Showing papers in "Astrophysics and Space Science in 1978"

A double layer review

Abstract: A review is given of the main results on electrostatic double layers (sometimes called ‘space charge layers’ or ‘sheaths’) obtained from theory and laboratory and space experiments up to the spring of 1977. The paper begins with a definition of double layers in terms of potential drop, electric field, and charge separation. Then a review is made of the theoretical results obtained so far. This covers, among other things, necessary criteria for existence and stability, quantitative estimates of charge separation and thickness, and some probable cause of DL-formation in terms of an instability. Next, experimental results obtained in the laboratory are compared with the theoretical results. Due to recent progress in experimental technique, the interior of a double layer can now be studied in much more detail than was possible before. By means of barium jets and satellite probes, double layers have now been found at the altitudes that were previously predicted theoretically. The general potential distribution above the auroral zone, suggested by inverted V-events and electric field reversals, is corroborated.

Linearized limb-darkening coefficients for use in analysis of eclipsing binary light curves

Abstract: Linear limb-darkening coefficientsu required in the analysis of eclipsing binary curves, are tabulated for a wide range of effective temperature (50 000° to 4000°), wavelength λ (02 μ≤λ≤22 μ), and gravitiesg (20≤logg≤50) The computation is based on the comprehensive range of model atmospheres of Carbon and Gingerich (1969) The results are compared with the theoretical values of Hosokawa (1957), Kopal (1959) and Grygaret al (1972), and examined in relation to empirically determined values ofu from analyses of eclipsing binary light curves An improved agreement between theory and observation for the calculated limb-darkening coefficients of the present work is noted

Spurious results from Fourier analysis of data with closely spaced frequencies

Abstract: It is shown how erroneous results can occur using some period-finding methods, such as Fourier analysis, on data containing closely spaced frequencies The frequency spacing accurately resolvable with data of lengthT is increased from the standard value of about 1/T quoted in the literature to approximately 15/T

Decametric survey of discrete sources in the Northern sky: I. The UTR-2 radio telescope. experimental techniques and data processing

Abstract: The paper describes the antenna system, principles of the electrical beam steering, checks of the pattern parameters and the receiving facilities of the UTR-2 radio telescope. Principal characteristics of the instrument are given and possible errors analysed for the operating frequencies of 10, 12.6, 14.7, 16.7, 20 and 25 MHz. The methods of survey work on discrete radio sources are described with an analysis of the flux measurement errors specific for the decametre band. A computer algorithm of data processing is presented.

A cometary ionosphere model for Io

Abstract: A source for the ionosphere of Io is proposed based on the assumption that the satellite is rather moonlike but continuously bombarded by intense fluxes of energetic particles, which makes its surface electrically conducting so that a significant Birkeland current is drawn up along magnetic field lines from Jupiter's ionosphere. It is suggested that the ion current is neutralized upon contact with Io's surface and that subsequent sputtering of this material from the surface supplies the satellite's neutral atmosphere. A model for the generation and maintenance of Io's ionosphere is outlined, according to which the structure of the ionosphere is determined by the impact of energetic trapped electrons from the Jovian magnetosphere and the ram pressure of the corotational magnetospheric wind. The first of these two processes provides the main ionization mechanism, while the second compresses the upstream (or 'nighttime') ionosphere via Alfven's critical-velocity phenomenon. It is concluded that Io's ionosphere is more nearly analogous to the coma and tail of a comet in the solar wind than to the earthlike case of a permanent gravitationally bound ionosphere.

The role of electrostatic instabilities in the critical ionization velocity mechanism

Abstract: The role of electrostatic instabilities in the critical ionization velocity mechanism is investigated. The analysis is based on the theory developed by Sherman, which interprets Alfven's critical velocity in terms of a circular process. This process involves the acceleration of electrons by a two-stream instability modified by the presence of a magnetic field. A general expression for the energy and momentum of ions and electrons associated with an electrostatic mode is derived in terms of the plasma dielectric constant. This is used in the case of the modified two-stream instability to determine the distribution of energy between ions and electrons. An extrapolation from the linear phase then gives an estimate of the energy delivered to the electrons which is compared to that required to ionize the neutral gas.

The neutrino radiation for a hot neutron star formation and the envelope outburst problem

Abstract: With the equations of neutrino heat conductivity being used, the neutrino light curve is calculated for the spherically symmetrical collapse of an iron-oxygen 2M ⊙ star (Figure 1) up to the formation of a hot hydrostatically equilibrium neutron star. The total energy, radiated in the form of muon and electron neutrinos, is 5.8×1053 erg (0.16Mc 2). The mean neutrino particle energy is ∼12 MeV for all the time the collapse proceeds. The maximum neutrino luminosity value is equal to 3×1053 erg s−1. For a 10M ⊙ star collapse, the luminosity maximum 3×1054 erg s−1 takes place just at the moment of the formation of a black hole inside the collapsing star. The total radiated energy in this case is about 0.08Mc 2. The set of calculations, allowing for the deposition of momentum by means of neutrino-nuclear coherent scattering, brings us to a conclusion that the envelope outburst is only possible if the scattering cross-section is 50 times larger than the value experimentally accepted (inequality 20)).

TheF N method for solving radiative-transfer problems in plane geometry

Abstract: TheFN method of solving problems in radiative transfer for plane-parallel media with anisotropic scattering is established. The method utilizes properties of the ‘exact’ solution and leads to final equations that are particularly concise and easy to use.

Decametric survey of discrete sources in the northern sky

Abstract: The paper describes the antenna system, principles of the electrical beam steering, checks of the pattern parameters and the receiving facilities of the UTR-2 radio telescope. Principal characteristics of the instrument are given and possible errors analysed for the operating frequencies of 10, 12.6, 14.7, 16.7, 20 and 25 MHz. The methods of survey work on discrete radio sources are described with an analysis of the flux measurement errors specific for the decametre band. A computer algorithm of data processing is presented.

The Sunyaev-Zel'dovich effect in the millimetric region

Abstract: We analyse the spectral shape and possible spurious distortions of the Sunyaev-Zel'dovich effect, arising from the interaction of the cosmic background radiation with hot gases of rich clusters, in the millimetric region. We show that one can measure this effect with high accuracy with present-day infrared detectors, obtaining a larger amount of information than by using radio systems. In particular, one can accurately measure the temperature of the cosmic background radiation without the usual absolute calibration of the detector.

Plasma flow in a curved magnetic field

Abstract: A beam of collisionless plasma is injected along a longitudinal magnetic field into a region of curved magnetic field Two unpredicted phenomena are observed: The beam becomes deflected in the directionopposite to that in which the field is curved, and itcontracts to a flat slab in the plane of curvature of the magnetic field The plasma is produced by a conical theta-pinch gun and studied by means of high speed photography, electric and magnetic probes, ion analyser, and spectroscopy The plasma beam is collisionless and its behaviour is, in principle, understood on the basis of gyro-centre drift theory A fraction of the transverse electric fieldE=−v×B, which is induced when the beam enters the curved magnetic field, is propagated upstream and causes the reverse deflection byE×B drift The upstream propagation of the transverse electric field is due to electron currents The circuit aspect on the plasma is important The transverse polarization current in the region with the curved field connects to a loop of depolarization currents upstream The loop has limited ability to carry current because of the collisionless character of the plasma; curlE is almost zero and electric field components arise parallel to the magnetic field These play an essential role, producing runaway electrons, which have been detected An increased electron temperature is observed when the plasma is shot into the curved field Runaway electrons alone might propagate the electric field upstream in case the electron thermal velocity is insufficient The phenomenon is of a general character and can be expected to occur in a very wide range of ensities The lower density limit is set by the condition for self-polarization,nm i /ɛ 0 B 2≫ 1 or, which is equivalent,c 2/v A 2 ≫;1, wherec is the velocity of light, andv A the Alfven velocity The upper limit is presumably set by the requirement ω e τ e ≫ 1 The phenomenon is likely to be of importance, for example, for the injection of plasma into magnetic bottles and in space and solar physics The paper illustrates the complexity of plasma flow phenomena and the importance of close contact between experimental and theoretical work

Interstellar clouds and the formation of stars

Abstract: Part I gives a survey of the drastic revision of cosmic plasma physics which is precipitated by the exploration of the magnetosphere throughin situ measurements. The ‘pseudo-plasma formalism’, which until now has almost completely dominated theoretical astrophysics, must be replaced by an experimentally based approach involving the introduction of a number of neglected plasma phenomena, such as electric double layers, critical velocity, and pinch effect. The general belief that star light is the main ionizer is shown to be doubtful; hydromagnetic conversion of gravitational and kinetic energy may often be much more important. In Part II the revised plasma physics is applied to dark clouds and star formation. Magnetic fields do not necessarily counteract the contraction of a cloud; they may just as well ‘pinch’ the cloud. Magnetic compression may be the main mechanism for forming interstellar clouds and keeping them together. Part III treats the formation of stars in a dusty cosmic plasma cloud. Star formation is due to an instability, but it is very unlikely that it has anything to do with the Jeans instability. A reasonable mechanism is that the sedimentation of ‘dust’ (including solid bodies of different size) is triggering off a gravitationally assisted accretion. A ‘stellesimal’ accretion analogous to the planetesimal accretion leads to the formation of a star surrounded by a very low density hollow in the cloud. Matter falling in from the cloud towards the star is the raw material for the formation of planets and satellites. The study of the evolution of a dark cloud leads to a scenario of planet formation which is reconcilable with the results obtained from studies based on solar system data. This means that the new approach to cosmical plasma physics discussed in Part I logically leads to a consistent picture of the evolution of dark clouds and the formation of solar systems.

Cosmic-Ray Streaming Perpendicular to the Mean Magnetic Field. II: The Gyrophase Distribution Function

Abstract: The distribution function of cosmic rays streaming perpendicular to the mean magnetic field in a turbulent medium is re-examined. Urch's (1977) discovery that, in quasilinear theory, the flux is due to particles at 90° pitch angle is discussed and shown to be consistent with previous formulations of the theory. We point out that this flux of particles at 90° cannot be arbitrarily set equal to zero, and hence we dismiss the alternative theory which proceeds from this premise. A further, basic inconsistency in Urch's new transport equation is demonstrated, and the connection between quasilinear theory and compound diffusion is discussed.

An improved and generalized theory for the collisional evolution of Keplerian systems

Abstract: A calculation of collisional integrals with a higher accuracy yields excellent agreement between computer simulations and the collisional theory of Keplerian systems. Inclusion of axial rotation of particles modifies the evolution but does not introduce qualitatively new phenomena. Friction between the particles has a stabilizing influence, while deviations from an exactly spherical shape produce an opposite effect. The rotation of spherical or irregular bodies cannot prevent a final flattening of the system into a monolayer without also causing its disintegration. Computer simulations with a small number of particles do not represent the typical collisional evolution. They provide a test for the theory, but may sometimes lead to a misinterpretation of astronomical phenomena.

Observation of Birkeland currents with the TRIAD satellite

Abstract: The intimate connection between geomagnetic storms and the aurora was appreciated by many early scientists including Edmund Halley and Anders Celsius, but the first serious study of this phenomena was made by Kristian Birkeland who, during his polar expeditions of 1902–1903, determined that large-scale ionospheric current were associated with the aurora. Birkeland was also the first to suggest that these currents originated far from the Earth and that they flowed into and away from the polar atmosphere along the geomagnetic field lines. The existence of such field-aligned orBirkeland currents was widely disputed because it was not possible to unambiguously identify current systems that are field-aligned (Alfven, 1939, 1940) and those which are completely contained in the ionosphere (Vestine and Chapman, 1938) only from a study of surface magnetic field measurements. During the last decade, the presence of Birkeland currents has been absolutely confirmed with particle and magnetic field observations acquired from a variety of rocket and satellite instruments. The vector magnetometer on the low-altitude (∼800 km) polar orbiting TRIAD satellite has been used to determine for the first time the flow direction, spatial distribution, and intensities of Birkeland currents in the north and south auroral regions. These observations support the mechanism originally proposed by Alfven (1939, 1940)-later expanded by Shieldet al. (1969)-to drive Birkeland currents in the auroral regions, and they demonstrate the important role that these intense currents (ranging between 106 and 107 amperes) play in the coupling of energy between the magnetosphere and the lower ionosphere and atmosphere.

Experimental investigation of the critical ionization velocity in gas mixtures

Abstract: The critical ionization velocity which is of cosmogonic and astrophysical interest has hitherto mainly been investigated for pure gases. Since in space we always have gas mixtures, it is of interest also to study gas mixtures. The present report, which is a summary of a more detailed report (Axnas, 1976), summarizes the results of systematic experiments on the critical ionization velocity as a function of the mixing ratio for binary gas mixtures of H2, He, N2, O2, Ne and Ar. The apparatus used is a coaxial plasma gun with an azimuthal magnetic field. The discharge parameters are chosen so that the plasma is weakly ionized. In some of the mixtures it is found that one of the components tends to dominate in the sense that only a small amount (regarding volume) of that component is needed for the discharge to adopt a limiting velocity close to that for the pure component. Thus in a mixture between a heavy and a light component having nearly equal ionization potentials, the heavy component dominates. Also, if there is a considerable difference in ionization potential between the components, the component with the lowest ionization potential tends to dominate.

The magnetic-binaries problem: Motion of a charged particle in the region of a magnetic-binary system

Abstract: The combination of the two-body problem with that of the motion of a charged particle in a magnetic-dipole field (Stormer's problem) is treated in this article. Thus, in a long series of articles, a first step is taken to study the behavior of a charged particle in a magnetic-binary system. This first step includes a description of a simple model of such a system, the derivation of the equations of motion of the particle in the system, and a discussion on this foundation of the problem.

The interaction of the spiral density wave and the Sun's galactic orbit

Abstract: The galactocentric radial motion of the Sun introduces another periodicity to the encounters between the Sun and the spiral density wave. We describe a model simulation of the effect and present the associated periods.

Stellar polarization and the structure of the magnetic field of our galaxy

Abstract: The stellar polarization data have been examined using a new catalogue containing accurate stellar distances. On the assumption of a magnetic alignment hypothesis, correlations on the larger distance scale indicate the existence of a dominant regular magnetic field, although its characteristics are difficult to determine. Within 500 pc its direction is towardsl≈45° and beyond this towardsl≈60°, though it is clear that such a longitudinal model is too simple. There is also some evidence for an inclination of this field to the galactic plane. The distribution of the polarization vectors away from the galactic plane has been examined and it is proposed that the two largest loop structures, previously identified as Supernova remnants, are linked by the regular field. Incremental polarization maps have been produced but they show little correlation with the spiral structure. The polarization appears to be saturated at about 1 kpc from the Sun, which is explained as the result of an observational selection effect. On the smaller distance scales an autocorrelation analysis in different directions has revealed no obvious coherence in the irregular component on scales greater than 50 pc.

Self-similar space-times: I: Three solutions

Abstract: The equations of general relativity are reduced to a set of simultaneous ordinary differential equations in one dimensionless variable by an appeal to self-similar symmetry. Analytic solutions are given representing (i) a static, spherically symmetric matter distribution with density ϱ ∝R−θ and θ=2, and (ii) a non-static dust cloud with θ dependent on epoch. (iii) For a ‘stiff’ equation of state, the topology of the solutions has been investigated numerically and we identify a uniquely defined ‘critical’ solution. We argue that self-similarity implies a canonical space-time cosmologically, and we also briefly indicate where these space-times may apply astrophysically.

Production of the light elements due to neutrinos emitted by collapsing stellar cores

Abstract: The cross-sections of interaction of neutrinos emitted by collapsing stellar cores with matter of an expelled envelope are estimated (Table I). The neutrinos are shown to be able to produce isotopes of light elements (lithium, beryllium and boron) effectively in the stellar envelope and also, possibly, to create some deuterium. The neutrino-induced production of light elements accounts quite naturally for the observed enhanced abundances of odd isotopes of lithium and boron, as compared with even ones. It is emphasized that the reliable quantitative evaluation of the light element yields has to be based on the thoroughly elaborated hydrodynamical models of the envelope ejection. A slow expulsion of the envelope due to non-thermal forces seems to be the most favourable case for the neutrino-induced production of light elements. The proposed mechanism for the production of light elements can be verified observationally. The enhanced abundances of lithium and boron with respect to their cosmic values are expected to be characteristic of Crab nebula filaments. Also, it is of interest to search for the lines of light elements in the spectra of supernovae.

Fourier analysis of the light curves of eclipsing variables, XIV

Abstract: The aim of the present paper will be to utilize the results obtained in the preceding papers of this series for the development of practical procedures for obtaining the elements of any eclipsing system from the observed photometric data by their analysis in the frequency-domain, for any type of eclipses, any proximity of the two components, and any degree of the law of limbdarkening of the eclipsed star. In Section 2, which follows a brief introduction to the subject, procedures will be developed which should permit us to perform such an analysis — by hand or automatic machine computation — for the case of mutual eclipses in binary systems the components of which can be regarded as spheres; and whose apparent discs are characterized by an arbitrary radially symmetrical distribution of surface brightness. In Section 3 we shall generalize these procedures to systems consisting of arbitrarily distorted stars.

Double Radio Sources and the new Approach to Cosmical Plasma Physics

Abstract: The methodology of cosmic plasma physics is discussed. It is very hazardous to try to describe plasma phenomena by theories which have not been carefully tested experimentally. One present approach is to rely on laboratory measurements andin situ measurements in the magnetosphere and heliosphere, and to approach galactic phenomena by scaling up the wellknown phenomena to galactic dimensions. A summary is given of laboratory investigations of electric double layers, a phenomenon which is known to be very important in laboratory discharges. A summary is also given of thein situ measurements in the magnetosphere by which the importance of electric double layers in the Earth's surrounding is established. The scaling laws between laboratory and magnetospheric double layers are studied. The successful scaling between laboratory and magnetospheric phenomena encourages an extrapolation to heliospheric phenomena. A further extrapolation to galactic phenomena leads to a theory of double radio sources.

Consistency of Weyl's geometry as a framework for gravitation

Abstract: Recent work, quoted in the introduction, concerning a scale covariant theory of gravitation and corresponding astrophysical tests, has called for a revival of the geometrical considerations put forward by Weyl half a century ago, together with a better understanding of the mathematical foundation of Weyl's geometry. The present paper is to be regarded as a contribution in that direction.

Energy changes of cosmic rays in the interplanetary region

Abstract: A clarification and discussion of the energy changes experienced by cosmic rays in the interplanetary region is presented. It is shown that the mean time rate of change of momentum of cosmic rays reckoned for a fixed volume in a reference frame fixed in the solar system is 〈p〉 =pV·G/3 (p=momentum,V is the solar wind velocity andG=cosmic-ray density gradient). This result is obtained in three ways: (i) by a rearrangement and reinterpretation of the cosmic-ray continuity equation; (ii) by using a scattering analysis based on that of Gleeson and Axford (1967); (iii) by using a special scattering model in which cosmic-rays are trapped in ‘magnetic boxes’ moving with the solar wind.

Decametric Survey of Discrete Sources in the Northern Sky. II: Source Catalogue in the Range of Declinations + 10° to +20°

Abstract: Coordinates and absolute flux densities of 329 discrete sources located in the interval of declinations between +10° and +20° are presented. The measurements were carried out at 10, 12.6, 14.7, 16.7, 20 and 25 MHz on the UTR-2 radio telescope. The coordinates are compared with the 4C catalogue and other high-frequency surveys. There are ten sources in the Grakovo catalogue which have not been detected elsewhere previously.

Comets, Ice Ages, and Ecological Catastrophes

Abstract: A total mass ∼ 1014 g added to the Earth’s upper atmosphere in the form of small particles of high albedo for visual wavelengths would produce an inverse greenhouse effect, shielding ground level from sunlight but permitting infrared radiation from the ground to escape into space. Such a mass of small particles might be acquired by the Earth in a close approach to a cometary nucleus. Ice ages and ecodisasters, such as that which occurred 6.5 × 107 years ago, could arise from the effects of such an addition of small particles.

The Alfvén-Carlquist double-layer theory of solar flares

Abstract: We use the Vlasov equations for ions and electrons to develop a theory of a double layer in which there are both free and trapped electrons and ions. We find the equations which replace the Langmuir condition and the Bohm conditions and by numerically solving the resultant differential equation we find for particular choices of distribution functions the potential distribution in the layer. We discuss the applicability of this theory to solar flares, and show that conditions in solar flares may be such that double layers can exist for which the free particles have a power-law energy distribution. These particles will be accelerated in a double layer and may in this way account for the production of high-energy particles during the impulsive phase of solar flares.

The gamma-ray luminosity of spiral galaxies. Its evolution and its contribution to the diffuse background above 100 MeV

Abstract: The contribution of normal spiral galaxies to the high galactic latitude gamma-ray background >100 MeV is examined in the light of the estimates of its flux from the SAS-II measurements. The gamma-ray luminosity of each object is inferred from the known Milky Way value normalized to the corresponding optical quantity. Several possibilities are considered for the responsible physical production mechanism both diffuse and localized; they are then set to evolve with the galactic age according to three well-known evolutionary models. A final space-time integration leads to results which are expressed in the same unit as the measured background. It is seen that the model presented here can play an important role in the region > 100 MeV where the information on the spectral shape of the radiation is still very poor. Experimental tests for future gamma-ray observations are presented.

Asymmetric supernova explosions and the origin of binary pulsars

Abstract: We investigate the effect of asymmetric supernova explosions on the orbital parameters of binary systems with a compact component. We relate such explosions to the origin of binary pulsars. The degree of asymmetry of the explosion is represented by the kick velocity gained by the exploding star due to the asymmetric mass ejection. The required kick velocity to produce the observed parameters of the binary pulsar PSR 1913+16 should be larger than ∼80 km s−1 if the mass of the exploding star is larger than ∼4M⊙. We examine the mean survival probability of the binary system ( ) for various degrees of asymmetry in the explosion. The rare occurrence of a binary pulsar does not necessarily imply that such a probability is low since not all pulsars have originated in a binary system. Assuming the birth rate of pulsars derived by Taylor and Manchester (1977), we derive that would be as high as 0.25. Such values of can be obtained if the mass of the exploding stars is, in general, not large (≲10M⊙).