A phenomenological interpretation of stellar chromospheres

An attempt is made to develop a phenomenological interpretation of stellar chromospheres. The following problems are examined: observed emission powers of magnesium chromospheres on stars based on the ultraviolet doublet, 2800 Mgii, observations; dependence of chromosphere emission on spectral and luminosity classes; stellar chromospheres as an accidental event; chromospheres of stars-components of binary systems; stars with the chromospheres of solar type (S) and nonsolar (NS) type; distribution of stars by means of the type of their chromosphere on luminosity class; stars with superpower magnesium emission; emission measures for both the magnesium and calcium chromospheres; interrelation between chromosphere, transition zone and corona; chromospheric activity and rotation of stars; possibility of the existence of chromospheres on hot stars; phenomenological picture of stellar chromospheres; stars without the line 2800 Mgii, in emission or in absorption; syndrome of red giant HD 4174. At the end, the problem of heating of stellar chromospheres is discussed.     

The stability parameters of three-body periodic orbits

Formulae containing the elements of the variational matrix are obtained which determine the linear iso-energetic stability parameters of periodic orbits of the general three-body problem. This requires the numerical integration of the variational equations but produces the stability parameters with the effective accuracy of the numerical integration. The procedure is applied for the determination of horizontally critical orbits among the members of sets of vertical-critical periodic orbits of the threebody problem. These critical-critical orbits have special importance as they delimit the regions in the space of initial conditions which correspond to possibly stable three-dimensional periodic motion of low inclination.     

Alternating cycle durations in dwarf novae

The cycle durationC in some dwarf novae underwent abrupt significant changes, as was found by using our own photometric data and the published moments of maxima. For the analysis, the DMRT method (Dumontet al., 1978) as well as least-squares (LS) routine were used. The DMRT method is more appropriate for a search for mean period, if the cycle numeration is unknown. Such changes may be possibly explained by a solar-type activity of convective secondary stars in cataclysmic binaries. However, the abrupt character of the changes ofC (i.e., the time of switchings from one cycle to another is sufficiently small as compared with the lifetime of a given cycle) is an observational puzzle which is to be interpreted theoretically.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Drift Kelvin-Helmholtz instabilities in space plasmas

We implement a complex-plane strategy and a multiple partition technique to the computation of polytropic models distorted by very strong and very rapid differential rotation. We also verify with our numerical results a heuristic relation between stability and virial theorem.     

Magnetic reconnection in a high-temperature plasma of solar flares

Simple self-consistent models for non-neutral current sheets are considered. Characteristics of high-temperature turbulent current sheets (HTCS) with a small transverse component of magnetic field are determined for conditions in the solar corona. The energy output of such an HTCS is much larger than that of a neutral sheet. This makes it possible to consider the HTCS as an energy source not only in long-lived X-ray loops but also in flaring loops during the not or main phase of a flare. In this case, the magnetic reconnection velocity agrees with the observed velocity of the loop rise. Thus, these phenomena can be interpreted as a result of magnetic reconnection, for example, between new flux emerging from under the photosphere and an old magnetic field.The role of a longitudinal magnetic field in a current sheet is less important for HTCS. As a result of the compression of a longitudinal field, there appears an electric current circulating around the sheet. This current may induce strong Joule heating, if the compression is large. This additional heating is realized because of the annihilation of the main component, not the longitudinal component of magnetic field. The effect is small for HTCS, but may be significant for preflare current sheets.     

Hydrodynamic models of Planetary Nebulae

This article tries to give a short review of the hydrodynamic modelling of Planetary Nebulae to date. I start by discussing the types of observations the modelling aims to reproduce and then discuss the results of different types of models. First the simplified models in which evolutionary changes in the stellar properties are neglected, then the evolutionary models in which these effects are taken into account. The simplified models can reproduce single shell nebulae with the right kinematic properties. The evolutionary models can reproduce multiple shells and some of the kinematic and morphological differences between various ions. I also briefly discuss some of the difficulties which have not been addressed and/or solved by the numerical modelling, such as the point-symmetric nebulae, some of the collimated phenomena, and clumpiness.Abbreviations AGB Asymptotic Giant Branch - PN Planetary Nebulae - YSO Young Stellar Object     

Electrostatic Wave Instability and Plasma Radiation from Coronal Loops

Polarization properties of solar and stellar radio emission require, in some cases, emission below the third or fourth coronal electron gyro level, < 3,_c; 4, _c. In the context of plasma radiation, the source parameters should be such that the intermediate magnetic field condition 1 < _p ² / _c ² < 3 is satisfied. Supposing this condition, we investigate the generation of electrostatic waves in a warm background plasma with a high-energy component of magnetically trapped electrons. We invoke the conversion of upper-hybrid waves and Bernstein waves into electromagnetic radiation as being responsible for intense radio emission from a coronal magnetic loop. Moreover, odd-half harmonic emissions in the solar radio spectrum as well as the o-mode polarization at the second harmonic of the plasma frequency are natural consequence of this proposed model.     

Planar orbits through second variations

The development and application of a predictor-corrector method for the computation of families of periodic motions as well as of singular periodic solutions from which branchings or change in the stability character occur, based on the use of second order variations is presented. Numerical results obtained by means of this method are also given. It is found that this algorithm consumes somewhat less computer time in determining orbits-members of a family of periodic orbits, while it represents a unique tool for the determination of branchings of various orders, as well as of the precise members of each family at which the orbits change from unstable to stable and viceversa.     

‘Dead’ pulsars: Cosmic-ray sources

Dead pulsars outnumber live pulsars by a factor of 10⁴. It is estimated that there are 3×10⁹ of them in our Galaxy. The exospheres of the atmosphere of dead pulsars are characterised by cosmic-ray energies per particle, as the result of accretion of cold particles from interstellar space. Velocities of particles in the exosphere tend to be Maxwellianised by collisions there. The temperature of the exosphere from which particles escape is of the order of 10¹² K while the temperature of the photosphere closer to the surface of the pulsar is of the order of 10⁷ K. Collisions in the exosphere result in Jeans's type escape of cosmic rays with GeV energies at infinity. Two braod ranges of conditions for the exospheres are considered (a) with no magnetic fields involved, and (b) with magnetic fields. Similar conclusions are reached regarding the escape of cosmic rays. Conditions are delineated such that the exospheres of dead pulsars might be major sources of cosmic rays.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.     

The development and production of new,non-standard methods of observations with the radio telescope RATAN-600

The recent modernization of the RATAN-600 radio telescope at the Special Astrophysical Observatory has provided an opportunity to develop new procedures, called relay and zoned relay, for utilizing the new cone-surface secondary reflector. The geometric area, the horizontal beamwidth spatial resolution, and the permissible radiometer bandwidth are discussed. We are also able to estimate the changes in the sensitivities of the radio telescope-radiometer systems. The new procedures, called relay and zoned relay, allow us to obtain two-dimensional images of radio objects and to track the development of rapidly varying sources in any azimuth.Presented at the CESRA-Workshop on Coronal Magnetic Energy Release at Caputh near Potsdam in May 1994.     

On the third integral of the galaxy

It is shown here that the third integral of the galaxy, whenever its constant is conserved, defines the same surface as the Hamiltonian, and thus does not constitute anynew integral, but a function of the already known integral of energy. In particular, the third integral and the Hamiltonian are found to possess collinear gradients, in accordance with Poincaré's theorem concerning the characteristic exponents in systems with multiple integrals.     

The ‘naked’ singularity in the scalar-tensor gravitation theory

The complete analysis of possible types of non-relativistic and ultra-relativistic motions in the field of the naked singularity of the STGT is given. The problem of the infall of a monochromatic radiation emitter on the naked singularity is studied. In spite of the absence of a freezing effect in the STGT, the naked singularity is not observable, for the brightness of the falling emitter decreases much faster than in the GTR. Atrr _g , the trajectory of particles and light are quite similar to that of the GTR, and due to this fact the naked singularity is observationally indistinguishable from a black hole.     

High-resolution photography of the solar chromosphere

High-resolution filtergrams of a sunspot and its surroundings, taken at seven wavelengths in H with the aid of a computer-controlled 1/8 Å filter, have been used to derive the contrast of ten sunspot superpenumbra fibrils as functions of wavelength. The observed contrast profiles are compared with profiles calculated on the basis of three theoretical models, namely, Beckers' (1964) cloud model, Athay's (1970) velocity model, and a pure Doppler shift model. However, none of these models in their present form account for all the observed profiles.Arguments are presented which suggest that, in addition to line-of-sight velocity, the height of a moving chromospheric structure relative to the region over which the H line is formed plays a decisive role in shaping the observed profile. Improvements to existing theoretical treatments are suggested.The nature of the fibril velocity field is discussed briefly.     

Microscopic spectral features in solar decametric bursts and coronal irregularities

A crossed Yagi antenna array at 35 MHz was employed in conjunction with a polarization switch so as to enable spectral observations of solar noise storm activity in R and L polarizations. Intense decametric solar noise storms were recorded during the third week of November 1975 and fourth week of March 1976 with the help of a high resolution spectroscope operating near 35 MHz.The paper describes some of the new microscopic spectral features observed during these two noise storms. Three sets of high resolution dynamic spectra of decametric solar bursts, two of which are explained in terms of induced scattering of Langmuir waves by thermal ions and the third in terms of additional propagation effects through dense coronal irregularities, are presented. The microscopic bursts, classified as inverted U U and dots, represent small-scale (10⁴ km) phenomena with durations of less than a second.Some burst spectra appear as chain of dots with individual bandwidths 40 kHz and durations 0.3 sec. It is suggested that the bandwidth of such dot emissions (40 kHz) provides an evidence that they might indeed be generated by the process of induced scattering of plasma waves which predicts emission bandwidth f × 10^–3, where f is the center frequency.Some bursts are observed as a chain of striations showing curvature along the frequency axis which is attributed to dispersion in propagation delays through the dense coronal irregularities.     

Solar flare nomenclature

The evolution of solar flare nomenclature is reviewed in the context of the paradigm shift, in progress, from flares to coronal mass ejections (CMEs) in solar-terrestrial physics. Emphasis is placed on: the distinction between eruptive (Class II) flares and confined (Class I) flares; and the underlying similarity of eruptive flares inside (two-ribbon flares) and outside (flare-like brightenings accompanying disappearing filaments) of active regions. A list of research questions/problems raised, or brought into focus, by the new paradigm is suggested; in general, these questions bear on the interrelationships and associations of the two classes (or phases) of flares. Terms such as eruptive flare and eruption (defined to encompass both the CME and its associated eruptive flare) may be useful as nominal links between opposing viewpoints in the flares vs CMEs controversy.     

Mass and particle ‘loading’ in plasma-neutral gas interaction

Qualitative considerations of mass and particle loading in plasma-neutral gas interaction are made by the help of a one-dimensional steady model. In the case of particle loading the plasma flow behind a strong shock is accelerated as in the case of mass loading regarded by Biermannet al. (1967) whereas behind a weak shock the flow is decelerated. As an example, one-dimensional time-dependent flows with ionization of the neutral gas by Alfvén's critical velocity effect are calculated. Because of the acceleration of the subsonic flow by loading processes, the existence of a tangential discontinuity in the flow around comets seems questionable.     

On phase and ray directions of magnetosonic waves

The behavior of phase speed for the slow and fast magnetosonic waves is well documented in the literature. Not so well documented is the behavior of the ray direction and its relation to the phase direction — indeed we have not found the ray behavior recorded in most of the standard plasma physics texts. We rectify this situation and point out some of the curiosities associated with the direction of the slow ray relative to the direction of the slow phase wave. These calculations have been performed as a necessary basis for discussion of phase and ray evolution of magnetosonic waves in differentially shearing plasmas, which subject is the topic of a later paper.     

The fast explosive evaporation of mini black holes and flares at magnetospheres: An attractive speculative analogy

A number of suggestive coincidences are found in a purely speculative analogy between the by-products, of a minihole fast evaporation in a plasma, and by-products from flares at magnetospheres. Solar flares might serve as good examples. From an observational point of view our attention is drawn to the considerable extent of the blank time interval existing between known before and after evidences on flares, during which some exotic energy conversion processes could be invoked to explain the large amount of energy released in time-scales that might be very short. We suggest that fundamental progress in the knowledge of energy conversion, processes in flares require observational data with a time resolution several orders of magnitude, better than the currently available data, particularly, for hard X-rays.     

Solar magnetic fields and convection

The flux-rope-fibre model of solar magnetic fields is developed further to cover post-spot evolution of the fields, faculae, and the influence of magnetic fields on some convective motions. (i) Unipolar magnetic regions of a strongly dominant polarity are explained, as are some fields outside the network, and some tiny reversed polarity fields. (ii) The migration of magnetic regions is explained: the following regions to the poles where most of the flux just vanishes and the preceding towards the equator. (iii) The model explains the rotation of the gross pattern of background fields with a period of 27 days. It explains the puzzling features of active longitudes and of magnetic longitudes extending across the equator. (iv) The magnetic model provides a framework for the various chromospheric fine structures, the rosettes, bushes, double chains, mottles and spicules. It provides qualitative models of these features and points the way to a very complicated quantitative model of the network. (v) Several new convective patterns are described and explained in terms of magnetic stresses. The first is the moat around sunspots, which replaces the supergranule motions there. The second is the long-lived (4–7 days) supergranule cell enclosed by strong fields. The third is a small-scale () convective motion, and the fourth is aligned or long granules, both caused by small-scale magnetic fields. (vi) Photospheric line faculae and photospheric continuum faculae are different phenomena. The former, like the chromospheric faculae, are caused by Alfvén-wave heating. The latter are caused by a new small-scale convective motion. (vii) A model of the 3-min oscillation is described.     

Геометрическая интерпретация необходимых условий устойчивости треугольных точек либрации общей задачи трех тел

. . . , , m ₁, m₂, m₃, n ( .

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The conditions of stability of the constant libration solutions of the general three-bodies problem obtained by E. Routh through investigation of the linearized perturbed motion equations are analised. To the values of the masses of the three bodies which satisfy boundary conditions of the region of stability the locus of all corresponding centers of masses is laid down in accordance with. It occurs that this locus is a circle, its centre coinciding with the geometric centre of the trianglem ₁, m₂, m₃ and its radius being a function of exponent in the law of attraction of the bodies. The motion may be stable only if the centre of masses of the bodies lies outside the circle mentioned above. In the case of the Newtonian law of attraction the radius of this circle equals 0,943 |r_max| where |r_max| is a distance of the vertex from the centre of the trianglem ₁, m₂, m₃. Thus stability is possible (if it is generally possible) inside a very small region in this case.