Important progress in the field of the Interstellar Medium (ISM) in general, and the Local Interstellar Medium (LISM) in particular, has been largely due to the recent launch of orbiting satellite missions and earth-bound observations with unprecedented sensitivity and high spectral or spatial resolution. However, the interpretation of a wealth of new data is still based to a great extent on models that were designed for the previous generation of observations. Thus there remains a clear gap between our theoretical understanding of the basic physics of the ISM and the detailed information available. In such a situation it seems appropriate to focus on some key observations and discuss their implications for modelling.

Constraints on the ambient (circumsolar) interstellar medium can be derived from observations of interstellar neutrals or their derivatives in the heliosphere. New results have been recently obtained which, when combined with optical and UV observations of the nearby stars (HST-GHRS, EUVE), remove long-standing contradictions and allow us to infer for our local cloud, pressure, ionization and limits on the magnetic field. The electron density in the circumsolar gas is found to be between 0.04 and 0.15 cm−3. Its total thermal pressure is within the interval 1700 – 2600 cm−3 K. If the local magnetic field is nearly perpendicular to the interstellar wind flow, which is likely, then its intensity is smaller than 3.6 μG. Our Sun is located very close to the edge of the local cloud (the volume of gas which has the same physical properties as the circumsolar gas), while there are at least 5 other cloudlets within 10 pc. Abundances vary from cloudlet to cloudlet. How they are located and whether they are separated by tenuous gas or shock discontinuities is not clear yet. Semi-hot (105 K) gas has been detected in absorption towards nearby stars (Wood et al, 1996), which probably originates in the “H walls” surrounding our heliosphere and other asterospheres. This shows that semi-hot gas is not necessarily linked with cloud interfaces with the hot gas of the Local Bubble.

We propose a new method to constrain the actual state of the interstellar cloud that surrounds the solar system. Using Voyager UVS Lyman-α sky maps and the powerful principle of invariance, we derive the H distribution all along the spacecraft path. Provided current models of the heliopause interface between the solar and the interstellar winds, we extrapolate this distribution to farther distances from the Sun and infer in a self consistent way key parameters of the local cloud. Our findings are a high interstellar hydrogen density of ~ 0.24 cm−3 and a weak ionization .

Ultraviolet absorption lines from carbon atoms within a neutral cloud inside or near the edge of the Local Bubble can reveal the thermal pressure of the gas, since the relative populations of the fine-structure levels are determined by a balance between collisional excitation and radiative decay. Features from a C I multiplet appearing in the uv spectrum of the star δ Cyg (l = 79°, b = +10°, d = 52 pc) were observed with the echelle spectrograph of the Goddard High Resolution Spectrograph on HST. An interpretation of absorptions detected from the two lowest levels gives a thermal pressure range 102.7 < p/k < 103.7 cm−3K.

We report on the status of a long-term program of mapping the velocity structure of the local ISM using ultra-high resolution observations of Call K-line absorption towards nearby B and A-type stars. Absorption components common to the local interstellar cloud (LIC) have been detected in many (but not all) lines-of-sight. Our preliminary results indicate that the local ISM has a complex velocity pattern of inhomogeneous absorption, indicative of a shocked filamentary nature.

The EURD instrument has been designed to measure diffuse emission in the extreme ultraviolet (350–1100 Å). This new design provides an unprecedented 4–5 Å spectral resolution and 200 photons/sec/cm2/sr sensitivity after only 100 hours of observations. One of the goals of this project is to search for spectral lines of highly ionized species from the high temperature component (105 – 106 K) of the interstellar medium that fills the Local Bubble. It is expected that EURD will detect lines due to a thermalized hot component of the interstellar medium, and it will also provide critical diagnostics of the physical properties of this gas. With EURD data we could also detect a spectral line due to the decay of massive neutrinos as well as study oxygen lines from the upper atmosphere airglow. EURD is on board the Spanish MINISAT-01 satellite.

New results from EUVE are reviewed, with emphasis on the ionization state of the ISM and the question of thermal pressure balance between warm clouds and the hot intercloud medium. A description of what may be expected from the next generation of experiments, some to be launched shortly, will also be presented.

We present results from a search for FUV emission from the diffuse ISM conducted with an orbital FUV spectrometer, DUVE, which was launched in July, 1992. The DUVE spectrometer, which covers the band from 950 Å to 1080 Å with 3.2 Å resolution, observed a region of low neutral hydrogen column density near the south galactic pole for a total effective integration time of 1583 seconds, the only emission line detected was a geocoronal hydrogen line at 1025 Å . We were able to place upper limits to several emission features that provide constraints to interstellar plasma parameters. We were also able to place continuum limits in this band. We use these upper limits to place constraints upon the emission measure vs. temperature distribution of this gas using an isothermal Landini and Fossi model.

We have used the Deep Survey telescope of EUVE to investigate shadows in the diffuse EUV/Soft X-ray background cast by clouds in the interstellar medium. We confirm the existence of a shadow previously reported, and provide evidence for two new shadows. We used IRAS data to identify the clouds producing these shadows and to determine their optical depth to EUV radiation. The EUV-absorbing clouds are optically thick in the EUV, and all EUV emission detected in the direction of these shadows must be produced from material in front of the clouds. We obtained new optical data to determine the distance to these clouds. We use a new differential cloud technique to obtain the pressure of the interstellar medium. These results do not depend on any zero level calibration of the data. Our results provide evidence that the pressure of the hot interstellar gas is the same in three different directions in the local interstellar medium, and is at least 8 times higher than derived for the local cloud surrounding our Sun. This provides new evidence for large thermal pressure imbalances in the local ISM, and directly contradicts the basic assumption of thermal pressure equilibrium used in almost all present models of the interstellar medium.

The first X-ray-opaque cloud in the direction of Auriga lies at 180 pc. A tenuous cloud, whose distance is derived from IUE absorption line studies as well as from an analysis of ROSAT data, is embedded at 55 pc. The X-ray emitting gas has a temperature of 105.9−6.0 K. The pressure is 1.6 104 K cm−3.

We have studied the EUV spectra of 13 DA white dwarfs, observed by the EUVE satellite, paying attention to the possible sources of absorbing material along the lines-of-sight in both the local interstellar medium and in the photospheres of the stars themselves. The range of interstellar column densities seen are consistent with our previous understanding of the local distribution of material. Absorption from interstellar He II is found in the direction of five stars, allowing us to measure directly the He ionization fraction and estimate, indirectly, that of H. The weighted mean ionization fractions along these lines-of-sight are 0.27 ± 0.04 and 0.35 ± 0.1 respectively. Where He II is directly detected, the observed ionization fractions are not correlated with direction or with the volume/column density of material along the line-of-sight. Furthermore, the limits on the amount of He II established in all other directions completely encompass the range of observed values. Indeed, all the data can be consistent with more or less constant He and H ionization fractions throughout the local ISM. However, observation of very hot DA stars, indicating higher He II columns, might contradict this picture if the material is not photospheric or circumstellar.

During the second flight of the ORFEUS-SPAS satellite (Nov./Dec. 96) high resolution (λ/∆λ = 10,000) Echelle-spectra of BD+28° 4211 in the wavelength regime 912–1400 Å have been taken. Deuterium can be clearly identified in the ORFEUSII Echelle-spectra of this star. For the first time it was possible to take spectra of faint, not redshifted objects in the far ultraviolet with a sufficient spectral resolution to study the deuterium column density in the whole spectral range of the Lyman-series down to the Lyman-limit. We obtained a value of log(ND) = 14.7 (±0.3) towards BD+28° 4211. The hydrogen column density has been determined using ORFEUS Echelle- and IUE-spectra of Ly-α (log(NH) = 19.8 (±0.2)). Thus a value of 8 × 10−6 can be obtained for the D/H-ratio on the line-of-sight towards BD+28° 4211.

Observations of the diffuse EUV background towards 138 different directions using the spectrometers aboard the Extreme Ultraviolet Explorer satellite (EUVE) have been combined into a spectrum from 150Å to 730Å and represent an effective exposure of 18 million seconds. There is no significant evidence for any line flux from any source other than the geocorona. These results are inconsistent with the Wisconsin C and B broad-band surveys assuming the source is a logT = 5.8 – 6.1 hot plasma in ionization equilibrium with solar abundances, confirming the previous result of Jelinsky, Vallerga & Edelstein (1995) (hereafter Paper I) using an observation along the ecliptic with the same instrument. To make these results consistent with the previous broad-band surveys, the plasma responsible for the emission must either be depleted in Fe by a factor of ~ 6, be behind an absorbing slab of neutral H with a column of 2×l019cm−2, or not be in collisional ionization equilibrium (CIE). One particular non-CIE model (Breitschwerdt & Schmutzler, 1994) that explains the soft x-ray results is also inconsistent with this EUV data.

The Diffuse X-ray Spectrometer (DXS) obtained spectra of the low energy X-ray (44 – 83 Å) diffuse background near the galactic plane from galactic longitudes 150° ≲ l ≲ 300° with ≲ 3 Å spectral resolution and ~ 15° angular resolution. Thus, DXS measured X-ray spectra that arise almost entirely from within the Local Bubble. The DXS spectra show emission lines and emission-line blends, indicating that the source of the X-ray emission is thermal – hot plasma in the Local Bubble. The measured spectra are not consistent with those predicted by standard coronal models, either with solar abundances or depleted abundances, over the temperature range 105 – 107 K. The measured spectra are also inconsistent with the predictions of various non-equilibrium models. A nearly acceptable fit to DXS spectra can be achieved using a hybrid model that combines the Raymond & Smith ionization balance calculation with recently calculated (by DAL) ionic emission lines.

Competing models for the origin of the local component of the diffuse X-ray background span more than an order of magnitude in temperature. We propose to test these models with a new instrument, the GRating Array Diffuse EUV Spectrometer (GRADES), which is sensitive to diffuse emission in the 90 – 260 Å bandpass, and offers a peak spectral resolution of about λ/127.

We plan to observe hot gas in the local interstellar medium (LISM) at high resolution using a rocket-borne spectrograph that can simultaneously observe OVI, NV and CIV at resolutions of 1 – 2kms−1. Data from a single flight will have sufficient signal to noise ratio to detect a broad 50kms−1 component down to a column density of N ≥ 0.5 × 1013cm−2. High-velocity components as fast as 300kms−1 can be detected. New high resolution observations of hot gas absorption line profiles will probe the kinematics of the local bubble, and comparison between the carbon, nitrogen and oxygen profiles will allow discrimination between the various production models.

Nearby late-B and early-A stars show clear IS absorptions in Mg II, C II and other lines (IUE spectra). Some of them, in the Sco Cen direction, also show absorption features at C IV and Si IV line wavelengths. We discuss the possible formation of these lines in an intermediate temperature gas region between the dense and cold H I wall and the hot ISM, a region that could be the collision interaction between the Local Bubble and the Loop I supershell. Some individual stellar spectra are presented and discussed.

The X-ray sky at ¼ keV is completely dominated by diffuse emission. It has become clear that it originates as at least three separate components: local emission within the nearest ~ 100 pc from the Sun, halo emission from beyond most of the neutral material of the Galactic disk, and the superposition of unresolved extragalactic sources. The only way to determine the temperatures and relative emission measures of the hot plasma responsible for the Galactic components is to use the X-ray intensity variations due to column density variations in the intervening Hi to separate the components. “Shadowing” studies have been pursued for individual objects using ROSAT data, from both pointed observations and the all-sky survey with considerable success.

This paper presents the results of an all-sky analysis of the ¼ keV background from the ROSAT survey. A Local Hot Bubble is found consistent with, although somewhat smaller than, previous models. It has a temperature of 106.1 K and an emission measure which varies by a factor of ~ 3.3 over large angles. The halo emission has a temperature near 106.0 K with an emission measure which varies from near zero to more than five times that of the local emission.

The soft X-ray background (SXRB, 0.1 – 2.0 keV) is generally believed to be comprised of a local component, the Local (Hot) Bubble (LHB), which is only marginally absorbed, and of distant emission (galactic halo, extragalactic background). It is vital to know the foreground emission (spectrum, intensity) if one wants to disentangle and determine the 3-dimensional structure of the X-ray emitting regions. During the ROSAT PSPC All-Sky Survey non-cosmic enhancements of the measured X-ray count rate have been observed. These lasted typically for up to 8 hours (i.e. a couple of orbits). Using PSPC data it has been shown that these so-called “long-term enhancements” (LXE) are related to geomagnetic storms and variations of the solar wind. Moreover, during an observation of the comet Hyakutake indications for a correlation of the X-ray background with the X-ray brightness of the comet have been found. This can be interpreted in terms of an X-ray emitting region around the Earth. Although the variable component of these enhancements could be subtracted from the sky maps an uncertainty remained concerning an undetected and relatively constant contribution which might be present in soft X-ray observations. As a consequence this could explain or contribute to the high PSPC count rate observed from the dark side of the Moon.