The numerous discussions that took place at the colloquium have reemphasized the primary importance of polarization observations and their interpretation for understanding the magnetospheric structure and the radiation mechanisms of pulsars. I have tried to take them into account in the summary at the end of this written version of my review of polarization phenomena, where I also attempt to address some of the questions raised by both observers and theorists as to what message polarization has for the planning of future observations and for the development of theories to explain pulsar radiation mechanisms. Because I feel it is relevant I shall begin with a little history to put things in perspective.

We report measurement of pulse profiles at 34.5 MHz of 8 pulsars using the Decameter-wave Radio Telescope at Gauribidanur, India. The dispersion measures of these pulsars range from 3 to 35 pc cm–3. None of the pulsars show any significant interpulse emission at this frequency which conflicts with earlier claims from 25-MHz observations (Bruck and Ustimenko 1976, Bruck and Ustimenko 1977a, Bruck and Ustimenko 1979). The intrinsic pulse width of PSR 1919+21 at 34.5 MHz appears to be smaller than at higher frequencies. These observations were made during the period 1984-86 and were reported by Deshpande (1987).

The true nature of the association between pulsars and supernova remnants has remained an intriguing and poorly understood problem even after all these years of research on them. We attempt in this review to marshal all the evidence one has on this question, and to see what conclusions we can draw.

Dr. Radhakrishnan outlined a geometrical theory of a rotating pattern of field lines, which would account for the pattern of polarisation observed in the integrated radio pulses from pulsars.

Several attempts were made to detect the possible radio recombination lines of positronium near the galactic center. An absorption feature seen at λ6cm, in the D-configuration of the VLA was not confirmed by subsequent observations at λ6cm and λ20cm using the B and C configurations of the VLA. An observation at λ3mm using the IRAM 30m telescope also did not detect any line. On the basis of one recombination line photon for every positron (McClintock 1984), our non-detections imply an upper limit to the positron production rate of < 3.1 × 1043 s−1, within about 2″ of the galactic center.

It is now recognized that diffuse matter in space plays a decisive role in the evolution of our Galaxy and of similar galaxies. Primordial gas—together with gas ejected in planetary nebulae, stellar winds, novae, supernovae, and other types of stars—has accumulated to form a complex medium containing regions with densities ranging from 10–3 to 106 particles cm-3 and with temperatures ranging from 10 K to 106 K. From time to time, part of the interstellar medium collapses to form stars. In order to understand the evolution of the Galaxy, it is essential to understand how energy, mass, trace elements, and dust grains are deposited into the interstellar medium by stars. It is also essential to understand the mechanisms that initiate star formation in certain regions, and how the ensuing collapse develops in space and time.

An extensive survey of HI absorption in the spectra of discrete radio sources has been in progress for the past three years at the Parkes radio observatory. One part of the survey comprising sources along the galactic plane north of declination —50° has recently been completed. In this communication we discuss some conclusions concerning the distances of 10 sources drawn from a preliminary analysis of the data.

The lack of any direct measurements on the spin temperature of the neutral hydrogen in the Galaxy has led to considerable controversy in the past. Estimates of the temperature have depended strongly on whether they are based on emission or absorption studies. The widely accepted value of 125°K based on emission studies dates back to Schmidt. He adopted this figure on the premise that the maximum observed brightness temperatures in the galactic plane were in directions of high optical depth. The brightness temperature was then equated with the spin temperature on the assumption that the temperature did not fluctuate very much in a large region around the Sun.

Towards the end of February 1968 the astronomical world was staggered by a paper from the Milliard Radio Observatory at Cambridge announcing the discovery of an astonishing periodic phenomenon. The characteristics of the pulsating radio source—or pulsar as it came to be called—involved a fantastic multiplicity of time-scales. The duration of the individual events was measured in tens of milliseconds, the repetition rate was of the order of a second, the pulse amplitude showed drastic variations over times of seconds, minutes, hours and even months and, lastly, the stability of the basic periodicity indicated a time-scale of millions of years. A series of pulses from CP 1919, the first pulsar, is shown in Figure 1, and one notices here both the regularity of the pulses and the variation in their amplitude with time. When the individual pulses were observed on an expanded time-scale it was found that the pulses were made up of sub-pulses (Figure 2) and that there was considerable structure even down to a millisecond time-scale.

It has become more evident during the last three years that the study of interstellar matter is paramount to understand the evolution of the universe and its constituents. From observations of the present state of the interstellar medium, in our galaxy, in other galaxies, and between galaxies, it is possible to test theories of: evolution of the universe, formation and evolution of galaxies, formation and evolution of stars and of the evolution of the interstellar medium itself. The amount of information on the interstellar medium that has been gathered during the 1982-1984 period has been very large and the theoretical models that have been ellaborated to explain these observations have been very numerous, these facts show that the subject of our Commission constitutes a very active field of astronomical research.

The high intensity anomalous OH emission which has been detected in the neighbourhood of HII regions exhibits remarkable polarization characteristics, narrow spectral features, and unusual ratios of line strengths. All of these unusual properties are generally believed to be attributable to maser action of some form. A knowledge of the structure of the emitting regions and their brightness temperature is necessary for the development of a satisfactory theory of the emission mechanism.

The subject of interstellar matter continues to be one of the most active fields of present day astronomical research. The advent of new instruments operating in the different parts of the electromagnetic spectrum has resulted in a phenomenal increase both in the amount of observational material, and in the theoretical work attempting to interpret or model the observations. This has made it increasingly difficult to cite all of the published work in the field, and to have room to report even in brief on the conclusions of these studies.

We discuss the distribution of source- and receiver- noise in radio synthesis images and show that the source-noise is maximum at the position of the source but also appears in the off-source region because of the sidelobes. Analytical expressions are derived for the rms noise at any location of both “total-power” and “correlation” images. We show that under certain conditions, deconvolution can remove the source noise from the off-source region in snap-shot images. Some of the results are verified experimentally.

The subject I have chosen for my talk today will come of age on the last day of this General Assembly. The first detection of the remarkable objects that we call pulsars, was on 28th November 1967 when Jocelyn Bell (now Bell-Burnell) who was working with Prof. Hewish at Cambridge discovered a new class of radio sources that put out pulses of radio radiation about once a second but with clock-like regularity. Two other major discoveries in the same decade using the radio spectrum were, of course, Quasars and the Cosmic microwave background. Those two took us to extremes in time and distance, and the amount of energy radiated, whereas the discovery of pulsars, situated near at hand by comparison, led us to extremes in the physical state of matter.

The 21-cm evidence for an intercloud medium is reviewed. The observations include the sky distribution and velocity structure of 21-cm emission profiles, self-absorption features in emission profiles, and absorption profiles in the directions of discrete sources. It is concluded that the intercloud medium in the solar neighbourhood has a temperature between 103 and 104 K, a column density of ~1.4 × 1020 cosec |b| atoms cm-2, and a brightness temperature of ~4.4 cosec |b| K wherever the line of sight does not intersect optically thick cold concentrations.

We use the recently introduced concept of a “window” of magnetic field strengths in which pulsars can be active to explain the variation in morphology of supernova remnants. Neutron stars created with field strengths of a value permitting pulsar activity result in particle production and Crab-like centrally concentrated remnants. Other field values lead to strong magnetic dipole radiation and consequent shell formation, e.g., Cas A.

I am afraid the best I can do on the next few pages is to convey the personal impressions that I have been left with after listening to sixty odd talks on almost every topic connected with pulsars. It is obviously impossible even to mention, leave alone comment on, each one of the papers that have been presented in these four packed days. Those few I touch upon will naturally reflect my personal biases for which I ask that you please bear with me.

An explanation is offered for the complexity and stability of integrated pulse profiles. We propose that the pulse structure arises from surface irregularities at the polar caps, and we indicate how the surface relief affects the number of charges which are released into the magnetosphere and create the observed radio radiation. The electrons produced in the vacuum break-down in the polar gap carry enough energy to continually modify the surface relief of the polar caps, and we suggest one way in which the surface could be modified in a timescale of a million years.

Triggering of the spark discharges in the Ruderman and Sutherland model by background gamma rays is shown to be effective only within a narrow range of neutron star magnetic fields centred on 2.5 × 1012 gauss. This calculated range of field strengths is in good agreement with ‘observed’ values, suggesting that (a) such triggering is operative, and (b) that neutron stars with much stronger fields do not function as pulsars.

In the last decades many techniques have been proposed to manufacture thin (<50µm) silicon solar cells. The main issues in manufacturing thin solar cells are the unavailability of a reliable method to produce thin silicon foils with contained material losses (kerf-losses) and the difficulties in handling and processing such fragile foils. A way to solve both issues is to grow an epitaxial foil on top of a weak sintered porous silicon layer. The porous silicon layer is formed by electrochemical etching on a thick silicon substrate and then annealed to close the top surface. This surface is employed as seed layer for the epitaxial growth of a silicon layer which can be partially processed while attached on the substrate that provides mechanical support. Afterward, the foil can be bonded on glass, detached and further processed at module level. The efficiency of the final solar cell will depend on the quality of the epitaxial layer which, in turn, depends on the seed layer smoothness.

Several parameters can be adjusted to change the morphology and, hence, the properties of the porous layer, both in the porous silicon formation and the succeeding thermal treatment. This work focuses on the effect of the parameters that control the porous silicon formation on the structure of the porous silicon layer after annealing and, more specifically, on the roughness of the top surface. The reported analysis shows how the roughness of the seed layer can be reduced to improve the quality of the epitaxial growth.