The Extreme Ultraviolet Explorer (EUVE), because of its sky survey strategy, performed two observations of each point along a 180° by 1° strip of the ecliptic during the initial survey phase of the mission. One observation used the deep survey telescope, and another, 90 days earlier or later, used the all sky scanner telescopes, two of which have a nearly identical passband to that of the ecliptic deep survey. Since the completion of the initial sky survey, EUVE has been used to carry out deep, pointed observations of selected targets. Many areas of the sky have therefore been observed two or more times, allowing us to compare count rates for some objects over a long temporal baseline. Objects with significantly varying count rates for widely separated times are of particular astrophysical interest.

With this technique, we have discovered one such object, which appears in the First EUVE Source Catalog as EUVE J2056-171. We present upper and lower limits on how frequently other highly-variable objects will be detected by EUVE in future observations.

Los Alamos National Laboratory’s ALEXIS satellite (a wide area EUV monitoring instrument) was launched April 25, 1993. Due to the damage sustained at launch by the satellite, the ALEXIS project team has had to spend over a year devising new methods to determine spacecraft attitude knowledge, essential for putting photons back on the sky correctly. These efforts have been successful and currently the ALEXIS attitude solutions are precise to better than 0.5 degree close to the original 0.25 degree pre-flight specification. This paper will discuss the number and types of point sources that have been revealed in the ALEXIS data to date. We will also discuss ALEXIS observations of the June, 1994 super outburst of the Cataclysmic Variable VW Hyi, a program to look for simultaneous EUV emission from Gamma Ray Bursts, as well as an effort to detect EUV transients with a 12 – 24 hour response time.

We present optical identifications of nine previously unidentified extreme ultraviolet (EUV) sources discovered during the Extreme Ultraviolet Explorer (EUVE) satellite surveys. The all-sky survey detected four of the sources and the more sensitive deep survey detected the other five sources. Three of the four all-sky survey sources, EUVE_J1918+59.9, EUVE_J2249+58.5, and EUVE_J2329+41.4, are listed in present catalogs as having possible associations with optical counterparts but without spectral class. The first two of these sources are hot DA white dwarfs showing an optical spectrum with broad Balmer lines. The source EUVE_J2329+41.4 is listed as having a possible association with an unclassified M star. We show that a pair of dMe stars are actually optical counterparts located within the error circle of the EUVE source position. The EUVE_J2114+503 remains unidentified even though all the possible candidates have been studied. Based on the count rates we predict a fainter white dwarf or a cataclysmic variable counterpart for this candidate. All five sources discovered with the EUVE deep survey, EUVE_J0318+184, EUVE_J0419+217, EUVE_J2053−175, EUVE_J2056−171 and EUVE_J2233−096, have been identified as late-type stars. The spectral classes, distances, visual magnitudes, and estimated hydrogen column densities for these EUVE sources are presented.

We have carried out optical and Far UV studies of the field around the EUV source EUVE J1027+323. We find two sources which contribute to this flux which are spatially unresolvable with EUVE. One is a non-cataloged QSO and one is a “hidden” hot white dwarf. Reasonable scenarios ascribe the majority of the flux to the white dwarf.

I discuss recent advances in EUV solar astronomy. The new work is primarily a result of current solar space missions such as the Yohkoh high energy solar physics mission, as well as upcoming space missions such as the ESA Solar and Heliospheric Observatory (SOHO). I discuss spectroscopic and atomic physics work, and new results concerning solar flares that are directly relevant to stellar research.

Systematic differences between elemental abundances in the corona and in the photosphere have been found in the Sun. The abundance anomalies are correlated with the first ionization potentials (FIP) of the elements. The overall pattern is that low-FIP elements are preferentially enhanced relative to high-FIP elements by about a factor of four; the transition occurs at about 10 eV. This phenomenon has been measured in the solar wind and solar energetic particle composition, and in EUV and X-ray spectra of the corona and flares. The FIP effect should eventually offer valuable clues into the process of heating, ionization and injection of material into coronal and flaring loops for the Sun and other stars. The situation for the Sun is remarkably complex: substantial abundance differences occur between different types of coronal structures, and variations occur over time in the same region and from flare to flare. Anomalies such as enhanced Ne/O ratios, distinctly at odds with the basic FIP pattern, have been reported for some flares. Are the high-FIP elements underabundant or the low-FIP elements overabundant with respect to hydrogen? This issue, which has a significant impact in physical interpretation of coronal spectra, is still a subject of controversy and an area of vigorous research.

We report on the results of a multi-frequency program to study coronal X-ray, EUV, and microwave activity of solar-type (G0-5 V) stars of greatly different ages. These stars are of interest as proxies for the Sun for ages from 50 Myr to 10 Gyr. Coronal temperatures decrease with X-ray/EUV luminosity and with increasing age and rotation period according to power-law relations. The young Sun had an extremely luminous corona with LEUV,X = 100 − 600LEUV,X (present Sun) and temperatures up to 10 MK; this information is pivotal for the study of the young planetary atmospheres. The findings further suggest an intimate connection between the presence of very hot coronal plasma and non-thermal radio-emitting particles.

Recent R-matrix calculations of electron impact excitation rates for Fe XIII are used to derive the theoretical electron density sensitive emission line ratios R1 = I(318.12 Å)/I(320.80 Å) and R2 = I(256.42 Å)/I(251.95 Å), which are found to be up to 50% different from earlier diagnostics. A comparison of the current line ratios with both solar flare and active region observations, obtained by the Naval Research Laboratory’s S082A spectrograph on board Skylab, reveals generally good agreement between densities deduced from Fe XIII and those estimated from diagnostic line ratios in species formed at similar temperatures. This provides experimental support for the accuracy of the line ratio calculations, and hence the atomic data adopted in their derivation. In Extreme Ultraviolet Explorer satellite (EUVE) spectra the Fe XIII emission lines are found to be severely blended. However, an analysis of these lines measured in the spectra of Procyon and α Cen demonstrates that they still allow very approximate values of the electron density to be inferred. Moreover, it should be possible to increase the accuracy of the measured line fluxes, and hence of the inferred densities, if longer exposures of the stars concerned can be obtained.

Theoretical electron density sensitive emission line ratios involving transitions in the 186–383 Å wavelength range are compared with observational data for a solar active region and a subflare, obtained by the Solar EUV Rocket Telescope and Spectrograph (SERTS). Electron densities derived from the majority of the ratios are consistent with one another, and are also in good agreement with the values of density estimated from diagnostic lines in other species formed at similar temperatures to Fe XII. These results provide observational support for the general accuracy of the diagnostic calculations. In addition, our analysis indicates that a line at 283.70 Å in the active region spectrum is the 3s23p32D3/2−3s3p42P1/2 transition in Fe XII, the first time (to the best of our knowledge) that this line has been identified in the solar spectrum. Several of the line ratios considered are predicted to be relatively insensitive to the adopted electron temperature and density, and the generally good agreement found between theory and observation for these provides evidence for the reliability of the SERTS instrument calibration. The application of the Fe XII diagnostics to EUVE observations of the F5 subgiant Procyon is briefly discussed.

Recent calculations of electron and proton impact excitation rates in Ne VI are used to derive the intensity ratios of lines in the ~402–1006 Å wavelength range as a function of electron temperature (Te) and density (Ne). These results are presented in the form of ratio-ratio diagrams, which should in principle allow both Ne and Te to be deduced for the Ne VI line emitting region of a plasma. Electron temperatures and densities derived from ratio-ratio diagrams involving the 562.7, 997.4, 999.6 and 1006.1 Å lines, in conjunction with observational data for a sunspot obtained with the Harvard S-0555 spectrometer on board Skylab, are found to be compatible, and in good agreement with plasma parameters determined using other methods. This provides some support for the diagnostic calculations presented in this paper, and hence the atomic data used in their derivation. However agreement between theory and observation is very poor for other Ne VI lines in the sunspot spectrum, and for most transitions observed in S-0555 active region and flare data, which is probably due to blending with lines from N III, Mg VI and Mg VII. The application of the calculations to non-solar EUV sources is discussed.

The Arcetri-95 spectral code for optically thin plasmas computes the continuum and line emission of the ions of the most abundant elements. It includes the most updated atomic models and the main atomic processes for ions from Fe IX to Fe XXIII and for ions of the Belike, C-like, and N-like isoelectronic sequences. The power emitted per unit emission measure is produced as a function of temperature and density.

Comparison with observations requires the knowledge of the differential emission measure (DEM) as a function of temperature. A numerical code evaluates the best DEM distribution that satisfies observations and theoretical predictions. The spectral code together with the DEM code allows to compute synthetic spectra for any specified temperature distribution model of the plasma.

We address the inversion problem of deriving the differential emission measure (DEM) distribution D(T) = nenHdV/d log T from the spectrum of an optically thin plasma. In the past we have applied the iterative Withbroe-Sylwester technique and the Polynomial technique to the analysis of EXOSAT spectra of cool stars, but recently we have applied the inversion technique discussed by Craig & Brown (1986) and Press et al. (1992) in the analysis of EUVE spectra of cool stars. The inversion problem-a Fredholm equation of the first kind-is ill-posed and solutions tend to show large, unphysical oscillations. We therefore apply a second-order regularization, i.e., we select the specific DEM for which the second derivative is as smooth as is statistically allowed by the data. We demonstrate the importance of fitting lines and continuum simultaneously, discuss the effect on the DEM of continuum emission at temperatures where no line diagnostics are available, and address possible ways to check various model assumptions such as abundances and photon destruction induced by resonant scattering.

A great deal of effort in recent years has gone into the development of spectroscopic techniques to probe the physical parameters of solar, stellar and other astrophysical plasmas. One aspect of this work is the calculation of plasma emission codes which are used to study EUV spectral lines. These codes require the input of a large amount of atomic data. In this paper, we present an overview of the atomic processes involved and an assessment of the accuracy of the parameters which are incorporated into different emission codes.

Systematic and large-scale calculation of accurate plasma opacities and atomic data by the Opacity Project and the Iron Project has applications in many areas of astrophysics. Analysis of EUVE observations using monochromatic opacities of elements calculated by the Opacity Project is described. Theoretical methods and atomic calculations are discussed briefly. Recent work related to ionization balance, photoionization and recombination, and the modeling of plasmas in local thermodynamic equilibrium (LTE) and non-LTE are discussed. New calculations for the important iron ions, under the Iron Project, are also described and their relevance to the more extensive non-LTE calculations is pointed out. The comprehensive radiative and collisional data sets from the Opacity and the Iron Projects should be applicable to a large number of sources in the EUV and other wavelength regions.

Recent progress in large scale computations of photoionization cross sections, radiative transitions probabilities, and collision strengths for the lowest ionization stages of Iron, i.e., Fe I-VI, are reported. These results are part of an international collaboration called the IRON Project. The present results exhibit large differences with respect to data currently in use and is expected to be of importance in the study of astrophysical objects in the UV and the EUV.

The 4 configuration, distorted wave calculation of Bhatia & Doschek (1995) (hereafter referred to as BD95), together with the ground transition calculation of Pelan & Berrington (1995) are here used to predict the intensities of the Fe X EUV lines, which are then used to derive electron densities from several solar spectra, including the recent SERTS spectra.

Recent developments of EUV diagnostics from transitions in Boron-like ions, C II, N III, O IV, Ne VI, Mg VIII, Al IX, Si X, and S XII, are discussed based on new atomic data (Zhang, Graziani & Pradhan (1994)) and systematic theoretical calculations of line intensity ratios (Peng & Pradhan 1995). The available EUV and UV lines which are good temperature and density diagnostics are summarized. Possible applications of these lines to the investigation of the physical conditions in astrophysical objects are proposed.

A bibliography has been produced of the most reliable emission and absorption line ratio diagnostic calculations currently available for application to the spectra of astrophysical sources in the UV and EUV wavelength region (50-3000 Å). References are listed containing diagnostics for species in the Li through P isoelectronic sequences, as well as the iron ions Fe II-Fe XXIII and nickel ions Ni XVII-Ni XXV. Also given is the wavelength range for which diagnostic calculations are presented in each reference, along with the type of diagnostic considered. These include, for example, emission line ratios for determining electron temperatures and densities, and absorption line diagnostics for evaluating hydrogen densities.

In order to promote research using data from NASA’s Extreme Ultraviolet Explorer (EUVE) satellite, the Center for EUV Astrophysics has implemented a Guest Investigator (GI) Science Program. The purpose of the GI Program is to provide researchers with information, services, and training in the use of public EUVE data sets; in effect, it offers to the research community the technical experience and intricate knowledge of the EUVE data sets resident at CEA. All interested researchers are encouraged to participate as GIs.

The new Extreme Ultraviolet Explorer (EUVE) Public Right Angle Program (RAP) offers the opportunity for researchers to obtain observations with the EUVE imaging telescopes, oriented at right angles to the EUVE spectrometer used for the NASA Guest Observer Program. Scientists may submit proposals electronically through the World Wide Web or e-mail using a template form to list specific targets and present the scientific motivation for the work. The RAP electronic proposal process is streamlined from proposal submission through data delivery and is a prototype for a system to be used for all guest observers during the EUVE extended mission starting in February, 1996.