Galactic X-rays observed with X-ray astronomy satellite ‘Hakucho’

Highlights of the results obtained with Japanese X-ray astronomy satellite Hakucho are reviewed. After a brief account of instrumentation (Section 2), some new features of non-bursting, non-pulsating objects are presented (Sections 3–5). The main part of the present review is devoted for X-ray bursts which are found more complex than one might have thought (Sections 6–11). The observation of X-ray pulsar, including a change of spin rate of Vela X-1, is described (Section 12). The main results obtained in the first two years are summarized in Section 13.     

A synthesis of our present knowledge of interstellar dust

The present state of knowledge as regards interstellar dust is reviewed in Section 1 (Introduction); Section 2 (Composition of Dust Grains: graphite, silicate, dirty-ice, diamond); Section 3 (Size of Grains: mainly r 10^–6 cm); Section 4 (Charge and Temperature of Grains: charge varies from 1–10 electrons (H i clouds) to 500 electrons (H ii clouds); temperature of grain material is about 10–20 K); Section 5 (Distribution and Origin of Grains: confined mainly to discs and arms of spiral galaxies, having had a passive origin by efflux from late-type stars or carbon-stars); Section 6 (Cosmogonical and Cosmological Aspects of Interstellar Grains: accretion by electrical-image forces of one dust grain onto a similarly-charged grain links up the absence of dust and gas in elliptical galaxies with the absence of a magnetic field of the type found in spirals. The origin of the 3 K background radiation field could be produced by a population of rotating silicate grains of r 10^–7 cm); Section 7 (Conclusion).     

Nonlinear Kinetic Theory of Cosmic-ray Acceleration in Supernova Remnants

A review of kinetic nonlinear theory for cosmic-ray (CR) acceleration and subsequent -ray production due to CR nuclear component in supernova remnants (SNRs) is presented. The correspondence of the expected spectrum and composition of CRs produced inside SNRs in the Galaxy with the experimental data is discussed. Possible explanations of negative results in searching high energy -ray emission from nearby SNRs are analyzed.     

Solar flares and particle acceleration

Energy release in solar flares occurs during the impulsive phase, which is a period of a few to about ten minutes, during which energy is injected into the flare region in bursts with durations of various time scales, from a few tens of seconds down to 0.1 s or even shorter. Non-thermal heating is observed during a short period, not longer than a few minutes, in the very first part of the impulsive phase; in average flares, with ambient particle densities not larger than a few times 10¹⁰ cm^–3 it is due to thick-target electron beam injection, causing chromospheric ablation followed by convection. In flares with larger densities the heating is due to thermal fronts (Section 1). The average energy released in chromospheric regions is a few times 10³⁰ erg, and an average number of 10³⁸ electrons with E 15 keV is accelerated. In subsecond pulses these values are about 10³⁵ electrons and about 10²⁷ erg per subsecond pulse. The total energy released in flares is larger than these values (Section 2). Energization occurs gradually, in a series of fast non-explosive flux-thread interactions, on the average at levels about 10⁴ km above the solar photosphere, a region permeated by a large number ( 10) of fluxthreads, each carrying electric currents of 10¹⁰–10¹¹ A. The energy is fed into the flare by differential motions of magnetic fields driven by photospheric-chromospheric movements (Section 3). In contrast to these are the high-energy flares, characterized by the emission of gamma-radiation and/or very high-frequency (millimeter) radiobursts. Observations of such flares, of the flare neutron emission, as well as the observation of ³He-rich interplanetary plasma clouds from flares all point to a common source, identified with shortlived ( 0.1 s) superhot ( 10⁸ K) flare knots, situated in chromospheric levels (Section 4). Pre-flare phenomena and the existence of homologous flares prove that flare energization can occur repeatedly in the same part of an active region: the consequent conclusions are that only seldom the full energy of an active region is exhausted in one flare, or that the flare energy is generated anew between homologous flares; this latter case looks more probable (Section 5). Flare energization requires the formation of direct electric fields, in value comparable with, or somewhat smaller than the Dreicer field (Section 6). Such fields originate by current-thread reconnection in a regime in which the current sheet is thin enough to let resistive instability originate (Section 7). Particle acceleration occurs by fast reconnection in magnetic fields 100 G and electric fields exceeding about 0.3 times the Dreicer field at fairly low particle densities ( 10¹⁰ cm^–3); for larger densities plasma heating is expected to occur (Section 8). Transport of accelerated particles towards interplanetary space demands a field-line configuration open to space. Such a configuration originates mainly after the gradual gamma-ray/proton flares, and particularly after two-ribbon flares; these flares belong to the dynamic flares in Sturrock and vestka's flare classification. Acceleration to GeV energies occurs subsequently in shock waves, probably by first-order Fermi acceleration (Section 9).     

Organic constituents of the carbonaceous chondrites

From a brief discussion of forms of meteorite carbon it is concluded that almost all the carbon in the carbonaceous chondrites is present as organic matter. Attempts to extract and identify this organic matter are then reviewed. It is shown that only 25 per cent has been extracted and only about 5 per cent chemically characterized. Of this 5 per cent most is a complex mixture of hydroxylated aromatic acids together with various hydrocarbons of the paraffin, naphthene and aromatic series. Small amounts of amino acids, sugars and fatty acids also are present. The possible chemical nature of the major fraction is discussed. It is suggested to be a mixture of high-molecular weight aromatic and hydrocarbon polymers.Possible sources of contamination of the meteorites are described and evidence indicating a general lack of organic contaminants is presented. It is concluded that most of the organic constituents are indigenous to the meteorites and are extra terrestial in origin. Synthetic processes for the compounds are mentioned and it is concluded that the organic material is probably of abiogenic origin.A brief review on studies of organized elements contained within the meteorites is presented. Difficulties of identification are discussed and photographs of some micro-structures of several carbonaceous chondrites are presented. No final conclusion about the nature of these objects is possible, but some appear to be various indigenous organic and mineral structures, while others are terrestrial contaminants.Contribution from the Chemistry Section, Space Science Division of Jet Propulsion Laboratory.     

Abundances of Deuterium and Helium-3 in the Protosolar Cloud

The mass spectrometric determinations of the isotopic composition of helium in the solar wind obtained from (1) the Apollo Solar Wind Composition (SWC) experiment, (2) the Ion Composition Instrument (ICI) on the International Sun Earth Explorer 3 (ISEE-3), and (3) the Solar Wind Composition Spectrometer (SWICS) on Ulysses are reviewed and discussed, including new data given by Gloeckler and Geiss (1998). Averages of the 3He/4He ratio in the slow wind and in fast streams are given. Taking account of separation and fractionation processes in the corona and chromosphere, 3He/4He = (3.8 ± 0.5) × 10-4 is derived as the best estimate for the present-day Outer Convective Zone (OCZ) of the sun. After corrections of this ratio for secular changes caused by diffusion, mixing and 3He production by incomplete H-burning (Vauclair, 1998), we obtain (D + 3He)/H = (3.6±0.5) × 10-5 for the Protosolar Cloud (PSC). Adopting 3He/H = (1.5±0.2) × 10-5 for the PSC, as is indicated from the 3He/4He ratio in the planetary gas component of meteorites and in Jupiter (Mahaffy et al., 1998), we obtain (D/H)protosolar = (2.1 ± 0.5) × 10-5. Galactic evolution studies (Tosi, 1998) show that the measured D and 3He abundances in the Protosolar Cloud and the Local Interstellar Cloud (Linsky, 1998; Gloeckler and Geiss, 1998), lead to (D/H)primordial = (2 - 5) × 10-5. This range corresponds to a universal baryon/photon ratio of (6.0 ± 0.8) × 10-10, and to b = 0.075 ± 0.015.     

Shock acceleration of energetic particles in the heliosphere

The theory of shock acceleration of energetic particles is briefly discussed and reviewed with an emphasis on clarifying the apparent distinction between the V × B and Fermi mechanisms. Attention is restricted to those situations in which the energetic particles do not themselves influence the given shock structure. In particular, application of the theory to the acceleration of energetic particles in corotating interaction regions (CIR) in the solar wind is presented. Here particles are accelerated at the forward and reverse shocks which bound the CIR by being compressed between the shock fronts and magnetic irregularities upstream from the shocks, or by being compressed between upstream irregularities and those downstream from the shocks. Particles also suffer adiabatic deceleration in the expanding solar wind, an effect not included in previous shock models for acceleration in CIRs. The model is able to account for the observed exponential spectra at Earth, the observed behavior of the spectra with radial distance, the observed radial gradients in the intensity, and the observed differences in the intensity and spectra at the forward and reverse shocks.Calculations and resulting energy spectra are also presented for shock acceleration of energetic particles in large solar flare events. Based on the simplifying assumption that the shock evolves as a spherically symmetric Sedov blast wave, the calculation yields the time-integrated spectrum of particles initially injected at the shock which eventually escape ahead of the shock into interplanetary space. The spectra are similar to those observed at Earth. Finally further applications are suggested.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

Interdisciplinary summary of solar/interplanetary events during August 1972

The solar/interplanetary events in early August 1972 are summarized in Section 1 (Introduction), Section 2 (August 1972 Events in the Solar Cycle 20), Section 3 (Evolution of Solar Active Region: McMath region No. 11976 and its flare-activity), Section 4 (Radio, X-ray, and Proton Characteristics of Four Major Solar Flares: F-1 at 0316 UT on 2 August, F-2 at 1958 UT on 2 August, F-3 at 0626 UT on 4 August, and F-4 at 1522 UT on 7 August), Section 5 (Interplanetary Shock Waves: observations of the shock waves generated from the four major solar flares at several points in interplanetary space, the Earth, Pioneer-9, Pioneer-10, etc.; interplanetary scintillations; shock trajectories in the heliosphere), Section 6 (Variations of Solar and Galactic Cosmic Rays: four solar proton events observed in the vicinity of the earth and at the Pioneer-9 location in the course of interplanetary disturbances; Forbush decreases of cosmic ray intensity; the spikeshaped variation in solar and galactic cosmic rays on 5 August), and Section 7 (Conclusions).     

3C 273: A review of recent results

3C 273 is the most extensively studied quasar both from the ground and from space. Recent satellite observations have given important information on the overall electromagnetic spectrum of 3C 273 in the -ray, X-ray, and UV ranges. The most salient results are: (i) the energy per decade of frequency emitted by 3C 273 is nearly constant between 6000 Å and 500 MeV and is 20 × 10⁴⁶ erg s^-1 for H = 50 km s^-1 Mpc^-1; (ii) there is no absorption in the soft X-ray range in contrast to the X-ray spectrum of Seyfert nuclei; (iii) the optical and UV spectra cannot be fitted by power-law spectra only, and the energy distribution in this range suggests that a substantial fraction of the energy in the UV is emitted as back-body radiation at 20 000 K. If the peculiar shape of the UV spectrum is indeed caused by black-body radiation, then an estimate of the energy emitted under this form is 2.5 × 10⁴⁶erg s^-1, corresponding to an optically thick disk of 10¹⁶ cm in diameter.The UV spectrum of 3C 273 shows absorption lines at zero redshift caused by interstellar matter in the disk and halo of our Galaxy. The strength of C iv 1550 in absorption indicates the presence of a hot outer region in the halo. Extragalactic objects with mostly continuous UV spectra, such as 3C 273, are very promising UV sources which allow us to observe the absorbing material over the entire line of sight throughout the galactic halo.     

Magnetic field reconnection theory and the solar wind — Magnetosphere interaction: A review

This review considers the theory of the magnetic field line reconnection and its application to the problem of the interaction between the solar wind and the Earth's magnetosphere. In particular, we discuss the reconnection models by Sonnerup and by Petschek (for both incompressible and compressible plasmas, for the asymmetric and nonsteady-state cases), the magnetic field annihilation model by Parker; Syrovatsky's model of the current sheet; and Birn's and Schindler's solution for the plasma sheet structure. A review of laboratory and numerical modelling experiments is given.Results concerning the field line reconnection, combined with the peculiarities of the MHD flow, were used in investigating the solar wind flow around the magnetosphere. We found that in the presence of a frozen-in magnetic field, the flow differs significantly from that in a pure gas dynamic case; in particular, at the subsolar. part of the magnetopause a stagnation line appears (i.e., a line along which the stream lines are branching) instead of a stagnation point. The length and location of the stagnation line determine the character of the interaction of the solar wind with the Earth's magnetosphere. We have developed the theory of that interaction for a steady-state case, and compare the results of the calculations with the experimental data.In the last section of the review, we propose a qualitative model of the solar wind — the Earth's magnetosphere interaction in the nonsteady-state case on the basis of the solution of the problem of the spontaneous magnetic field line reconnection.     

Simulation of three-dimensional solar wind disturbances and resulting geomagnetic storms

A kinematic method of representing the three-dimensional solar wind flow is devised by taking into account qualitatively the stream-stream interaction which leads to the formation of a shock pair. Solar wind particles move radially away from the Sun, satisfying the frozen-magnetic field condition. The uniqueness of the present approach is that one can incorporate both theoretical and observational results by adjusting the parameters involved and that a self-consistent data set can be simulated. One can then infer the three-dimensional structure of the solar wind which is vital in understanding the interaction between the solar wind and the magnetosphere, and it is for this reason that the present kinematic method is devised. In the first part of this paper, the present kinematic method is described in detail by demonstrating that the following solar wind features can be simulated: (i) Variations of the solar wind quantities (such as the solar wind speed, the density and the IMF vector), associated with the solar rotation, at the Earth; (ii) the solar wind flow pattern in the meridian planes; (iii) the three-dimensional structure of the corotating interaction region (CIR); and (iv) the three-dimensional structure of the warped solar current sheet.In Section 2, the three-dimensional structure of solar wind disturbances are studied by introducing a flare-generated high speed stream into the two-stream model of the solar wind developed in Section 1. The treatment of the stream-stream interaction is generalized to deal with a flare-generated high speed stream, yielding a shock pair. The shock pair causes three-dimensional distortion of the solar current sheet as it propagates outward from the Sun. It is shown that a set of characteristic time variations of the solar wind speed, density, the interplanetary magnetic field magnitude B and angles (theta) and gf (phi) result at the time of the passage at the location of the Earth for a given set of flare conditions. These quantities allow us to compute the solar wind-magnetosphere energy coupling function . Time variations of the two geomagnetic indices AE and Dst are then estimated from . The simulated geomagnetic storms are compared with observed ones.In the third part, it is shown that recurrent geomagnetic storms can reasonably be reproduced, if fluctuating components of the interplanetary magnetic field (IMF) are superposed on the kinematic model of the solar wind developed in the first part. As an example, we simulate the fluctuating components by linearly polarized Alfvén waves and by random variations of the IMF angle (theta). Characteristics of the simulated and observed geomagnetic storms are discussed in terms of the simulated and observed AE and Dst indices. If the fluctuating components of the IMF can generally be identified as hydromagnetic waves, they may be an important cause for individual magnetospheric substorms, while the IMF magnitude B and the solar wind speed V modulate partially the intensity of magnetospheric substorms and storms.     

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.     

Guided propagation of HF radio waves in the ionosphere

A review is given of the work done in the field of ionospheric guided propagation of HF radio signals from artificial satellites. The possible theoretical approaches to the problem are summarized with more details on the analysis in terms of ray-tracing based on a geometric-optics treatment.A survey is done of the experimental observations performed through satellite transmissions, with particular emphasis on the results obtained through three ad hoc developed experiments: Orbis, S. Marco and OV 4.     

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.     

A new view of the galactic plane with EXOSAT

A new X-ray image of the galactic plane has been produced using the 45 arcmin square field of view of the Medium Energy Instrument on EXOSAT. This image shows a total of 64 sources including 18 new ones which include the first observation of persistent emission from the globular cluster bursters Terzan 1 and Terzan 5. The most important discovery from this image is a 2° wide ridge of diffuse emission symmetrical about the plane and extending from the galactic centre to 1=±40°. The spectrum of this emission appears to be hard ( 1.2) with no significant absorption.     

Modelling of the interstellar hydrogen distribution in the heliosphere

The detailed knowledge of the distribution of neutral interstellar hydrogen in the interplanetary space is necessary for a reliable interpretation of optical and H⁺ pickup ions observations. In the paper, we review the status of the modelling efforts with the emphasis on recent improvements in that field. We discuss in particular the role of the nonstationary, solar cycle-related effects and the consequences of hydrogen filtration through the heliospheric interface region for its distribution in the inner Solar System. We demonstrate also that the use of the simple cold model, neglecting the thermal character of the hydrogen gas (T 8000 K), is generally incorrect for the whole region of the inner heliosphere (R < 5 AU) since it leads to a substantial underestimation of the local hydrogen density and thus influences the derivation of the H properties in the outer heliosphere/LISM. Referring to recent Ulysses measurements, we point out also the need to consider in the modelling the effects of the latitudinal asymmetry of the ionization rate.     

High energy cosmic ray observations during August 1972

A series of spectacular cosmic ray events which included two relativistic solar particle enhancements and three major Forbush decreases were registered by ground-based cosmic ray monitoring stations beginning 4 August, 1972. These were associated with four major proton flare events on the Sun and with large interplanetary magnetic field disturbances and high velocity shock waves. This review attempts to discuss and interpret the high energy cosmic ray phenomena observed during this period in the light of the known behaviour of low energy particulate flux, interplanetary plasma and field observations and other associated solar and terrestrial effects recorded during this period.The first Forbush decrease event FD-1 occurred in the early hours of 4 August, exhibiting very strong north-south and east-west anisotropies. Immediately following the onset of FD-1, the first ground level solar particle enhancement occurred. This event, which had its onset almost 6 h after the flare event on 4 August, had a very steep rigidity spectrum. The major Forbush event of the series which had its onset at 2200 UT on 4 August, exhibited extremely interesting and complex behaviour, the prominent features of which are a precursory increase prior to the onset (PI-1), a large decrease (FD-2), the largest observed to date, followed immediately by an abrupt square wave like enhancement (PI-2). Interplanetary space during this entire period was highly disturbed by the presence of large low energy particulate fluxes and shock waves, at least one of which had a velocity exceeding 2000 km s^-1. Large north-south and east-west anisotropies existed throughout the event. Both FD-2 and PI-2 were characterized by almost the same rigidity spectrum, with a power law index of -1.2 ± 0.2, and a predominant anisotropy along the sunward direction. The square wave-like spike PI-2 during the recovery of FD-2 was associated with a similar abrupt change in low energy particle flux in space, as well as an abrupt decrease in the interplanetary magnetic field value from 50 to 10 .Based on the available particle, field and plasma observations, an unified model is presented to explain the Forbush event in terms of a transient modulating region associated with the passage of a narrow magnetic shock front. In this model, the reflection of particles from the approaching shock front account for the precursory increase PI-1. The main Forbush event is caused when the magnetic barrier at the shock front sweeps past the Earth. The square wave increase is due to the enhanced flux contained in the magnetic well just behind the shock front and bounded by magnetic discontinuities, which is explained as due to the transverse diffusion of particles into this region from the interplanetary space which have easy access to this region. In situ plasma, field and low energy particle observations are reviewed to support the model.Also Professor at Physical Research Laboratory, Ahmedabad 380009, India.     

The solar flare plasma: Observation and interpretation

In the past several years, X-ray observations of the Sun made from rockets and satellites have demonstrated the existence of high temperature (20 × 10⁶ – 100 × 10⁶ K), low density plasmas associated with solar flare phenomena. In the hard X-ray range ( < 1 ), spectra of the flaring plasma have been obtained using proportional and scintillation counter detectors. It is possible from these data to determine the evolution of the hard X-ray flare spectrum as the burst progresses; and by assuming either a non-thermal or thermal (Maxwellian) electron distribution function, characteristic plasma parameters such as emission measure and temperature (for a thermal interpretation) can be determined. Thermal interpretations of hard X-ray data require temperatures of 100 × 10⁶ K.In contrast, the soft X-ray flare spectrum (1 <<30 ) exhibits line emission from hydrogen-like and helium-like ions, e.g. Ne, Mg, Al, Si,... Fe, that indicates electron energies more characteristic of temperatures of 20 × 10⁶ K. Furthermore, line intensity ratios obtained during the course of an event show that the flare plasma can only be described satisfactorily by assuming a source composed of several different temperature regions; and that the emission measures and temperatures of these regions appear to change as the flare evolves. Temperatures are determined from line ratios of hydrogen-like to helium-like ions for a number of different elements, e.g., S, Si, and Mg, and from the slope of the X-ray continuum which is assumed to be due to free-free and free-bound emission. There is no obvious indication in soft X-ray flare spectra of non-thermal processes, although accurate continuum measurements are difficult with the data obtained to date because of higher order diffraction effects due to the use of crystal spectrometers.Soft X-ray flare spectra also show satellite lines of the hydrogen-like and helium-like ions, notably the 1s ²2s ² S-1s2s2p ² P transition of the lithium-like ion, and support the contention that in low density plasmas these lines are formed by dielectronic recombination to the helium-like ion. Also, series of allowed transitions of hydrogen-like and helium-like ions are strong, e.g., the Lyman series of S up to Lyman-, and ratios of the higher member lines to the Lyman- line can be compared with theoretical calculations of the relative line strengths obtained by assuming various processes of line formation.This review will discuss the X-ray spectrum of solar flares from 250 keV to 0.4 keV, but will be primarily concerned with the soft X-ray spectrum and the interpretation of emission lines and continuum features that lie in this spectral range.     

Variations of Solar Ultraviolet Irradiance Observed by the UARS SOLSTICE — 1991 to 1999

The Solar Stellar Irradiance Comparison Experiment (SOLSTICE) is one of ten instruments on the Upper Atmosphere Research Satellite (UARS) — one of two instruments measuring the solar ultraviolet irradiance. The instrument is a three channel spectrometer covering the spectral range 120 to 420nm with a spectral resolution of approximately 0.2nm. It has been successfully operating since October 1991, and has now provided more than eight years of data, extending from near the peak of solar cycle 22, through solar minimum and into the new cycle. The data provide time series that display solar variations over time scales from a few days up to the 11-year solar cycle. Quantitative estimates of amplitudes of both rotational modulation and the solar cycle variation in the 1991–1999 epoch are given for the UV spectrum between 119 and 300nm.