Soft X-ray spectra of aluminum and titanium and some other elements were observed in laser produced plasmas. The hydrogen and helium-like ion resonance lines and their satellites were obtained for one laser pulse by means of a concave mica crystal spectrograph. The experimental wavelengths were compared with theoretical ones. Time dependence of the aluminum spectra was studied using the electron optical camera and scintillator as the X-ray detector. The duration time of the resonance line emission is approximately equal to that of laser pulse for helium-like ions and is noticeably shorter for hydrogen-like ions. In the latter case this time is rather different for each laser pulse.

Spectra emitted by laser produced plasmas of the light elements Be to 0 when observed with spatial resolution show the H-like Lyman lines strongly broadened and asymmetrically self-absorbed especially near the target. The first three lines, Lα, Lβ and Lγ of Be IV, B V, C VI, and the Lα of N VII and 0 VIII have been analyzed in detail. Temperature, density and plasma dimensions have been spectroscopically determined. A model for the emitting plasma that tkes into account Stark and Doppler broadening, motion Doppler shift and optical opacity has been devised that successfully explains the observed profiles. The diagnostic possibilities of such observations are discussed.

When calculating the Stark widths of isolated lines of multiply charged ions with several bound electrons, radiated by dense, hot plasmas, the existing theories of Stark broadening which have been applied mainly to lower-temperature measurements on singly-ionized species, need to be modified in several respects. These modifications involve taking more careful account of the complexities of the particular atomic structure: e.g. deviations from LS coupling, configuration mixing, and optically forbidden transitons. Earlier work, showing explicitly the relationship between the Stark widths and the oscillator strengths for the collision-induced transitions Which contribute to the width, is extended. Some of the principles involved will be discussed and, if time permits, comparison will be made between calculations and the results of several critically selected experiments on spectral lines in the near UV (2000-3500 Å).

The ultraviolet interstellar extinction curve from 2740Å to 1350Å has been obtained using data from the S2/68 satellite experiment. The extinction increases into the ultraviolet and shows a pronounced peak at 2200Å. This is interpreted as a general scattering continuum with a strong absorption feature superposed on it at 2200Å. The profile of the feature appears to be symmetrical and has a half-width of 360Å. There is a strong correlation between the strength of the feature and the scattering part of the curve in both the ultraviolet and the visible. On a broad scale the shape of the extinction curve is constant showing no variation with distance from the sun, direction around the galaxy, and height above the galactic plane.

In order to interpret soft X-ray spectra of cosmic X-ray sources, it is necessary to know the photoabsorption cross-section of the intervening interstellar material. Current models suggest that the interstellar medium contains two phases which make a substantial contribution to the X-ray opacity: cool, relatively dense clouds that exist in pressure equilibrium with hot, tenuous intercloud regions. We have computed the soft X-ray photoabsorption cross-section of each of these two phases. The calculations are based on a model of the interstellar medium which includes chemical evolution of the galaxy, the formation of molecules and grains, and the ionization structure of each phase.

This paper wil describe recent calculations which attempt to pin down the relative importance of collisional excitation, recombination, diffusion, and departures from a Maxwell-Boltzmann electron velocity distribution on the formation of lines and continua of He I and He II in the Sun. There will be some comments on the relevance of these calculations to other stars.

The procedure will be described that is developed for assigning wavelengths to the UV stellar spectra obtained in 1976 by means of a balloon-borne instrument consisting of telescope with starpointing system, echelle spectrograph and SEC vidicon. The instrument covers the spectral region of 2000 Å to 3400 Å with a wavelength resolution of 0.1Å. The wavelength assignment aims at an accuracy of 0.01 Å. Results will be presented and difficulties encountered, related with the use of echelle spectrograph and SEC vidicon detector, will be analysed.

For both observations and laboratory measurements the use of television-type detectors is increasing. The Space Research Laboratory at Utrecht is, in collaboration with Johnson Space Center at Houston, carrying out high resolution spectroscopy from a balloon borne observatory using an echelle spectrograph and a SEC-vidicon detector. The calibration of such a system is not trivial. This paper deals with the procedures required to achieve a proper intensity calibration of the system in use.

New ultraviolet observations of nine Wolf-Rayet stars have been obtained with the S2/68 experiment of the ESRO satellite TD-1. These low resolution (Δλ ∼35Å) spectra have been studied yielding line identifications and strengths of the numerous emission lines observed in both WN and WC stars. The ultraviolet data are combined with complementary ground based photoelectric measurements to yield new estimates of the interstellar reddening corrections required. The combined ultraviolet and visible WR energy distributions, corrected for interstellar extinction, yield colour temperatures ∽30000 K for all nine stars. these temperatures are close to those derived from a Zanstra analysis of the He II 1640 Å line observed in emission in single WN and WC stars.

the observed helium, carbon and nitrogen lines in the ultraviolet and visible spectra of four WR stars have been analysed using a non-LTE treatment and Escape Probability methods to determine the abundances of these species in the WR atmospheres. The analysis of low excitation carbon and nitrogen lines in the ultraviolet allows for the first time accurate abundances for these species to be determined. It is found that in the WC star studied the C/He and N/He ratios are close to the normal cosmic values. In the WN stars the N/He ratio is slightly higher than normal but the C/He ratio is much reduced. In both sequences the H/He ratio is very low. Thus it appears that the carbon abundance is the controlling factor in determining the spectral characteristics of both sequences. It is clear that the WR stars are in the helium burning phase, with the WN sequence being less evolved than the WC sequence. These results in many ways support the recent suggestion of Conti (1976) that the of stars are the progenitors of the WR class.

Theoretical spectra have been computed (in LTE) for B type stars, in the wavelength range 2000-3000 Å. The results show a good agreement with observations (S59 and Copernicus).

It will be shown that the most important factor that influences the resulting computed spectrum - at a resolution of 1.8 Å or lower - is the completeness of the list of spectral lines used in the computations, owing to the large (number) density of lines in the ultraviolet.

At a resolution of 1.8 Å a large error is tolerated on gf values, while identification of observed line features from the theoretical spectra remains possible.

The accuracy of the gf values becomes increasingly more important if one goes to higher spectral resolution, as will be shown by comparison of theoretical spectra with observations.

Our present knowledge of oscillator strengths is sufficient for. identification of a large number of spectral lines in high resolution spectra; for a detailed analysis more accurate data are preferable in many cases.

An extensive study of ultraviolet spectra of Ap stars observed with the S2/68 TD-1 experiment has shown a large depression at 1400 Å in the spectra of Si and SrCrEu stars which is not present in the spectra of normal stars. This feature is attributed to Si II autoionization lines. More such lines seem to appear at longer wavelength in the UV and in the visible. We discuss the interest of an astro-physical plasma to detect and study autoionization lines in absorption.

A four hour balloon observation of HERC X-l during the 'On-state' in the 35 day cycle was performed on May 3rd, 1976. The 1.24 second pulsations show a pulsed fraction of 58 ± 8% in the 18-31 KeV interval. A pulsed flux (1.24 sec) was discovered in the 31-88 KeV interval with a pulsed fraction of 51 ± 14%. The spectrum of the pulsed flux can be represented up to 50 KeV by an exponential distribution with KT approximately 8 KeV. At approximately 58 KeV a strong and narrow line feature occurs which we interpret as electron cyclotron emission (ΔN = 1 Landau transition) from the polar cap plasma of the rotating neutron star. The corresponding magnetic field strength is approximately 5 x 1012 Gauss, neglecting gravitational red shift. There is evidence for a second harmonic at approximately 110 KeV (ΔN = 2 ).

The astrophysical application of this discovery will be discussed in some detail.

A spectrum covering Fe XXV to Fe XXII x ray lines from a tokamak (PLT) plasma has been obtained. Results indicate similarity with the flare spectrum in this region - the forbidden lines also appear in the PLT spectrum.

Te was about 1.8 keV- a value confirmed by intensity of dielectronic recombination lines. Ti was found to be not more than 0.7 keV, by Doppler width of the He like singlet 1s2 –ls2p 1S0 -1P0. A Johansson crystal was used, with the spectrograph adapted for an extended X-ray source.

The spectrum of the TFR Tokamak has been observed in the range 5-100 Å. Identification of Mo XIV to Mo XXXIII lines originating from the limiter was achieved by comparison with the Mo spectrum emitted from a low-inductance vacuum spark, and with relativistic calculations. Some of the TFR lines were identified as forbidden transitions (electric quadrupole). The appearance of these lines depending on plasma parameters is discussed.

Excitation rates for Be-like Ne VII have been measured using a theta-pinch. The absolute intensity of the intercombination line allows the metastable level population density to be determined. The sum of the population densities of the ground and metastable levels is found by solving the time dependent ionisation-recombination rate equations knowing the temperature and density of the discharge as a function of time. The ratio of metastable level to ground level population densities thus obtained is compared to values obtained theoretically.

The experimental rates are compared to theory and the accuracy of using them for line ratios measurements of temperature and density are discussed.

A column of sodium vapor with a significant fraction of the atoms in the 3p2p state (excited state density of the order of 1015 atoms/cm3 ) was prepared by axially irradiating a sodium heat-pipe oven with a pulsed dye laser tuned to the 3s-3p resonant frequency. This technique has recently been shown to produce nearly complete ionization of the illuminated column in ∼ 600 nsec. By passing continuum VUV light down the pipe 150 nsec after initiating the laser pulse, an absorption spectrum of neutral excited state sodium is obtained in the region of autoionizing levels containing the 2p53s3p and 2p53d3p configurations. The continuum is produced by a BRV-type triggered spark that gives a light pulse of ∼ 100 nsec duration in the wavelength range of observation of ∼ 200-400 A. About 25 absorption lines are attributed to these configurations based on predicted wavelengths obtained from an intermediate coupling calculation of the autoionizing levels, including configuration interaction. This appears to be the first observation of these even parity levels, which are inaccessible to photabsorption from the ground state by electric dipole transitions.

Non-polarized one photon excitation in atoms from a lower state with non-zero total angular momentum J will always reach opposite parity final states with three different total angular momentum, J' = J, J ± 1. In the case of heavy elements, e.g. Rare Earth or Actinides atoms, there are more than two electrons in the un-occupied open valence shells and in turn, many series are generated for one J' alone. All these make the population of levels close to ionization potential to be extremely dense, besides, accidental degeneracy will further defy the resolution and identification of the levels. An experiment is proposed to select final states with one J only and therefore simplifies the spectra by a factor of 3. The J-selection spectroscopy incorporated with Multichannel Quantum Defect Theory can thus make the complex spectral analysis easy. Uranium spectra is investigated as an example.

Knowledge of the (differential) emission measure function (with temperature), Y(Te), is likely to be of fundamental significance in attempts to understand the behaviour of any optically thin plasma - and this is certainly true of present studies of the solar atmosphere. In this latter case the gross temperature structure of material below 106K has been deduced from uv line measurements since the early 1960's and, since 1968, similar analyses of the material above 106K have been made using emission lines at x-ray wavelengths. In this latter case the validity, and even desirability, of published results has recently been quite strongly challenged. Comment is made upon those criticisms of early results and upon their relevance to present day analyses. In this paper the crucial value of fine temperature resolution in the determination of the emission measure function of this coronal material is argued.

The intensity ratio of the sum of forbidden (3S - 1S) and intersystem (3S - 1S) lines to the resonance (1P - 1S) line of helium-like ions can serve as a useful measure of optical depth due to resonance scattering. The temperature dependence of this ratio for a low density, optically thin plasma, is examined in detail for the case of 0 VII. Collisional excitation rates with exchange contributions as well as cascades and the effect of unresolved satellite lines resulting from dielectronic recombination are included. These results are compared to spatially resolved measurements of the solar 0 VII spectrum and the difference interpreted in terms of resonance scattering into or out of the line of sight.

Soft X-ray images of solar active regions obtained by the S-054 experiment on Skylab have been compared with simultaneous interferometric observations at 2.8 cm. The radio data consist of one-dimensional scans with a spatial resolution of 16 arcseconds in the East-West direction. The resolution, although lower than the X-ray telescope resolution, is high enough for a detailed comparison.

We have found that there is a general correspondence in position and size between X-ray and radio sources, but relevant differences are also present. In particular, very bright, narrow components at 2.8 cm appear coaligned with regions of very weak X-ray emission. These strong radio components appear to be localized directly above sunspot umbrae.

Models of active regions are investigated both for the atmosphere directly over the sunspot umbra and for regions above the adjacent plage. The presence of the magnetic field is taken into account and its effects on the energy dissipation and on propagation of radio waves are discussed.