to get information about the development of a spatially limited perturbation as seen at a fixed location. The consideration concerns both the evolution of the perturbed quantities inside a domain where the perturbation initially localizes and spreading the perturbation outside this domain. Inside the initial perturbation domain, the conditions for the exponential growth/decay are found to coincide with the well-known Field's criteria, ensuing the analysis of the normal modes. However, as soon as the modal isentropic stability criterion is satisfied the perturbation outside its initial domain asymptotically spreads out with a subsonic velocity not depending on the initial perturbation field. It enables the gas flow to carry the disturbances away and leads to an improved stability criterion for inhomogeneous thermally balanced flows where the modally unstable region appears to be spatially bounded. The spreading velocity, playing a key role in the new stability criterion, is calculated as a function of the same derivatives of the heating/cooling function as the modal instability criteria exploit.     

The Flare-Energy Distributions Generated by Kink-Unstable Ensembles of Zero-Net-Current Coronal Loops

It has been proposed that the million-degree temperature of the corona is due to the combined effect of barely detectable energy releases, called nanoflares, that occur throughout the solar atmosphere. Unfortunately, the nanoflare density and brightness implied by this hypothesis means that conclusive verification is beyond present observational abilities. Nevertheless, we investigate the plausibility of the nanoflare hypothesis by constructing a magnetohydrodynamic (MHD) model that can derive the energy of a nanoflare from the nature of an ideal kink instability. The set of energy-releasing instabilities is captured by an instability threshold for linear kink modes. Each point on the threshold is associated with a unique energy release; thus we can predict a distribution of nanoflare energies. When the linear instability threshold is crossed, the instability enters a nonlinear phase as it is driven by current sheet reconnection. As the ensuing flare erupts and declines, the field transitions to a lower energy state, which is modelled by relaxation theory; i.e., helicity is conserved and the ratio of current to field becomes invariant within the loop. We apply the model so that all the loops within an ensemble achieve instability followed by energy-releasing relaxation. The result is a nanoflare energy distribution. Furthermore, we produce different distributions by varying the loop aspect ratio, the nature of the path to instability taken by each loop and also the level of radial expansion that may accompany loop relaxation. The heating rate obtained is just sufficient for coronal heating. In addition, we also show that kink instability cannot be associated with a critical magnetic twist value for every point along the instability threshold.     

Stability of particulate discs

Collisionally-induced amplification of density fluctuations can also produce non-axisymmetric local condensations in particulate discs if the optical thickness is between definite values. Gravitational instability occurs above this interval. The theory of both phenomena is derived from collisional equations. The conventional criterion for gravitational instability in a gaseous medium cannot be used for particulate discs, in which the equilibrium depends on the collisional energy loss. These instabilities can produce an unbounded growth in density or a gravitational coagulation of particles, but the typical consequence is the formation of highly elongated clouds which are denser than the background matter and have a relatively long lifetime before decay. The third type of instability, the thermal one, appears at low values of velocity dispersion. It only affects the random motion of particles without producing condensations.     

Magnetic and vertical shear instabilities in accretion discs

The stability properties of magnetized discs rotating with angular velocity Ω = Ω( s ,  z ), dependent on both the radial and the vertical coordinates s and z , are considered. Such a rotation law is adequate for many astrophysical discs (e.g., galactic and protoplanetary discs, as well as accretion discs in binaries). In general, the angular velocity depends on height, even in thin accretion discs. A linear stability analysis is performed in the Boussinesq approximation, and the dispersion relation is obtained for short-wavelength perturbations. Any dependence of Ω on z can destabilize the flow. This concerns primarily small-scale perturbations for which the stabilizing effect of buoyancy is strongly suppressed due to the energy exchange with the surrounding plasma. For a weak magnetic field, instability of discs is mainly associated with vertical shear, whilst for an intermediate magnetic field the magnetic shear instability, first considered by Chandrasekhar and Velikhov, is more efficient. This instability is caused by the radial shear which is typically much stronger than the vertical shear. Therefore the growth time for the magnetic shear instability is much shorter than for the vertical shear instability. A relatively strong magnetic field can suppress both these instabilities. The vertical shear instability could be the source of turbulence in protoplanetary discs, where the conductivity is low.     

Laboratory simulation of energy release in solar flares

A laboratory simulation method is proposed for energy release processes occurring in a fragment of the flare current sheet on the Sun. The method relies on the assumption that the spatial scale of such processes is represented by the current sheet's thickness whose values can be close for both the solar and laboratory conditions. It is shown that in an extended current sheet, current dissipation on anomalous resistivity that ensures the specific power of energy release close to that observed in a flare, is the main energy release mechanism. A rapid compression of the sheet by external magnetic fields can provide the condition for switching on a powerful energy release. The tearing instability developing in a homogeneous neutral sheet, leads to the formation of magnetic islands in which the energy release is localized.     

Linear and nonlinear cyclotron instability and VLF emissions in the magnetosphere

A theoretical study is made of the whistler mode cyclotron instability both in linear and nonlinear regimes in conjunction with the generation of VLF emissions in the magnetosphere. For the nonlinear treatment, a well-established quasilinear method is used and some physical processes of the cyclotron instability viz. energy conservation, mechanism of instability and frequency change of the excited emissions are clarified. The results are applied to some types of the triggered VLF emissions; whistler triggered emissions and artificially stimulated emissions (ASE). It is found that whistler triggered emissions excited around the upper cutoff frequencies of whistlers may be explained by the whistler mode cyclotron instability by a model distribution function inferred from satellite data. In order to see a nonlinear evolution of the whistler mode cyclotron instability, computer simulations were carried out and it is shown that the change of frequency with time of whistler triggered emissions as well as characteristics of ASE are well explained by resonant nonlinear behaviour of whistler mode cyclotron instability considered in the present paper.     

Cosmic Ray Nonlinear Processes in Space Plasma: Applications to the Dynamic Heliosphere

Influence of cosmic ray pressure and kinetic stream instability on space plasma dynamics and magnetic structure are considered. It is shown that in the outer Heliosphere are important dynamics effects of galactic cosmic ray pressure on solar wind and interplanetary shock wave propagation as well as on the formation of terminal shock wave of the Heliosphere and subsonic region between Heliosphere and interstellar medium. Kinetic stream instability effects are important on distances more than 40–60 AU from the Sun: formation of great anisotropy of galactic cosmic rays in about spiral interplanetary magnetic field leads to the Alfven turbulence generation by non isotropic cosmic ray fluxes. Generated Alfven turbulence influences on cosmic ray propagation, increases the cosmic ray modulation, decreases the cosmic ray anisotropy and increases the cosmic ray pressure gradient in the outer Heliosphere (the later is also important for terminal shock wave formation). This revised version was published online in July 2006 with corrections to the Cover Date.     

On the Kelvin-Helmholtz instability of structured plasma layers in the magnetosphere

The hydromagnetic Kelvin-Helmholtz (K-H) instability problem is studied for a three-layered system analytically by arriving at the marginal instability condition. As the magnetic field directions are taken to vary in the three regions, both the angle and finite thickness effects are seen on the instability criterion. When the relative flow speed of the plasmas on the two sides of the interfaces separating the inner and the surrounding layers is U < U_c, where U_c is the critical speed, the system is stable both for symmetric and asymmetric perturbations. However, unlike the case of the interface bounded by two semiinfinite media, U_c is no longer the minimum critical speed above which the system will be unstable for all wavenumbers; another critical speed U^* > U_c is introduced due to the finiteness of the system. When U_c < U < U^*, the instability can set in either through the symmetric or asymmetric mode, depending on the ratio of the plasma parameters and angle between the magnetic field directions across the boundaries. The instability arises for a finite range of wavenumbers, thus giving rise to the upper and lower cut-off frequencies for the spectra of hydromagnetic surface waves generated by the K-H instability mechanism. When U > U^*, both the modes are unstable for short wavelengths. The results are finally used to explain some observational features of the dependence of hydromagnetic energy spectra in the magnetosphere on the interplanetary parameters.     

Evolution of astrophysical jets generated by a vortex mechanism

The nonlinear dynamics of a rotating jet is examined following its ejection from a compact gravitating object by a vortex mechanism. A scenario is described in which a dense stream expands and is subsequently transformed into a nonstationary vortex consisting of a cylindrical core and a “sheath.” At this stage of development, a converging radial flow of matter in the differentially rotating nonuniform sheath collimates the jet and speeds up the rotation of the core, as well as the flow of matter along the jet, in accordance with a power law or “explosive” instability, until the velocity discontinuity at the surface of the core approaches the sound speed. Flows of this type have low energy dissipation and can serve as unique channels for the acceleration and collimation of jet eruptions from young stars, active galactic nuclei, and quasars. Translated from Astrofizika, Vol. 52, No. 1, pp. 135–145 (February 2009).     

A physical mechanism for the production of solar flares

The weakly ionized photospheric layer in the sunspot environment satisfies certain dynamo inequalities (Equations (11) and (13)) resulting in photospheric Hall current systems. The corresponding Joule dissipation is associated with the surrounding plage area. For critical values of the driving or convective winds (speeds 1 km/s), two stream instability results. The computed energy is of the order of that found in solar flares.     

A Manifestation of Negative Energy Waves in the Solar Atmosphere

Magnetosonic modes of magnetic structures of the solar atmosphere in the presence of inhomogeneous steady flows are considered. It is shown that, when the speed of the steady flow exceeds the phase speed of one of the modes, the mode has negative energy, and can be subject to an over-stability due to the negative energy wave instabilities. It is shown that registered steady flows in the solar atmosphere, with speeds below the threshold of the Kelvin–Helmholtz instability, can provide the existence of the magnetosonic negative energy wave phenomena. In particular, in isolated photospheric magnetic flux tubes, there are kink surface modes with negative energy, produced by the external granulation downflows. Dissipative instability of these modes due to finite thermal conductivity and explosive instability due to nonlinear coupling of these modes with Alfvén waves are discussed. For coronal loops, it is found that only very high-speed flows (>300 km s^-1) can produce negative energy slow body modes. In solar wind flow structures, both slow and fast body modes have negative energy and are unstable.     

Instabilities in a nonstationary model of self-gravitating disks. I. Bar and ring perturbation modes

The instabilities of bar and ring mode perturbations against the background of a disk oscillating nonlinearly in its own plane are examined in a disk model which is a nonstationary generalization of the well known Bisnovatyi-Kogan-Zel'dovich model. Nonstationary analogs corresponding to a dispersion relation are found for these two oscillation modes. The results of the calculations are presented in the form of critical dependences of the initial virial ratio on the degree of rotation. A comparative analysis of the growth rates of the gravitational instability for these modes is also carried out. The bar mode instability occurs if the initial total kinetic energy of the disk is no more than 10.4% of the initial potential energy. The mechanism is associated with an instability in the radial motions which is aperiodic for small values of the rotation parameter Ω < 0.1, but is otherwise oscillatory. Calculations show that a ring structure can be formed as a result of an instability in the radial motions if the initial total energy of the model is no more than 5.2% of the initial potential energy, regardless of the value of Ω. __________ Translated from Astrofizika, Vol. 51, No. 3, pp. 487–499 (August 2008).     

Large-scale circulations and energy transport in contact binaries

A hydrodynamic model for the energy transport between the components of a contact binary is presented. Energy is transported by a large-scale, steady circulation carrying high entropy matter from the primary to secondary component. The circulation is driven by the baroclinic structure of the common envelope, which is a direct consequence of the non-uniform heating at the inner critical Roche lobes due to unequal emergent energy fluxes of the components. The mass stream flowing around the secondary is bound to the equatorial region by the Coriolis force and its width is determined primarily by the flow velocity. Its bottom is separated from the underlying secondary's convection zone by a radiative transition layer acting as an insulator. For a typically observed degree of contact the heat capacity of the stream matter is much larger than radiative losses during its flow around the secondary. As a result, its effective temperature and entropy decrease very little before it returns to the primary. The existence of the stream changes insignificantly specific entropies of both convective envelopes and sizes of the components. Substantial oversize of the secondaries, required by the Roche geometry, cannot be explained in this way. The situation can, however, be explained by assuming that the primary is a main-sequence star whereas the secondary is in an advanced evolutionary stage with hydrogen depleted in its core. Such a configuration is reached past mass transfer with mass ratio reversal. Good agreement with observations is demonstrated by model calculations applied to actual W UMa-type binaries. In particular, a presence of the equatorial bulge moving with a relative velocity of  10–30 km s^−!  around both components of AW UMa is accounted for.     

The solar wind-magnetosphere dynamo and the magnetospheric substorm

In a quiet condition, the solar wind kinetic energy is converted into electrical energy. A small part of this energy is dissipated as heat energy in the polar ionosphere. We identify at least three types of magnetospheric disturbances which are not associated with an increase of the heat production and call them reversible disturbances, while the magnetospheric substorm is an irreversible disturbance which is associated with a large increase of the heat production.The magnetosphere appears to have an inherent internal instability by which a large amount of heat energy is sporadically produced in the polar upper atmosphere at the expense of the magnetic energy in the magnetotail. A positive feed-back process may be responsible for the growth of the instability and for the expansive phase, while the recovery phase sets in when some process begins to suppress the positive feed-back process.