Constraints on Coronal Outflow Velocities Derived from UVCS Doppler Dimming Measurements and in-Situ Charge State Data

We constrain coronal outflow velocity solutions, resolved along the line-of-sight, by using Doppler dimming models of H I Lyman alpha and O VI 1032/1037 Å emissivities obtained with data from the Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO. The local emissivities, from heliocentric heights of 1.5 to 3.0 solar radii, were determined from 3-D reconstructions of line-of-sight intensities obtained during the first Whole Sun Month Campaign (10 August to 8 September 1996). The models use electron densities derived from polarized brightness measurements made with the visible light coronagraphs on UVCS and LASCO, supplemented with data from Mark III at NCAR/MLSO. Electron temperature profiles are derived from 'freezing-in' temperatures obtained from an analysis of charge state data from SWICS/Ulysses. The work concentrates on neutral hydrogen outflow velocities which depend on modeling the absolute coronal H I Lyα emissivities. We use an iterative method to determine the neutral hydrogen outflow velocity with consistent values for the electron temperatures derived from a freezing-in model.     

Observed redshifts of transition region/corona lines

Solar UV observations reveal a redshifted emission at transition region temperatures, commonly interpreted as a net downflow of plasma. In earlier investigations the magnitude of the redshift has been found to increase with temperature, reaching a maximum at T=10⁵ K, and then to decrease towards higher temperatures. These observations, mostly from Skylab, suggested no significant shift of the O V line at 1218 Å formed at 2.4×10⁵ K. The variation of the downflow velocity with temperature is, however, uncertain since there are few reliable observations of lines formed at higher temperatures.Using spectrograms from the High Resolution Telescope and Spectrograph — HRTS we find an average net redshift of the O V lines at 1218 Å and 1371 Å at all locations extending from disk center to solar limb. A discrepancy between the observed flow velocity in the two lines is probably caused by uncertainty in the available laboratory wavelength of the intercombination line at 1218 Å (2s^{2 1}S₀-2s2p³P₁).The observed shift in O V is compared with corresponding measurements of lines formed at other temperatures (Si IV, C IV, N IV, O IV, and Fe XII). Large variations in the shift are found along the instrument slit. Thus, blueshifts are also observed with the sites of the largest upflow located in the sunspot umbrae and in a quiet region close to an active region.     

Swan Observations of the Solar Wind Latitude Distribution and its Evolution Since Launch

SWAN is the first space instrument dedicated to the monitoring of the latitude distribution of the solar wind by the Lyman alpha method. The distribution of interstellar H atoms in the solar system is determined by their destruction during ionization charge-exchange with solar wind protons. Maps of sky Ly-α emission have been recorded regularly since launch. The upwind maximum emission region deviates strongly from the pattern that would be expected from a solar wind that is constant with latitude. It is divided in two lobes by a depression aligned with the solar equatorial plane, called the Lyman-alpha groove, due to enhanced ionization along the neutral sheet where the slow and dense solar wind is concentrated. The groove (or the anisotropy) is more pronounced in 1997 than in 1996, but it then decreases between 1997 and 1998. This revised version was published online in June 2006 with corrections to the Cover Date.     

The X-ray imaging telescopes on Exosat

The instrument configuration and performance characteristics of the X-ray imaging telescopes on EXOSAT are described. The instrument comprises two fully independent Wolter I imaging telescopes. Each telescope can be used in either of two principal modes: (i) an imaging mode with either a position sensitive proportional counter or a channel multiplier array plate in the focal plane, (ii) a spectrometer mode which features a 500 lines/mm and/or a 1000 lines/mm transmission grating as dispersive element.Preliminary results from the calibration of the fully integrated experiment indicate an ultimate angular resolution of 8.5 arc sec full width at half maximum or 17.5 arc sec half-power beam width. The ultimate wavelength resolution in the spectrometer mode ranges from 1Å for wavelengths below 50Å, to 5Å at wavelengths near 300Å.A method for estimating the telescope performance is given which reasonably accounts for the influence of the X-ray source spectrum and the degree of interstellar absorption on the counting statistics.A comparison between EXOSAT and the EINSTEIN telescope in terms of band width/resolution and minimum source detectability shows an enhanced potential for EXOSAT relative to EINSTEIN for sources with T 10⁷K and low column densities (< 4 × 10²⁰cm^–2) and a reduced potential for sources with hard, or heavily cut-off, spectra.     

Ultraviolet spectrometer experiment for the Voyager mission

The Voyager Ultraviolet Spectrometer (UVS) is an objective grating spectrometer covering the wavelength range of 500–1700 Å with 10 Å resolution. Its primary goal is the determination of the composition and structure of the atmospheres of Jupiter, Saturn, Uranus and several of their satellites. The capability for two very different observational modes have been combined in a single instrument. Observations in the airglow mode measure radiation from the atmosphere due to resonant scattering of the solar flux or energetic particle bombardment, and the occultation mode provides measurements of the atmospheric extinction of solar or stellar radiation as the spacecraft enters the shadow zone behind the target. In addition to the primary goal of the solar system atmospheric measurements, the UVS is expected to make valuable contributions to stellar astronomy at wavelengths below 1000 Å.     

Das thermische plasma als intensitäts normalstrahler im wellenlängenbereich von 1100 bis 3100 å

In this paper a standard intensity source is described consisting of a thermal argon plasma with small additives of hydrogen, nitrogen, and carbon dioxide. These gases are introduced in such a concentration to produce a series of optically thick spectral lines in the region 1100 Å to 3100 Å. The intensity in an optically thick line center is given by the Kirchhoff-Planck-function B (T) which is determined if the plasma temperature is known. The continuous character of the function allows for interpolation between the spectral lines.At longer wave lengths above 1800 Å only two carbon I-lines are found to be optically thick. Here the optically thin argon continuum is calibrated against the carbon I-lines and is used as an additional standard emission.By this procedure an absolute intensity scale is constructed at every wave length between 1100 Å and 3100 Å.The precision of the absolute intensities depends essentially on the precision of the temperature measurement for the plasma. Four different spectroscopic methods are used for this determination giving a temperature of 12540 K with an error of ±1 % to ±2 %. This means an error of ±10 % to ±20 % for the absolute intensity scale. The internal consistency of the measurements is better than ±2 %A plasma with these properties is produced in a wall-stabilized cascade arc burning through a central hole in a stack of refrigerated discs. By an adequate flow pattern of argon, hydrogen, nitrogen, and carbon dioxide an argon window is formed through which the radiation of the added gases leaves the arc axially without self-absorption.The arc is burning at a pressure of 1 atm, and a windowless light path to a vacuum spectrograph is formed by a 3 chamber differential pumping stage allowing continous observation.As a result the absolute intensities of the optically thick lines observed at high resolution are given. Moreover, the total spectral energy flux of the light source is presented for a lower resolution.In the range 2500 Å to 3100 Å the absolute scale overlaps the useful range of the carbon arc which is used as an intensity standard in the visible and near ultraviolet. A comparison of the two sources yields a compatibility within the experimental errors of a few percent.     

Stars and Stellar Systems at z > 5: Implications for Structure Formation and Nucleosynthesis

Enough UV radiation was generated before z = 5 to have ionized the intergalactic medium. If this comes from stars (probably aggregated in systems of subgalactic scale), one straightforwardly calculates that the associated nucleosynthesis would be sufficient to produce a universal abundance of order 1 percent of solar. The first pre-galaxies may eventually be detectable by their direct UV emission, with characteristic spectral features at Lyman alpha; high-z supernovae may also be detectable. Other probes of the IGM beyond z = 5, and of the epochs of reheating and reionization, are discussed, along with possible links between the diffusion of pregalactic metals and the origin of magnetic fields.     

Le spectre ultraviolet des etoiles

Sommaire Les travaux expérimentaux stellaires dans le proche UV sont rappelés brièvement: ils ont conduit à la détection de nébulosités entourant les étoiles chaudes (vers 1300 Å) et à l'observation, pour ces mêmes étoiles chaudes, d'un affaiblissement important de la brillance pour < 2400 Å. Les différentes interprétations et discussions sont passées en revue. Une discussion plus complète est donnée dans le cas de l'hypothèe d'un nuage de particules circumstellaires, qui semble susceptible de fournir une interprétation aux deux types d'observation mentionnées.

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Summary The stellar observational data obtained in the near UV are briefly described. They led to the detection of nebulosities around early type stars (circa 1300 Å) and to the observation, in the spectrum of these stars, of a strong reduction of the brightness at wave lengths shorter than 2400 Å. The several interpretations and discussions are reviewed. A somewhat more complete discussion is presented for the case of the circumstellar dust cloud hypothesis, which seems able to make understandable both types of observational data.

     

The importance of spectroscopy in the 80–800 å region for plasma diagnostics in the solar atmosphere

We discuss the importance of the spectral range from about 80 to 800 Å for determining physical conditions in different regions of the solar atmosphere. We give examples of line ratios that may be used to determine electron densities in quiet Sun regions, active regions, and flares. We discuss the possibility of determining electron temperatures from line ratios in the EUV.We show that profiles as well as intensities of spectral lines must be obtained for a proper interpretation of the spectra. We give approximate parameters for a solar grazing incidence spectrograph suitable for the study of the 80–800 Å wavelength region.Given as an invited review paper at the EGAS Meeting in Munich, Germany, 11–14 July 1978.     

First UV observations of the optical counterpart of the X-ray source 2S0114+650

The X-Ray Source 2S0114+650 discovered by SAS-3 in 1978 has been associated with a highly reddened star showing broad H emission.We observed this star with the LWR spectrograph on board the I.U.E. Satellite on Sep. 1980 and Feb. 1981. An Ultraviolet flux was detected longward of the 2200 Å bump, consistent with the B0.5 spectral classification and the values of V=11 and E(B-V)=1.48, The UV extinction in the region around 2200Å seems to deviate from the standard interstellar extinction law.European Space Agency Satellite Tracking Station P.O.Box 54065 — Madrid — SpainBased on observations by the International Ultraviolet Explorer, collected at the Villafranca Satellite Tracking Station of the European Space Agency.     

X-Ray spectroscopic investigation of the coronal structure of capella

The binary system Capella (G6 III + F9 III) has been observed on 1979 March 15 and on 1980 March 15–17 with the Objective Grating Spectrometer (OGS) onboard theEinstein Observatory. The spectrum measured with the 1000 l/mm grating covers the range 5–30 Å with a resolution < 1 Å. The spectra show evidence for a bimodal temperature distribution of emission measure in an optically thin plasma with one component 5 million degrees and the other one 10 million degrees. Spectral features can be identified with line emissions from O VIII, Fe XVII, Fe XVIII, Fe XXIV, and Ne X ions. Good spectral fits have been obtained assuming standard cosmic abundances. The data are interpreted in terms of emission from hot static coronal loops rather similar to the magnetic arch structures found on the Sun. It is shown that the conditions required by this model exist on Capella. Mean values of loop parameters are derived for both temperature components.     

The solar grazing incidence (GRIST) and optical (SOT) telescopes joint accommodation

Some of the problems foreseen for the joint accommodation and operation of the Grazing Incidence Solar Telescope (GRIST) under study by ESA to operate in the extreme ultraviolet region (90 < < 1700 Å), and the Solar Optical Telescope (SOT), developed by NASA to operate in the ultraviolet, optical and infrared region (A > 1100 Å) on a Spacelab mission are described.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.     

High and medium resolution spectroscopy of the X-ray transient 4U1543-47

Preliminary results of an EXOSAT observation of the transient X-ray source 4U1543-47 are presented. The source was observed in August 1983, during a high state, following a Tenma alert that the source was again active. Results from the GSPC and the LE 1000 l/mm grating are presented. The spectrum is complex, but in the 2–10 keV energy range can be well described by a Comptonised thermal distribution. Extrapolating the same model into the lower energy band of the grating requires an absorption column density equivalent to 2 × 10²¹ H cm². A marked under-abundance of Oxygen and overabundance of Nitrogen, along with a strong, unidentified line feature at 9.8 Å, are necessary to model the grating data.     

Spartan 201 coronal spectroscopy during the polar passes of ulysses

Spartan 201 is a shuttle deployed spacecraft that is scheduled to perform ultraviolet spectroscopy and white light polarimetry of the extended solar corona during two 40 hour missions to occur in September 1994 and August 1995. The spectroscopy is done with an ultraviolet coronal spectrometer which measures the intensity and spectral line profile of HI Ly up to heliocentric heights of 3.5 solar radii. It also measures the intensities of the OVI doublet at 1032 and 1037 Å and of Fe XII at 1242 Å. The HI Ly line profile measurements are used to determine the random velocity distribution of coronal protons along the line-of-sight. The absolute HI Ly intensities can be used together with electron densities from the white light coronagraph to estimate electron temperatures from hydrogen ionization balance calculations, and bulk outflow velocities from models of Doppler dimmed resonant scattering. Intensities of minor ion lines are used to determine coronal abundances and outflow velocities of O⁵⁺. Ultraviolet spectroscopy of extended coronal regions from the 11 April 1993 mission of Spartan 201 are discussed.     

Concerning the interpretation of the ultraviolet radiation from B stars

Measurements of the shape of the ultraviolet spectrum from B stars are compared with the theoretical spectra predicted from a homogeneous series of eight model atmospheres which are known to be close to a state of radiative equilibrium and to give a good representation of the ordinarily observed spectral region. The broad-band photometer measurements of Byram, Chubb, and Friedman in the region 1314 indicate that the stars become brighter in the ultraviolet as their temperature increases. The theoretical spectra reproduce this trend. However, the theoretical spectra are about three times as bright at 1314 relative to their brightness at 5560 as is observed.The spectral observations at 50Å resolution of Stecher and Milligan of six absorption-line stars are compared in detail with theoretical spectra. The observed shape of the spectrum is reproduced well by the models from 2600 to longer wavelengths. At wavelengths shorter than 2600 Å, the observed fluxes from B stars are less than the predicted fluxes. At 2000 the deficiency is between a factor two and a factor four. The spectrum of Canis Majoris is observed to have a different shape from that found for four other early-type stars. In the case of Canis Majoris the deficiency at 2000 is about a factor 13.The proper manner in which to compare theory and observation is discussed and some astrophysical terminology is explained. Theoretical fluxes, , are given in Table 1 for eight early B type model atmospheres at wavelengths between the Lyman limit and 6251. These fluxes have been computed without consideration of the opacity due to line blanketing. It is shown that line blanketing can probably account for the differences noted between predicted and observed ultra-violet spectra of B stars. It is not necessary at present to invoke unusual sources of opacity in the stellar atmosphere or in the space between the star and the earth in order to explain the observations. Spectra of B stars in the 2000 region at sufficient resolution to show the line spectrum would clarify the problem.     

Broadening of Fe X (6374 Å) profiles above the limb in a coronal hole

Profiles of the visible Fe X (6374 Å) coronal emission line as a function of height above the limb were obtained out to 1.16 solar radii in a coronal hole using the NSO/Sacramento Peak Observatory Coronagraph, Universal Spectrograph and a CCD camera. These are the first coronal line profiles obtained as a function of height in a coronal hole from the ground. Analysis of the line widths suggests a large component of nonthermal broadening which increases with height ranging from 40 to 60 km/s, depending upon the assumed temperature or thermal component of the profile.     

Photoelectric array detectors for use at XUV wavelengths

The characteristics of photoelectric detector systems for use at visible-light, ultraviolet, and X-ray wavelengths are briefly reviewed in the context of the needs of the Spacelab solar-physics facilities. Photoelectric array detectors for use at XUV wavelengths between 90 and 1500 Å are described, and their use in the ESA Grazing-Incidence Solar Telescope (GRIST) facility is discussed.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.     

Simulated observations of the electron coronal density irregularity\overline {n^2 } /(\bar n)^2

A technique to derive the coronal density irregularity factor , wheren is the electron density, has been proposed by Fineschi and Romoli (1993). This technique will exploit the unique UVCS capability of cotemporal and cospatial measurements of both UV line radiation and K-coronal polarized brightness,pB.The ratio of the measured H I Lyman (Ly-) line intensity to the resonant-scattering dominated H I Lyman (Ly-) intensity can be used to extract the collisional component of the Ly-. This component yields an estimate of . The quantity is then obtained from the UVCS white-light K-coronal measurements.We present simulated observations of the UVCS for coronal atmosphere models with different filling factors and electron density profiles, and for different coronal structures (e.g., coronal holes, streamers). These simulations will show how the proposed technique may be used to probe inhomogeneities of the solar corona.     

The Coronal Helium Abundance Experiment on Spacelab 2

The Coronal Helium Abundance Spacelab Experiment, (CHASE), basically consists of a grazing incidence telescope and spectrometer sensitive over the range 150–1335 Å. Whilst aimed primarily at deriving the solar helium abundance from measurements of coronal resonance scattering, its specification has been extended in order to provide a more general purpose solar XUV facility. The instrument will be flown on the Spacelab 2 Mission, currently scheduled for launch in November 1984.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.     

The dynamics of chromospheric spicules

We present the observational results on chromospheric spicules obtained at the Sayan observatory 50 cm coronograph. To investigate the evolution of chromospheric spicules, we analysed spicule spectra of strong chromospheric lines measured simultaneously at three altitudes above the solar limb during 5–60 min with a time resolution of 10 to 20 s. The spatial resolution was better than 1, and the spectral resolution was 0.03Å in 6563Å. The appearance of a spicule at a given altitude is preceded by an sharp increase in line-of-sight velocity and/or in line half-width at a lower level. Generally, the evolution has a non-monotonous impulsive character. Changes of line-of-sight velocities and other parameters of the line profile can be represented as the superposition of slow, evolutionary changes and fluctuations with periods of about 80 to 120 s. The amplitude of line-of-sight velocity fluctuations is 2–3 km/sec and tends to increase with height. By studying the phase delays of the fluctuations at different heights, we found that the propagation velocity exceeds 300 km s^–1, and that the disturbances do not necessarily propagate upwards.