The Sun is moving in respect to the nearby stars with a velocity of 20 km.s-1 in the direction of the Apex, α = 271° and δ = 30° (celestial coordinates). As the lights of a car illuminate the water droplets when driving in the fog, the Sun illuminates the Hydrogen and helium atoms of the interstellar medium which it travels through. As a result, the sun and the whole solar system are imbedded in a glow of the resonance lines of hydrogen (H Lyman α ; 121.6 nm) and helium (58.4 nm), which have been studied by several space instruments in the last 14 years.

From the intensity distribution of the glow in the solar system, one can derive the density of H and He in the L1SM and the direction of the relative motion between the sun and the LISM in the very vicinity of the sun. The velocity module Vw and the LISM temperature T are more adequately found from a measurement of the Lyman α line shape, which is an image of the velocity distribution of H atoms.

A summary of results will be presented, together with a discussion of the methods of interpretation and their difficulties. The vector is found to be 20 ± 1 km.s-1 in the direction α = 254 ± 3°, δ = - 17 ± 3°, quite different from the Apex direction. This means that the LISM is moving also in respect to the local frame of reference giving rise to the socalled Interstellar Wind. This wind blows in the galactic plane at 16 km.s-1, in the direction , significantly different from the direction found by interstellar absorption lines on stars within ≃ 100 pc, pointing to a local significance of this flow. The temperature of the LISM is T = 8,000 ± 1,000 K, the density n (H) ≃ 0.04 to 0.06 cm-3, and the helium density n (He) ≃ 0.015 to 0.020. The high helium/hydrogen ratio, in respect to the cosmological ratio, would imply that a substantial part of the hydrogen is ionized. Temperature, density and degree of ionization of the LISM are suggesting that the sun is now in an intermediate phase of the interstellar medium, at the Interface between a hot and tenuous gas, and a dense and cold cloud of gas.

The combination of Pioneer photometric and Voyager spectrometric observations of EUV interstellar-interplanetary emissions in the region beyond 5 AU have been applied to a determination of atomic hydrogen and helium densities. These density estimates obtained from direct measurement of scattered radiation depend on absolute calibration of the instruments, in the same way as other earlier determinations based on the same method. However, we have combined the spacecraft data with daily full sun averages of the H Lya 1216 A line obtained by the Solar Mesospheric Explorer (SME) satellite, to obtain a measure of atomic hydrogen density independent of instrument absolute calibration. The method depends on observations of long and short term temporal variability of the solar line over a 1 year period, and the fact that the ISM is optically thick. The density estimates from preliminary work on these observations are [H] = 0,12 cm-3 and [He] = .016 cm-3, giving a density ratio close to the cosmic abundance value, in contrast to some earlier results indicating a depletion of atomin hydrogen. We have obtained estimates of galactic background emissions in the signals of both spacecraft.

It is possible to deduce LISM properties from observations of interstellar neutral gases in the inner solar system. Parameters accessible by this method are the interstellar wind vector and the densities and temperatures of hydrogen and helium, implying also the deduction of the relative abundance ratios and the degree of ionization in the LISM. Direct inference from observations, for example resonance luminescence measurements of Ly-alpha and He-58.4 nm radiation, yields values appropriate only for the inner solar system, i.e. for the regions within the heliopause dominated by the solar wind plasma.

Particularly the subsonic LISM plasma interface ahead of the heliopause causes profound changes in the properties of the neutral LISM gas traversing this region. Mainly p-H charge exchange processes give rise to the destruction of primary hydrogen and the production of secondary hydrogen atoms, the net effect being a depletion of the neutral hydrogen component of the LISM by about 50%.

Details on the depletion mechanisms, the hydrogen and oxygen extinctions, and the consequences for the Ly-alpha resonance luminescence intensity interpretations are presented.

Neutral interstellar particles penetrating into the heliosphere, besides being subject there to specific loss processes, suffer elastic collisions with KeV-solar wind ions. The momentum transfer to the neutrals connected with these collisions leads to a loss of angular momentum with respect to the sun and to a fractional compensation of the effective solar gravity. The dynamical particle trajectories hence are changed into non-Keplerians leading to density and temperature distributions differing from those calculated in the past. This is found from a solution of the Boltzmann equation that linearizes the effect of this additional force. It is shown here that the HeI-584A resonance glow of the heliospheric helium cone lead to substantially lower interstellar helium temperatures if re-interpreted on the basis of this revised theory. These temperatures now seem to be in accordance with the derived temperatures for interstellar hydrogen.

Previous published absorption line studies performed at ultraviolet and visual wavelengths are combined with new ultraviolet data in order to map out the distribution of HI within 150 pc of the Sun. Newly presented data for distances less than 50 pc further support the local cloud model as presented by Bruhweiler (1982). The Sun is embedded, near the edge of a diffuse cloud with total column density of 2 x 1019cm-2. Most observed directions within 50 pc away from the cloud body reveal trace amounts of gas (N)HI) ~ 1018 cm-2) presumably arising in the outer skin of the local cloud. At greater distances (50 ≲ d ≲ 150 pc) most directions show significant absorption with N(HI)>1019 cm-2. Two directions, one toward the northern galactic pole (NGP), the other toward β CMa exhibit unusually low HI column densities out to distances of 150-200 pc. However, substantial amounts of gas, N(HI) > 1019 cm-2, are seen toward the NGP at greater distances. The implications of these results on astronomy at wavelengths shortward of 912Å are discussed.

During this workshop, a considerable amount of data was presented, some refined versions of earlier results and some entirely new. While no completely definite picture of the local interstellar medium can be presented yet, some general conclusions can be drawn. We attempt to draw together the most relevant data in Section III. Some new UV results are collated with published results to help state the physical properties of the gas. A morphological view of the local medium is given in Section III. We confine ourselves to the region within 100 pc with the caveat that the distance of much of the material under discussion is poorly known.

High-dispersion Copernicus and IUE observations of chromospheric Ly α emission are used to study the distribution of HI in the local interstellar medium. Interstellar parameters are derived toward 3 stars within 5 pc of the sun, and upper limits are given for the Ly a flux from 9 other stars within 10 pc.

We have combined Copernicus and IUE observations of 5 stars within 50 pc of the Sun to study the ionization of magnesium in the local interstellar medium (LISM). The high resolution Copernicus spectrometer was used to detect interstellar Mg I 2852 in the spectra of α Gru, α Eri, and α Lyr, while placing upper limits on Mg I in the spectra of α CMa and α PsA. Observations of Mg II 2795, 2802 for these stars were also obtained with IUE and Copernicus. The column densities of Mg I and Mg II are used to place constraints on the temperature of the LISM.

IUE spectra of Mg II h and k in late type dwarfs and giants have been used to detect and measure absorption components due to the LISM. This technique gives a method of probing the awkward range from d = 3 pc to d = 80 pc from the sun . In spite of interpretational uncertainties we can plot the HI component of the LISM well enough to confirm it as a cloud some 20-30 pc in extent, peaking sharply in density towards , moving towards the sun from , at 28 Km/sec. The “hole” towards is confirmed, suggesting a solar position close to the cloud's edge in this direction.

UV spectra with 15 km s-1 resolution of bright stars have been searched for Fell and Mgll interstellar lines. We present equivalent widths of absorption for 8 stars and find average line-of-sight densities n(FeII)= 4 10-8 and n(MgII)= 2 10-7 cm-3 . This represents approximately n(H)= 0.03 cm-3.

With the aim to sample well the Local Interstellar Medium (LISM),we propose to use A stars as targets. The Mg II UV lines seems to be the best interstellar absorption candidates. Several hundreths of A stars can be reached within 100 pc. First preliminary results (20 lines of sight) are presented, based on previous Copernicus and actual IUE observations.

The line of sight to βCMa has been probed by Copernicus observations. This particular line of sight is remarkable for the low mean densities. We find n̄H ∼ .002 cm-3. However we can distinguish two separate regions:

1) A local nearby HI region extends over a few parsecs from the sun with a density of the order of 0.1 cm-3 and a temperature of 11000 to 12500 K.

2) An HII region lies somewhere beyond the HI region and is spread over about 60pc. Its total hydrogen mean density is of the same order as the HI region, i.e. of ∼0.1 cm-3 and it contains only elements in low ionization state. All the data are coherent with the picture of a cloud in ionization equilibrium at T ∼ 23 000° K.

We underline the present situation of deuterium abundance evaluation in interstellar space and show that it should be < 10-5. Studiing in more detail the λ Sco line of sight and having observed two Nal interstellar components toward that star, we can show that the D/H evaluation made toward λ Sco is in fact related to the local interstellar medium (less than 10 pc from the sun). Because this evaluation is also < 10-5 it is in stricking contrast with the one made toward α Aur (D/H > 1.8 10-5 ) confirming the fact that the deuterium abundance in the local interstellar medium varies by at least a factor of two over few parsecs.

Discovery of interstellar Ge, Ga, and Kr is reported. Several intercombination lines of Fe II are detected. The depletion is most pronounced in Ca, Ti, and V. The highly ionized gas C IV and Si IV is not coextensive with the Si II and C II gas nor with the 0 VI gas. The molecular gas (CO) shows very small velocity dispersion (b~l km/sec).

The Voyager 1 and 2 ultraviolet spectrometers are sensitive over the wavelength range 500 to 1700 A. In the EUV, at wavelengths shortward of the Lyman limit (912 A), Voyager observations have detected emission from three out of a sample of 11 nearby hot DA white dwarfs. These observations imply very low HI column densities in the directions of the three stars detected. In the FUV, at wavelengths between 912 and 1200 A, Voyager observations of O and B stars can be used to study interstellar reddening at the shortest wavelengths and to provide useful estimates of interstellar H2 column densities.

Observations of diffuse, galactic Hα, [NII]λ6583, and [SII]λ6716 emission lines provide evidence for a warm (~ 104 K), primarily ionized component of the interstellar medium distributed throughout the galactic disk. This component of the interstellar gas has an electron density ≈ 0.1-0.2 cm-3 and occupies about 10-30% of the interstellar volume. Interstellar Hα emission near the galactic poles, the dispersion measure of a nearby pulsar, and observations of interstellar gas flowing into the solar system indicate that this ionized component is an important constituent of the interstellar medium in the solar neighborhood. The intensity of the Hα background at high galactic latitudes implies that this component is maintained by an average hydrogen ionization rate in the vicinity of the Sun of (2-4) x 106 s-1 per cm2 of galactic disk. The emission measure is 1.3-2.3 cm-6 pc toward the galactic poles. The sources of this ionization have not yet been identified but may include escaping Lyman continuum radiation from planetary nebulae, hot white dwarfs, and early type stars. Investigations of the regions surrounding ζ Oph (09V), the nearest (d ≃ 140 pc) 0 star, and a Vir (Bl IV), one of the nearest (d ≃ 87 pc) early B stars, have revealed areas of enhanced Hα emission extending 6°-12° from each star. However, it appears that these stars do not contribute significantly to the more diffuse ionization within the local interstellar medium.

A high-resolution spectroscopic investigation has been made of interstellar lines of sodium. From identifications of D1 and D2 line components concentrations of interstellar sodium gas have been studied. Some preliminary data are provided on the spatial distribution of stronger sodium concentrations.

The yellow D lines from neutral sodium are some of the most useful interstellar absorption features for tracing out low density optically thin interstellar material. Hobbs (1978) compared column densities of Nal as determined from the D lines with HI column densities derived from ultraviolet observations and found values of N(NaI)/N(H)<6.3x10-9 in 18 sight-lines with very low reddening, with the exact value of the ratio apparently a function of temperature. If this ratio limit holds for diffuse clouds in nearby space, we expect that observations of the Nal D2 lines at the 3 mA level can be used to trace out nearby clouds with column densities in the range of ≥2.5x1018 cm-2. Thus sensitive measurements of the Nal D lines are a useful way of observing nearby neutral cloud material. With this in mind, we have chosen high-resolution observations of the yellow sodium lines as a means of locating and mapping nearby low density interstellar clouds.

We observed the interstellar D-lines of Na I toward 49 stars in order to determine the distance (125 ± 25 pc) to a cold H I cloud. This sheet of gas may be part of the back side of the shell formed by the Loop I supernova.

Despite only a few percentages of the interstellar mass are contained in solid dust grains knowledge of its distribution is of importance per se, for dereddening purposes and because it probably correlates with the interstellar gas. General information on the bulk of the interstellar mass is thus obtainable from continuum absorption studies. Compared to diffuse emission data interstellar reddenings have the additional advantage by including an upper distance to the material observed.