The use of interstellar polarization as a measurement tool for dust or magnetic field presents practical difficulties (dust and magnetic field configurations inextricably mixed up; limited number of suitable stars). This general rule applies even more in the local environment, for which the polarizations are small and the influence of errors of observation changes in character. Because of this, the use of polarimetry and the design of polarimetric observing programmes for the local interstellar medium must be approached with even greater care than in the general case. I propose in this review to amplify this point, so that the reader can judge for himself to what extent he can use published results and can also, given the opportunity and instrumentation, design an observing programme that will really pay off.

Probable dust counterparts of arching structures are looked for in two cases, (a) the Eridanus expanding shell, Heiles (1976) and (b) the extended Sco-Cen feature, Weaver (1977). Apparently the color excess data may be used to derive distance and dimension estimates of these structures.

Wide-angle photographs of the Northern Milky Way reveal a complex system of dark clouds covering the galactic longitudes from 1 = 90° to about 1 = 200° (fig.). This Northern Dark Cloud System is a coherent structure of individual clouds of preferentially cellular appearance. The cloud system has to be assigned to the local spiral arm. This is substantiated by the large extent in galactic latitudes and its proximity to well-established tracers of the local spiral arm.

In the ultraviolet spectra of hot stars, we can see absorption lines from highly ionized species in the interstellar medium. Observations of these features have been very influential in revising our perception of the medium's various physical states and how gases in space are heated and ionized. The pervasiveness of 0 VI absorption lines, coupled with complementary observations of a diffuse background in soft x-rays and EUV radiation, shows that there is an extensive network of low density gas (n ≈ fewx10-3cm-3) existing at “coronal” temperatures, 5.3 ≲ log T ≲ 6.3. Thus, while we once thought that x-rays and cosmic rays were the dominant sources of excitation and heating of the interstellar gas, we now realize that shocks created by supernova explosions or mass loss from early-type stars can propagate freely through space and eventually transfer a large amount of energy to the medium. To create the coronal temperatures, the shocks must have velocities in excess of 150 km s-1; shocks at somewhat lower velocity (v ≲ 100 km s-1) can be directly observed in the lines of Si III. Observations of other lines in the ultraviolet, such as Si IV and C IV, may highlight the widespread presence of energetic uv radiation from very hot, dwarf stars. More advanced techniques in visible and x-ray astronomical spectroscopy may open up for inspection selected lines from atoms in much higher stages of ionization.

The Berkeley EUV telescope flown on the Apollo Soyuz mission in July, 1975 established the existence of a measurable flux of EUV radiation (100 ≤ λ ≤ 1000 Å) originating from sources outside the solar system. White dwarfs, flare stars and cataclysmic variables were discovered to be relatively intense compact sources of EUV photons. Moreover, this and other subsequent experiments have strongly suggested the presence of a truly diffuse component of the EUV radiation field possibly due to thermal emission from hot (T > 105 K) interstellar gas located in the general vicinity of the sun (r ≃ 100 parsecs.) Closer to the HI, 912 Å edge, the effect of a few hot O and B stars has been shown to be very important in establishing the interstellar flux density. All these results imply that the local ISM is immersed in a non-negligible EUV radiation field which, because of the strong coupling between EUV photons and matter, will play a crucial role in determining its physical structure. Conversely, of course, the local ISM is expected to leave a strong imprint on the EUV field reaching the terrestrial observer.

The objective of this review will be to assemble and critically analyze the available information on the local ISM derived from the limited EUV observations carried out so far. These include measurements of the spectra of bright EUV sources that reveal clear evidence of H photo absorption at λ > 400 Å and of the He ionization edge at 228 Å. The EUV diffuse background is found to convey interesting information on the density, temperature and possible location of the hot ISM component that can be profitably compared with similar data on far uv absorption lines, principally OVI. The results discussed in this context will be shown to be quite useful in illuminating the path that future EUV sensitive observatories such as EUVE and Columbus might profitably follow in the near future.

Detection of narrow ( δλ< 0.5 A ) absorption features in C IV at λ λ 1548 and 1550 have been made in the spectra of 4 late B dwarfs within 200 pc of the sun; the Si IV doublet at λ λ 1393 and 1403 shows up in two of them. We argue that it is difficult to account for the strengths, widths, shapes , and C IV/Si IV ratios in terms consistent with a circumstellar origin except possibly for an asymmetric C IV component in one star (HD 135037). The most probable source is “semi-torrid” gas (cf. Bruhweiler et al. 1980) in the 50,000 K range forming the interfaces between cooler H I clouds and the ambient medium at coronal temperatures. Our technique, using late B rapid rotators, is useful for LISM probing of this kind.

The spectra of 24 stars, including 5 at distances d < 200 pc, have been observed in the regions of the coronal [Fe X] λ6375 and [Fe XIV] λ5303 lines at detection limits near an equivalent width of 1 mÅ in the best cases. In general agreement with predictions based on a multi-phase model of the interstellar medium, no absorption which can be attributed to Fe X or Fe XIV ions in hot interstellar gas emitting the soft x-ray background is seen in any of these spectra, except for two. Toward λ Cephei an absorption line near λ6375 is measured with an equivalent width of 8.1 ± 2 mÅ, a width corresponding to 20 ± 5 km s-1 or a temperature T ≤ (0.5 ± 0.25) x 106 °K, and, if it is caused by Fe X ions, a radial velocity of -355 km s-1. On that hypothesis, the hot interstellar gas constitutes at least 63% of the column density of gas along this light path.

Observations of the diffuse X-ray background in the B and C bands (130-188 eV and 160-284 eV, respectively) provide convincing evidence for the existence of high-temperature interstellar gas. Since the opacity of normal interstellar material is very high, we must assume that the soft X-ray flux observed in the galactic plane originates within a few hundred parsecs of the Sun. The intensity and B/C ratio of this low-latitude flux can be provided by emission from an equilibrium plasma with normal abundances, T = 106.0 K, and 0.0019 cm-6 pc emission measure. More sophisticated nonequilibrium models of material heated by a supernova blast wave would reduce the required emission measure somewhat, but not by so much as a factor of two. Arbitrarily limiting the pressure to 104 cm-3 K gives a maximum density of 0.005 cm-3 and a minimum radius for the emitting region of 75 pc. This fits in well with UV interstellar absorption measurements which indicate that the ISM is very deficient in neutral hydrogen out to ~100 pc from the Sun.

The constancy of the observed B/C ratio implies that there are less than 5x1019 cm-2 variations in any cooler material lying between us and the bulk of the emission. This is consistent with the much smaller column densities of neutral or partially ionized material detected in the solar neighborhood through UV absorption measurements, but the organization of the cooler gas and its interaction with the coronal material require further investigation.

X-ray intensities at high latitudes are larger than those in the plane by as much as a factor of three. The fluctuations show a global anti-correlation with H I column density which suggests that they might be caused by variations in the transmission of X-rays from an extragalactic source, such as a galactic halo or corona. Such models are very difficult to reconcile quantitatively with existing H I measurements, however, and it seems more likely that the bulk of the high-latitude excess is produced by an extension of the unabsorbed interstellar emission in those directions with the remaining fluctuations produced by a combination of absorption and displacement by embedded cooler material, or possibly by transmission of flux from a hot halo.

While it would be most interesting to resolve this point because of its impact on the nature of the galactic halo and the high-latitude H I distribution, it does not greatly affect the amount of hot gas which apparently exists in the solar neighborhood: an isotropic emitting region with only the emission measure required by the average intensity near the galactic plane would account for about two-thirds of the integrated B and C band flux observed at the Earth. The total energy flux is ~1x10-6 ergs cm-2 s-1 if we assume a thermal equilibrium spectrum.

Comparison of the SAS-3 soft X-ray sky survey (F. Marshall and G. Clark 1984) with the 21-cm neutral hydrogen survey of Stark et al. (1984) confirms the well-known anticorrelation between the counting rates in the C-band (0.10-0.28 keV) and the column density of neutral hydrogen, and demonstrates that this anticorrelation is significant on all angular scales ranging from that of the general trend from the galactic equator to the poles down to the angular resolution of the detector (2°.7 FWHM). Included in this general anticorrelation are numerous instances of what appear to be soft X-ray “shadows” of nearby (100-300 pc) 21-cm features, and several bright X-ray regions coincident with “holes” in the ISM.

The spatial structure of the X-ray sky in the direction of the North Polar Spur was examined in two energy bands, the B band (0.10 - 0.18 keV) and the C band (0.15 - 0.28 keV). A model with two emitting regions, one local with unabsorbed emission, and the other more distant with emission partially absorbed by spatially varying amounts was investigated.

Using the distribution of atomic hydrogen as a measure of absorbing material, this model was used to predict the flux in the direction of the North Polar Spur. The predicted flux was compared to the data obtained from several sounding rocket flights. The derived flux was found to correlate well with the observed data.

The intensity of the X-ray background between 0.5 and 1.0 keV has surprisingly little dependence on galactic latitude. Possible mechanisms for the production of these X-rays include extragalactic emission and emission from dM stars, both of which should be strongly dependent on galactic latitude, and diffuse emission from hot gas (T ≃ 3 x 106 K) surrounding the Sun. These mechanisms can be distinguished by the presence or absence of absorption by gas within a few hundred parsecs of the Sun. We use X-ray data from the HEA0-1 LED detectors and HI data from the recent Crawford Hill 21 cm survey to place limits on the 0.6 keV intensity originating within 300 pc of the Sun in the general direction of (l,b) = (150°, -30°).

Maps of the diffuse soft X-ray background intensity are presented, spanning four energy intervals. The lowest energy interval (0.18-0.56 keV) is dominated by local emission, while the next two intervals (0.56-1.0 keV and 1.0-1.4 keV) reveal more distant and more sharply defined structures. Enlarged maps of several of these structures are presented, including the North Polar Spur, the Galactic Center region and the Eridanus Loop.

Preliminary results from a May 8, 1984 sounding rocket survey of the soft X-ray background are presented. The X-ray detectors are sensitive to X-rays in three soft X-ray bandpasses: 80-110 eV, 90-188 eV, and 284-532 eV (at 20% of peak response). The lowest energy X-rays in this range have a mean free path of order 1019 cm-2 and provide information about the local interstellar medium. The count rate in the 80-110 eV energy band (the Be band) tracks the 90-188 eV band (the B band) very well, indicating that the same ~1 million degree gas that is responsible for the B band emission may be responsible for the bulk of the Be band X-rays as well. We estimate for the flux in the Be band ~1 photon cm-2 s-1 sr-1 eV-1 , about a factor of four lower than that found by Stern and Bowyer (1979) and Paresce and Stern (1981) over a similar energy band.

A preliminary comparison of color excesses and HEA0-1, A2 0.25 keV counts have been performed on two angular levels in order to search for indications of local photoelectric absorption.

a. Color excesses and background counts are smoothed to a common 1.5 1.5 sq.deg. beam. Although distance information on the color excesses are available it is not used in this first attempt. The lines of sight to which the excesses pertain have however a length comparable to the expected viewing distance at these energies in the local medium. Figure 1 displays the resulting average counts/pixel versus average excess/pixel for the five areas investigated. A general decrease of the counts with excess is apparent. The wide scatter in the individual regions is probably due to excess variations on scales smaller than the collimated beam. The minimum counts on the diagram come from a region where an almost coherent sheet of dust is present within a distance ~200 pc. The constancy of the minimum counts in this particular direction indicates that the soft X-ray emission probably originate exclusively in front of the matter observed to be present. Note that the SGP counts show a shallow inverse dependence on excess. The latitude distribution of the five areas assures that the general inverse relation of the 0.25 keV counts on color excesses is not a latitude effect.

Observations at the 2.6 mm line of CO reveal the presence of a large number of molecular clouds at high galactic latitude. If the velocity dispersion of the clouds is a measure of their scale height, the mean distance of the ensemble we have detected is 100 pc. The clouds are unusual in that either they are not gravitationally bound or they are very deficient in CO relative to molecular hydrogen. These clouds represent a heretofore unrecognized component of the local interstellar medium. If they are pervasive in the Milky Way, they probably represent the small molecular cloud component of the interstellar medium.

It is now well established that molecular clouds are an important part of the interstellar medium, containing much or most of the dense, cold component of the gas, and producing the massive stars and supernovae responsible for the diffuse, hot component. It would therefore appear essential in formulating a complete picture of the local interstellar medium to have some knowledge of the distribution and properties of nearby molecular clouds. To this end we have used the Goddard-Columbia 1.2-meter telescope to carry out a wide latitude, low angular resolution survey of CO along most of the first galactic quadrant and a small part of the second. The survey is uniform and fully sampled in galactic longitude from 12° to 100°, and in latitude from -5° to +6°, with extensions to as high as +11° to include specific dark clouds; the total area surveyed, 1128 deg2, is much larger than any region previously studied in CO or any other interstellar molecule. In order to fully sample such a large area in a reasonable amount of time, angular resolution has been sacrificed to coverage and speed; an angular resolution of 1° was obtained synthetically by simply scanning a square 8 x 8 raster of points separated by 1/8°, the size of the primary beam, and summing the resulting 64 spectra at the end of an observation.

A catalogue of dark nebulae and globules has been compiled from a study of the ESO-B and SRC-J sky atlas for galactic longitudes 230°<1<360°. This catalogue closes the great southern gap open since the work of Lynds (1962), We have listed 489 dark nebulae and 311 globules. The catalogue contains positions, sizes, opacities and the van den Bergh classification (1972) on the filamentary morphology of dark clouds. We present statistics concerning the northern and southern distributions and sizes of the nebulae.

21-cm line observations with the Westerbork Synthesis Radio Telescope of a dust and molecular filament at the boundary of the Draco Nebula (l≈ 91°, b≈ 38°) reveal a jet-like neutral hydrogen feature funneling through an outlet in the low velocity shock front at the interface between the Draco Nebula and the surrounding gas. The jet-like feature is apparently connected with a high velocity filament at VLSR = -180 km s-1. We suggest that the soft x-ray emission observed in the area is thermal bremsstrahlung produced by the deceleration of high velocity gas in galactic gas.

Extended and very faint bright nebulae are found in high (b≳ 30°) galactic latitudes at the Palomar Observatory Sky Survey (POSS) (Lynds, 1965) and even more pronounced at a very sensitive photographic survey of the galactic polar caps by Sandage (1976). Such a nebula, located in the constellation Draco and called “Draco Nebula” or “Dracula”, was found to be in detailed positional coincidence with a 21-cm emission line feature at a LSR velocity of VLSR ≈ -22 km s-1 by Goerigk et al. (1983).