The rotation velocity of molecular gas in the halos of M82 and NGC4631 decreases with the height from the galactic plane. The slower rotation of halo gas can be explained if the gas is supplied from the central region of the galaxies due to some ejection.

Optical photographs of highly-tilted, dust-rich nearby spiral galaxies like NGC253 have revealed numerous vertical dark filaments which we call vertical dust jets (VDJ). The VDJ exdend more than a few kpc from the disk in an almost coherent manner, while they are as thin as a few tens of pc. They are most likely due to boiling-steaming galactic disk, which ejects gas into the halo. The coherency suggests that VDJ trace large-scale poloidal magnetic lines of force.

Observational constraints on the galactic potential and modelling aspects are reviewed. The following conclusions come out. The mass distribution generating the galactic gravitational potential should include a bulge to account for the innermost features of the rotation curve, an extensive massive corona responsible for a flat rotation at large radius, and a disc able to produce a rotation curve as given by Rohlfs and Kreitschmann (1988) and a vertical force law Kz as from Bienaymé et al. (1987). We still know very little about the potential more than 1500 pc above the galactic plane.

The stability of the gas-field system in the Galaxy is examined to show that it is difficult to maintain a stable configuration due to numerous energy releasing phenomenon occurring in the Galaxy. However, it is argued that, as in the case of our atmosphere, hydrostatic equilibrium is expected in the Galaxy on a global scale. A summary of the advances made in the study of the global equilibrium is presented. The major postulates and predictions from some of these studies are compared with the observations. The usefulness of detailed observation of nearby edge-on galaxies in radio and X-ray regime is brought out.

Although locally a static, stable equilibrium seems consistent with observations of the interstellar medium (ISM), the extrapolation towards other galactic regions is not straightforward. Basically, the variation with galactic radius of the gravitational acceleration cannot be reconciled with the approximate constancy of the HI scaleheight. Moreover, halo gas located in the outer galaxy is prone to thermal instability and subsequent collapse towards the galactic plane.

The observational evidence for chimney models of the interstellar medium is reviewed. The variation of the state of the interstellar medium of an external galaxy between the three-phase mode, the chimney mode and the two-phase mode as a function of: galaxy type, galactocentric radius within a given galaxy, and the time varying star formation rate is discussed.

In the context of the chimney models it is shown how the photoionization of the halo can be achieved by utilizing both direct ionizing rays from the central OB association and diffuse reradiation from the chimney walls that will create an extended low ionization halo around chimneys with SII/Hα ~ 0.5.

A number of considerations for future studies that are a direct result of the work discussed at this conference are outlined.

The dynamics of galactic halos is discussed with special emphasis on the role of Cosmic Rays. Whereas Cosmic Rays should at best play a minor role for galactic fountain flows they appear essential in driving a wind from galaxies like ours. Active galaxies with a very hot interstellar medium (T ≥ 107K) have winds due to the thermal gas alone.

By estimating the X-ray intensities of old supernova remnants in the disk-halo interface, deductions can be made about the ability of isolated supernovae to have created a coronal halo.

The dynamical halo of the Galaxy offers a natural explanation for the form of the variation of cosmic-ray path length with energy. The variation above 1 GeV per nucleon can be understood as due to the variation of the diffusion coefficient, and hence the resident time in the galaxy, with energy. The flattening of the curve below 1 GeV per nucleon is seen to mark a transition to a convection dominated regime where the variation of the diffusion coefficient is no longer a determining factor. It is possible that the random motion of the cosmic rays about the galaxy that prevents us from seeing their sources in a clear manner may enable us to extract information about the galaxy at large and learn something about its large scale motions.

The solution of the steady-state transport equation describing the propagation of relativistic electrons perpendicular to the galactic plane in a galactic wind is discussed for extended source distributions. The wind velocity is assumed to be zero in the plane and to increase with galactic height. The electrons undergo simultaneously diffusion, convection, adiabatic deceleration, radiative losses and injection. We contrast the resulting spectra with those derived for an infinitely thin source distribution and discuss the dependence of the variation of the spectral index inside the source distribution on the form of this distribution in any transport model. Apart from a new region at very small energies the asymptotic spectra agree with those of the line-source model (LS). Inside the source distribution the spectral index break is doubled from δγlin =0.5 to δγext=1.0.

On the basis of our present knowledge about Cosmic Ray (CR) propagation, it is argued that galactic halos should mainly consist of two parts, namely a lower region, extending from the disk-halo interface to a few kpc, in which CR diffusion prevails and an upper region, where convection dominates. The upper part is the possible site of galactic wind formation due to the strong coupling of the CRs to the thermal plasma via (mainly outwards propagating) Alfvén waves as a mediator. In the lower halo and also in the disk, the gas will be effectively static, the wave field will be almost random in its direction (due to stochastic gas motions), and the CRs must diffuse through the gas to escape. We present a model that describes both the upper and the lower halo and briefly discuss simple analytic solutions for the diffusion and numerical results for the convection region.

The recent observations of the nonthermal properties of the halo of our Galaxy at radio and γ-ray wavelengths are summarized. Radio and γ-ray data show a similar spectral flattening with Galactic height towards the anticenter direction, which is interpreted as a cosmic-ray effect. Several theoretical explanations for the flattening of the energy spectra of the radiating cosmic-ray electrons (in the radio) and nucleons (in γ-rays) are reviewed including propagation of cosmic rays in an accelerating Galactic wind and the presence of cosmic-ray sources with flat energy spectra in the halo.

We have compared theoretical HI models to a large area (18°x12°) HI channel map centered at 1 = 205°, b = 0°. One set of models were calculated in which uncorrected clouds of HI populate space. Of the cloud models considered, which include “Spitzer”-type clouds and the McKee-Ostriker formulation, none were able to reproduce the data successfully. In another class of models, holes and holes with shells permeate an otherwise continuous HI medium. Although not entirely successful, these models contain characteristics that are similar to the data. In order to reproduce the amplitude of the HI variations on angular scales of >1°, the volume occupied by ionized gas must be less than 30%, in sharp conflict with the models that suggest that the volume of the ISM is dominated by hot gas.

The presence of dust grains at high galactic latitude as well as in the Halo of external galaxies has received substantial observative support in the last few years. Besides intense hydrodynamics stirring phenomena, like supernovae expanding shells, stellar winds and Galactic fountains, the removal of dust from the Disk can be ascribed to the global galactic radiation field. The continuous sputtering in the hot Halo gas may explain the large scale height found for refractory elements (Edgar and Savage, 1989).

The recent progress of numerical studies on outflow phenomena from the galactic disk to the halo is summarized. Firstly, a galactic-scale outflow is considered. If the high-velocity cloud is formed from the radiatively cooled gas, which was originally ejected from the disk as a hot gas, the temperature and density at the base of the halo should be ~ 106 K and 10–3 cm–3. Next, recent results of numerical simulations of the evolution of superbubbles, through which hot gas flows out to the halo, are reviewed. In the case of a thin disk whose density scale height is H ≃ 100 pc, the shell begins to be accelerated upwardly after several dynamical time scales. After that, the polar cap of the shell is broken and the hot gas flows away into the halo. In the case of a thick disk (H ≃ 500 pc) or a magnetized disk with a magnetic field parallel to the disk (B ≃ 5μG), the shell is not accelerated and never shows blow-out.

Observations of galactic halo gas are consistent with an interpretation in terms of the galactic fountain model in which supernova heated gas in the galactic disk escapes into the halo, radiatively cools and forms clouds which fall back to the disk. The results of a new study of several large-scale gas dynamical effects which are expected to occur in such a model for the origin and evolution of galactic halo gas will be summarized, including the following: (1) nonequilibrium absorption line and emission spectrum diagnostics for radiatively cooling halo gas in our own galaxy, as well the implications of such absorption line diagnostics for the origin of quasar absorption lines in galactic halo clouds of high redshift galaxies; (2) numerical MHD simulations and analytical analysis of large-scale explosions and superbubbles in the galactic disk and halo; (3) numerical MHD simulations of halo cloud formation by thermal instability, with and without magnetic field; and (4) the effect of the galactic fountain on the galactic dynamo.

Two-dimensional MHD simulations are performed to study the nonlinear evolution of the Parker instability in galactic gas disks. When the most unstable mode grows, magnetic field lines kink across the equatorial plane of the disk and thin spur-like structures are formed above dense regions in magnetic pockets. In low β (= ρgas/ρmag < 3) disks, shock waves are produced at the footpoint of magnetic loops, while in high β (> 3) disks, nonlinear oscillations are excited and the loop length increases with time up to λc ≃ (3.5β + 6)H, where H is the half-thickness of the disk.

An impressively large amount of data was presented at this meeting and a review by an ordinary human cannot hope to do full justice to either the authors or the research. Accordingly, I shall concentrate on those aspects that particularly piqued my interest and apologize to those authors whom I overlook. I shall give references in the usual style except for references to papers presented at this meeting, for which I shall simply mention the author(s) with no date.