Some results of ionospheric investigations by means of coherent radiowaves emitted by satellites

In this paper we discuss theoretical expressions, determining the difference of Doppler shifts of various coherent radiowave frequencies emitted by a radiator moving in the ionosphere or interplanetary medium. The rotating Doppler effect (Faraday effect) caused by the Doppler shifts ±H of the ordinary and extraordinary waves is also considered. In a three-dimensional inhomogeneous ionosphere, stationary in time (N/t = 0), is determined in the general case, by an equation with three variables. The equation for proper depends only on the local value of the electron concentration N _c around the radiator and on integral values, determining, by means of additional calculations, the angle of refraction or its components, the horizontal gradients of electron concentration N/x and N/y, and in some cases, the integral electron concentration ₀ ^zcN dz. We describe the analysis of the measurements, made with the satellites Cosmos I, II and partially XI, assuming that N/t = N/y = 0, with a two variables equation. The expected errors are considered. The results coincide well for different points (Moscow, The Crimea, Sverdlovsk) and thus agree with the measurements of _H and with height-frequency ionospheric characteristics. The curve giving electron concentration versus height N (z) in the outer ionosphere (above the maximum of F2), shows a new maximum higher than the main maximum of the ionosphere N _MF2 at 120–140 km. At this maximum the value of N (z) is (0.9–0.95) N _MF2. The new data on the large-scale horizontal inhomogeneities of the ionosphere, exceed the previous ones by about a factor 10. By means of the irregular variations of the spectrum W() of the inhomogenous formation is determined. Three unknown constant maxima with values 16 to 18 km, 28 to 32 km and 100 to 120 km are found. The spectrum W () mainly characterizes the local properties of the ionosphere along the orbit of the satellite.     

Recent work on Cepheid variables

Recent work on Cepheids is reviewed in the areas of (1) the large-amplitude mode behavior, (2) convection, and (3) Cepheid masses. Initial-value type nonlinear calculations have not yet yielded true double-mode behavior. Yet we have the beginnings of a promising theory of modal selection. Theoretical calculations also yield reasonably located red edges to Cepheid (and Cepheid-like) instability regions.Recent observational results have led to increased values of the pulsation mass, so that this mass is now in fair agreement with evolution theory. The Wesselink mass is also satisfactory. Thus now only bump and beat masses are possibly discrepant. Some possible ways which have been suggested to alleviate these discrepancies are reviewed. The proposal of helium enrichment in the outer stellar layers can apparently satisfactorily resolve the beat (and perhaps also the bump) mass anomaly. A recent suggestion that part of the pressure in the envelope is due to a tangled magnetic field (not unusually strong) resolves the above mass anomaly about as well as the helium-enrichment idea does.Recent results regarding duplicity and period changes in Cepheids are reviewed.     

A New Mechanism of Coronal Heating

The interaction between network magnetic fields and emerging intranetwork fields may lead to magnetic reconnection and microflares, which generate fast shocks with an Alfvén Mach number M _A<2. Protons and less abundant ions in the solar corona are then heated and accelerated by fast shocks. Our study of shock heating shows that (a) the nearly nondeflection of ion motion across the shock ramp leads to a large perpendicular thermal velocity (v _th), which is an increasing function of the mass/charge ratio; (b) the heating by subcritical shocks with 1.1 M_A 1.5 leads to a large temperature anisotropy with T/T 50 for O⁵⁺ ions and a mild anisotropy with T_/T 1.2 for protons; (c) the large perpendicular thermal velocity of He⁺⁺ and O⁵⁺ ions can be converted to the radial outflow velocity (u) in the divergent coronal field lines; and (d) the heating and acceleration by shocks with 1.1 M_A 1.5 can lead to u(O⁵⁺) v _th(O⁵⁺) 460 km s^–1 for O⁵⁺ ions, u(He⁺⁺) v _th(He⁺⁺) 360 km s^–1 for He⁺⁺ ions, and u(H⁺) v _th(H⁺) 240 km s^–1 for protons at r=3–4 R . Our results can explain recent SOHO observations of the heating and acceleration of protons and heavier ions in the solar corona.     

Pulsations of the R Coronae Borealis stars

The radial pulsations of very luminous, low-mass models (L/M 10⁴, solar units), which are possible representatives of the R CrB stars, have been examined. These pulsations are extremely nonadiabatic. We find that there are in some cases at least one extra (strange) mode which makes interpretation difficult. The blue instability edges are also peculiar, in that there is an abrupt excursion of the blue edge to the blue for L/M sufficiently large. The range of periods of the model encompasses observed periods of the Cepheid-like pulsations of actual R CrB stars.     

Characteristics and interpretation of the photometric variability of eta carinae and its nebula

A short review is given on the history of the peculiar variable object Car and on a number of relevant references describing and discussing its physical characteristics and behaviour, based on different types of observational techniques. The star is known to be variable since the 17th century. The excessive mass loss to which it was subject during the 19th century is now visible as an ellipsoidal reflection nebula of 15 diameter: the so-called homunculus. The remainder of the paper is spent on different kinds of problems partly based on the results of a decade of photometric monitoring in the VBLUW photometric system of Walraven. Foreground reddening and reddening by dust in the homunculus are determined and amount to E(B - V) _J = 0 50 and < 6, respectively. Scanning of the homunculus revealed an estimate for the photometric characteristics of the central object, which presumably consists of a massive hot star surrounded by a cooler gas envelope. The total luminosity is derived using fluxes of various sources in the wavelength region 0.15 < < 175 n resulting in M _bol = - 12 3 ± 0 2. The total observed flux corrected for foreground extinction corresponds to a star with R 96 R if T _eff 30 000 K. The mass may be near 150 M . The excess luminosity in 1843, when the star was presumably bolometrically at least 2 5 brighter than at present, may have been caused by envelope-energized pulsations when the star's luminosity was close to its Eddington limit. The temperature should then have been 50 000 K. The mass loss rate, during the excess luminosity phase lasting 30 yr, is estimated to amount to M 4 × 10^-3 M yr^-1. At present the mass loss may be M 10^-4M yr^-1. Since the homunculus is mainly built up from material expelled in the 30 yr interval (from 1830 to 1860), its total mass amounts to M _hom 0.15 M . The historical observations of the colours of Car and a comparison with the characteristics of S Dor type stars, suggest that Car was subject to a number of S Dor type phases similar to those of P Cyg (in the 17th century), S Dor and others. A satisfactory explanation is found for the complete historical light curve. The decrease in light after the 1843 maximum by 9 ^m , presumably consists of a fading of the luminosity excess and the S Dor effect by 2 5 and 3^m, respectively, and a 3 5 extinction by circumstellar dust. The small amplitude light variations which Car showed during the last decade, were studied with the aid of the variations of the Balmer jump. They are presumably caused by temperature variations of the central star.Percy and Welch (1983) (Publ. Astron. Soc. Pacific 95, 491) have observed P Cyg on a number of nights in 1982 and found for the photometric variations a time scale of 30 to 50 days and an amplitude of 0 _. ^m 15.Based partly on observations collected at the ESO, La Silla, Chile.     

The scientific payload of the space-based Solar and Heliospheric Observatory (SOHO)

The space-based Solar and Heliospheric Observatory (SOHO) is a joint venture of ESA and NASA within the frame of the Solar Terrestrial Science Programme (STSP), the first Cornerstone of ESA's long-term programme Space Science — Horizon 2000. The principal scientific objectives of the SOHO mission are: a) a better understanding of the structure and dynamics of the solar interior using techniques of helioseismology, and b) a better insight into the physical processes that form and heat the Sun's corona, maintain it and give rise to its acceleration into the solar wind. To achieve these goals, SOHO carries a payload consisting of 12 sets of complementary instruments which are briefly described here.     

The structure of low-mass X-ray binaries

The present knowledge of the structure of low-mass X-ray binary systems is reviewed. We examine the orbital period distribution of these sources and discuss how the orbital periods are measured. There is substantial observational evidence that the accretion disks in low-mass X-ray binaries are thick and structured. In a number of highly inclined systems, the compact X-ray emitting star is hidden from direct view by the disk and X-radiation is observed from these only because photons are scattered into the line of sight by material above and below the disk plane. In such systems the X-ray emission can appear extended with respect to the companion star, which can lead to partial X-ray eclipses. There are substantial variations in the thickness of the disk rim with azimuth. These give rise to the phenomenon of irregular dips in the X-ray flux which recur with the orbital period, or to an overall binary modulation of the X-ray flux if the source is extended. The X-ray spectra of low-mass X-ray binaries can be used to probe the innermost emission regions surrounding the compact star. The spectra of the bright Sco X-1 variables can be fitted with two components which are provisionally identified as originating in the inner disk and the boundary layer between the disk and the neutron star respectively. The characteristic energy dependent flaring of the Sco X-1 sub-class may be a geometric effect triggered by an increase in the thickness of the inner disk or boundary layer. The X-ray spectra of the lower luminosity systems, including the bursters, are less complex, and in many cases can be represented by a single power law with, in some sources, a high energy cut-off. Iron line emission is a characteristic of most low-mass X-ray binaries, irrespective of luminosity.     

Characteristics of nearby interstellar matter

There is a warm tenuous partially ionized cloud (T10⁴ K,n(HI)0.1 cm^–3,n(Hii 0.22–0.44 cm^–3) surrounding the solar system which regulates the environment of the solar system, determines the structure of the heliopause region, and feeds neutral interstellar gas into the inner solar system. The velocity (V–20 km s^–1 froml335°,b0° in the local standard of rest) and enhanced Caii and Feii abundances of this cloud suggest an origin as evaporated gas from cloud surfaces in the Scorpius-Centaurus Association. Although the soft X-ray emission attributed to the Local Bubble is enigmatic, optical and ultraviolet data are consistent with bubble formation caused by star formation epochs in the Scorpius-Centaurus Association as regulated by the nearby spiral arm configuration. The cloud surrounding the solar system (the local fluff) appears to be the leading region of an expanding interstellar structure (the squall line) which contains a magnetic field causing polarization of the light of nearby stars, and also absorption features in nearby upwind stars. The velocity vectors of the solar system and local fluff are perpendicular in the local standard of rest. Combining this information with the low column densities seen towards Sirius in the anti-apex direction, and the assumption that the cloud velocity vector is parallel to the surface normal, suggests that the Sun entered the local fluff within the historical past (less than 10 000 years ago) and is skimming the surface of the cloud. Comparison of magnesium absorption lines towards Sirius and anomalous cosmic-ray data suggest the local fluff is in ionization equilibrium.Reason has moons, but moons not hers, Lie mirror'd on her sea, Confounding her astronomers, But, O! delighting me.Ralph Hodgson     

SOHO: The Solar and Heliospheric Observatory

The Solar and Heliospheric Observatory (SOHO), together with the Cluster mission, constitutes ESA's Solar Terrestrial Science Programme (STSP), the first Cornerstone of the Agency's long-term programme Space Science — Horizon 2000. STSP, which is being developed in a strong collaborative effort with NASA, will allow comprehensive studies to be made of the both the Sun's interior and its outer atmosphere, the acceleration and propagation of the solar wind and its interaction with the Earth. This paper gives a brief overview of one part of STSP, the SOHO mission.     

Voyager energetic particle observations at interplanetary shocks and upstream of planetary bow shocks: 1977–1990

The Voyager 1 and 2 spacecraft include instrumentation that makes comprehensive ion (E 28 keV) and electron (E 22 keV) measurements in several energy channels with good temporal, energy, and compositional resolution. Data collected over the past decade (1977–1988), including observations upstream and downstream of four planetary bow shocks (Earth, Jupiter, Saturn, Uranus) and numerous interplanetary shocks to 30 AU, are reviewed and analyzed in the context of the Fermi and shock drift acceleration (SDA) models. Principal findings upstream of planetary bow shocks include the simultaneous presence of ions and electrons, detection of tracer ions characteristic of the parent magnetosphere (O, S, O⁺), power-law energy spectra extending to 5 MeV, and large (up to 100:1) anisotropies. Results from interplanetary shocks include observation of acceleration to the highest energies ever seen in a shock ( 22 MeV for protons, 220 MeV for oxygen), the saturation in energy gain to 300 keV at quasi-parallel shocks, the observation of shock-accelerated relativistic electrons, and separation of high-energy (upstream) from low-energy (downstream) populations to within 1 particle gyroradius in a near-perpendicular shock. The overall results suggest that ions and electrons observed upstream of planetary bow shocks have their source inside the parent magnetosphere, with first order Fermi acceleration playing a secondary role at best. Further, that quasi-perpendicular interplanetary shocks accelerate ions and electrons most efficiently to high energies through the shock-drift process. These findings suggest that great care must be exercised in the application of concepts developed for heliosphere shocks to cosmic ray acceleration through shocks at supernova remnants.     

Some crucial X-ray observations — One or two SNR(s) in Crux?

A new EXOSAT (LE/CMA) observation of the region in Crux (R.A. 11h 45m, Dec. -62°) where Markert et al. (1981) reported the existence of two x-ray SNR's is presented. After cleaning the CMA field from the point source component, due to the UV emission of the numerous stars in the field, the smoothed x-ray contours are compared to the 408 MHz radio map of Caswell et al. (1983). The existence of two, well-separate x-ray emission regions is confirmed by EXOSAT, and the current x-ray/radio picture is not sufficent to distinguish clearly between the assumption of one or two (possibly interacting) SNR's in the region.     

Fine scale temporal structures in solar hard X-ray bursts observed by PHEBUS

We report here on preliminary results of a systematic study of fast temporal fluctuations in impulsive and extended solar X-ray bursts observed by PHEBUS at energies around 100 keV. Subsecond timescales are quite common in the impulsive events and are not observed in extended ones.     

Relations between ISM inside and outside the heliosphere

Thanks to remarkable new tools, such as the Goddard High Resolution Spectrograph (GHRS) on board the HST and the EUVE spectrometer on the interstellar side, and Ulysses particle detectors on the heliospheric side, it is possible now to begin to compare abundances and physical properties of the interstellar matter outside the heliosphere (from absorption features in the stellar spectra), and inside the heliosphere (from in situ or remote detection of the interstellar neutrals or their derivatives, the pick-up ions or the Anomalous Cosmic Rays detected by the two Voyager spacecraft).Ground-based and UV spectra of nearby stars show that the Sun is located between two volumes of gas of different heliocentric velocities V and temperatures T (see also Linsky et al, this issue). One of these clouds has the same velocity (V= 25.6 km s^–1 from = 255 and =8) and temperature (6700 K) as the heliospheric helium of interstellar origin probed by Ulysses, and is certainly surrounding our star (and then the Local Interstellar Cloud or LIC). This Identification allows comparisons between interstellar constituents on both sides of the heliospheric interface.Ly-alpha background data (absorption cell and recent HST-GHRS spectra) suggest that the heliospheric neutral H velocity is smaller by 5–6 km s^–1 than the local cloud velocity, and therefore that H is decelerated at its entrance into the heliosphere, in agreement with interaction models between the heliosphere and the ISM which include the coupling with the plasma. This is in favor of a non negligible electron density (at least 0.05 cm³). There are other indications of a rather large ionization of the ambient ISM, such as the ionization equilibrium of interstellar magnesium and of sodium. However the resulting range for the plasma density is still broad.The heliospheric neutral hydrogen number density (0.08–0.16 cm^–3) is now less precisely determined than the helium density (0.013–0.017 cm^–3, see Gloeckler, Witte et al, Mobius, this issue). The comparison between the neutral hydrogen to neutral helium ratios in the ISM (recent EUVE findings) and in the heliosphere, suggests that 15 to 70% of H does not enter the heliosphere. The comparison between the interstellar oxygen relative abundance (with respect to H and He) in the ISM and the heliospheric abundance deduced from pick-up ions is also in favor of some filtration, and thus of a non-negligible ionization.For a significant ISM plasma density, one expects a Hydrogen wall to be present as an intermediate state of the interstellar H around the interface between inside and outside. Since 1993, the two UVS instruments on board Voyager 1 and 2 indeed reveal clearly the existence of an additional Ly-alpha emission, probably due to a combination of light from the compressed H wall, and from a galactic source. On the other hand, the decelerated and heated neutral hydrogen of this H wall has recently been detected in absorption in the spectra of nearby stars (see Linsky, this issue).     

Theoretical modelling of Algol disks

A brief review of various theoretical approaches to model accretion disks is presented. Emphasis is given to models that determine self-consistently the structure of a disk together with the radiation field. It is argued that a proper treatment of the vertical structure is essential for calculating theoretical spectra to be compared with observations. In particular, it is shown that hot layers above an accretion disk (sometimes called disk chromospheres or coronae), whose presence is indicated by recent UV observations of strong emission lines of highly ionized species, may be explained using simple energy balance arguments.1987–88 JILA Visiting Fellow.This work was in part supported by a NASA grant ADP U-003-88 (Plavec and Hubeny). I also wish to thank the organizers of the IAU Colloquium 107 for the travel grant which enabled me to attend the meeting.     

Determination of the characteristics of the gamma-ray telescope Gamma-1

The Gamma-1 telescope has been developed through a collaboration of scientists in the USSR and France in order to conduct -ray astronomical observations within the energy range from 50 to 5000 MeV. The major characteristics of the telescope were established by Monte-Carlo simulations and calibrations made with the aid of electron and tagged -ray beams produced by an accelerator, and these have been found to be as follows: the effective area for photons coming along the instrument's axis varies from about 50 cm² at E = 50 MeV to approximately 230 cm² at E 300 MeV; the angular resolution (half opening of the cone embracing 68% events) is equal to 2.7° at E = 100 MeV, and 1.8° at E = 300 MeV; the energy resolution (FWHM) varies from 70% to 35% as the energy of the detected photons increases from 100 to 550 MeV; the telescope's field-of-view at the half-sensitivity level is 300–450 square degrees depending upon the spectrum of the detected radiation, and the event selection logic. Proceeding from the thus obtained characteristics it is demonstrated that a point source producing a photon flux J (E 100 MeV) = 3 × 10^-7 cm^-2 s^-1, can be detected with a 5 significance by observing it during 10⁶ s at the level of the Cygnus background, and a source having intensity J (E 100 MeV) = 10^-6 cm^-2 s^-1 can be detected to within a mean square positional accuracy of about 15.     

Detectors for infrared heterodyne mixing and detection

Conclusion For wavelengths < 50m fast and sensitive detectors are available. For wavelengths > 50m the available detectors are far from ideal. Research and development of far infrared detectors for the mixing purpose are highly recommended.     

Theoretical studies of the flux and energy spectrum of gamma radiation from the Sun

The purpose of this work is to study the various -ray-production mechanisms in solar flares and to calculate the flux, the spectrum, and the decay curves of radiation. Using the continuity equation and taking into account the energy losses for solar-flare-accelerated particles, we obtain the time-dependent particle distribution and thus the time behavior of the resulting rays. The important processes for producing rays in solar flares are found to be nonthermal electron bremsstrahlung, decay of neutral mesons, positron annihilation, neutron capture, and decay of excited nuclei. The results are applied to several known solar flares. For a large flare such as the class 3⁺ on February 23, 1956, continuous rays with energies up to 100 MeV from electron bremsstrahlung and neutral meson decays are observable at the orbit of the Earth by existing -ray detectors. Line rays from positron annihilation (0.51 MeV), neutron capture (2.23 MeV), and deexcitation of excited nuclei O¹⁶ (6.14 and 7.12 MeV) and C¹² (4.43 MeV) are particularly strong and well above the continuous -ray background due to electron bremsstrahlung. These lines can be detected at the Earth.NASA-NRC Resident Research Associate.     

A numerical study of the fission hypothesis for rotating polytropes

Conclusions Our results show that n = 3/2 and n = 1/2 polytropes are dynamically unstable to non-axisymmetric perturbations if t 0.30, in rough agreement with linear theory. Instead of fission as the direct end product of dynamic instability in rapidly rotating, centrally condensed stars, it appears that a rapidly rotating star can, through gravitational torques, eject some high angular momentum material in its equatorial plane and settle down into a dynamically stable (lower t) configuration. The central object, at least in the detailed evolution described above, ends up as a stable traixial star! The assumption of reflection symmetry through the rotation axis needs to be relaxed before this evolutionary picture can be considered realistic and before meaningful comparisons can be made with the results of Lucy (1977) and Gingold and Monaghan (1978, 1979), who found odd modes to be of crucial importance.We cannot escape mentioning that the ejected disk/ring of material in our models may have some connection with the formation of planetary systems.This research was supported by National Science Foundation Grant No. AST-7821449.     

Anomalous Cosmic Rays and Solar Modulation

We review aspects of anomalous cosmic rays (ACRs) that bear on the solar modulation of energetic particles in the heliosphere. We show that the latitudinal and radial gradients of these particles exhibit a 22-year periodicity in concert with the reversal of the Sun's magnetic field. The power-law index of the low energy portion of the energy spectrum of ACRs at the shock in 1996 appears to be -1.3, suggesting that the strength of the solar wind termination shock at the helioequatorial plane is relatively weak, with s 2.8. The rigidity dependence of the perpendicular interplanetary mean free path in the outer heliosphere for particles with rigidities between 0.2 and 0.7 GV varies approximately as R2, where R is particle rigidity. There is evidence that ACR oxygen is primarily multiply charged above 20 MeV/nuc and primarily singly-charged below 16 MeV/nuc. The location of the termination shock was at 65 AU in 1987 and 85 AU in 1994.     

β Cephei stars from a photometric point of view

This is an observational review, with an emphasis on photometric data and their interpretation. Two lists are presented, one containing Cephei stars, and the other, Cephei suspects. These lists then serve as a basis for discussing such topics as the location of Cephei stars in the observational and theoretical H-R diagrams, the evolutionary state of these stars, the period-luminosity and period-luminosity-color relations, and observational identification of pulsation modes. The paper also includes references to recent work connected with the theoretical discovery that an opacity mechanism is responsible for the excitation of Cephei-star pulsations. Finally, observational programs for verifying the consequences of this discovery are suggested.Belgian Fund for Scientific Research (NFWO).