Until early in 1968 radio sources were believed to be relatively constant and slowly evolving objects. Sporadic intensity variations had been found at high frequencies in some quasars but, except for the Sun and some flare stars which exhibited occasional bursts of radiation, the time scale of changing emission appeared to be very long.

Until comparatively recently solar activity was discussed in terms of independent centres around the Sun’s surface. These centres, which form a constantly but slowly changing pattern, were thought to be linked only beneath the photosphere by tangled magnetic fields, themselves dominated by the mechanical energy of mass motions at photospheric and lower levels. Recent discoveries now indicate that active centres interact with one another above photospheric level and especially through linkages within the corona. The linkages appear to be of at least two distinctive kinds: magneto-hydrodynamic shock waves capable of travelling vast distances through the corona at ∼103 km/s and triggering eruptions at distant centres; and magnetic field lines which loop high and wide in the corona and convey energy in the form of fast particles. These two kinds of interaction will be considered in turn.

The term white dwarf was originally introduced, some 50 years ago, to describe intrinsically faint A-type stars. Spectroscopic studies and theoretical considerations confirm the suggestion that these are degenerate objects. Photographic colorimetry of faint proper motion stars can readily distinguish the blue objects and in this way several hundred white dwarf candidates have been discovered, mainly by Luyten. Accurate photometry of about 1000 of these stars has led to accurate estimates for the space density of 1.5 = 10−3 pc−3. However, subluminous late-type stars are more difficult to detect among the multitude of main sequence stars and some additional criteria are needed.

A class 2 flare began on 1968 June 9 in the active region near 17°N 40°E, at about 00h15m U.T., maximum 00h37m, terminating about 02h30m. The event was recorded at Culgoora with the 5-inch flare-patrol and 12-inch chromo-spheric telescopes at solar diameters and frame intervals of 16 mm/10 s and 17 cm/3 s respectively. Both telescopes used Halle filters, pass bands about 0.5Å centred on Hα, and Duplopan 35 mm film. The seeing was mediocre, with occasional fairly good frames.

In 1961 Fokker and Roosen suggested that the concept of homology as applied to solar optical flares by Ellison, McKenna and Reid could be extended to flare-associated radio events as well, since successive flares within the same centre of activity sometimes produce radio bursts which look remarkably similar on single-frequency records. Teresa Fortini reported very similar ionospheric responses due to X-radiation from flares recurring in the same active region. In 1950 Wild noted similarities in the dynamic spectra of type III bursts occurring within periods of minutes.

Since the start of high-resolution observations with the radioheliograph at the Culgoora Solar Observatory it has been noted that some bursts which occur from separate active regions could be physically correlated. It was suggested that the time delay between the correlated bursts is of the order of seconds, and that fast electrons provide the trigger. Wild found that shortly before a large flare weak bursts, stronelv polarized in the same sense, appeared simultaneously in different sources after a quiet period and he discussed the phenomenon in the light of flare mechanisms.

With a number of important exceptions associated with transitory flare activity, most persistent sources of solar radio emission observed to date with the 80 MHz Culgoora radioheliograph have been stable in position over periods of tens of minutes or hours even though they fluctuated greatly in intensity. During several hours on 1968 November 11, however, we observed some interesting activity which showed frequent changes in the position of a source.

Since they were first interpreted, moving type IV bursts have been attributed to synchrotron radiation from electrons with energy ∼3 MeV radiating in weak magnetic fields (∼1 G) high in the solar corona. In this paper a description is given of 80 MHz radioheliograph observations of an outburst in which it was possible to isolate the moving type IV source and demonstrate that its circular polarization was strong (∼80%). Hence it is shown that the energy of the radiating electrons cannot exceed 6 = 105eV.

On 1968 October 30 a large solar flare (importance 3B to 4B) began at approximately 23h42m U.T. We present here a preliminary account of the associated radio emission.

The heights of solar radio sources at 1424 MHz and 696 MHz have been measured during the years 1965 and 1966. Solar activity at this time was near minimum. The number of radio sources appearing on the solar disk rarely exceeded three at any time and it thus was possible to resolve the majority of these with a high resolution grating interferometer. Many of the previous height measurements at these frequencies have been made near times of maximum solar activity and the confusion of sources within the beam has limited their accuracy. The number of sources studied here is quite considerably higher than in any previous investigation at these frequencies, and the period of observation has been continuous.

Recently a programme of observing 21 cm-λ solar radio bursts with high angular resolution has been commenced at the Fleurs Radio Field Station of the School of Electrical Engineering, University of Sydney. In some preliminary measurements fringes from two 2-element E-W interferometers, spaced at roughly 3800 and 1900 wavelengths, have been recorded simultaneously.

Recently I have carried out a two-dimensional radiative-transfer analysis of the data given by Edmonds on centrelimb relative rms intensity fluctuations in terms of horizontal temperature fluctuations in the solar photosphere. Edmonds himself attempted an analysis of his data and found a ΔTrms which increased monotonically from about 300°K at unit mean optical depth (i.e. at a given physical depth) to 500°K at optical depth 3.

In July 1967 and June 1968 observations of the central intensity variation in several chromospheric lines were obtained by P. R. Wilson and myself at Sacramento Peak Observatory, New Mexico using the 16-inch coronagraph coupled with the high-dispersion Littrow Spectrograph. Observations in the MgB lines (λ5184, 5173, 5167 Å) were obtained the first summer whilst the NaD lines (λ5889, 5896 Å) were studied in the second. Table I shows the mean heights in the solar atmosphere at which it is thought these lines are formed.

Parker realized that the solar corona would expand into a supersonic wind, whose flow could be described by hydrodynamics. Subsequently a number of such steady state models have been developed which include the energy transport equation. These models assume that the heating of the gas is confined to a thin shell at the base of the corona, but neglect the effects of solar rotation and the general magnetic field, which is carried into space by the solar wind.

Following the detection of a high flux density from Saturn at 21.2 cm by Davies and Williams and the confirmation by Kellermann, it has been known that the radio emission from Saturn is enhanced at long wavelengths. The Molonglo cross telescope has searched for this emission at 73.5 cm on two occasions, the first in 1967 November when the planet was stationary, and the second during the opposition of 1968 October. Between these dates, installation of preamplifiers on the EW arm doubled the sensitivity of the cross so that although the same procedures were used on both occasions, this paper is based almost entirely on the recent results.

The possibility of the occultation by Neptune of a star of similar apparent magnitude to itself was first pointed out by the amateur astronomer Ross A. Shuart. It was clear that such an event would provide data on Neptune’s position which would be of value to the observatories which produce the planetary ephemerides. The existing ephemeris of Neptune is known to contain small errors which have been accumulating over many years.

Photo-electric observations of the occultation of BD —17°4388 by Neptune on 1968 April 7 were made with instruments and techniques as listed in Table I. A preliminary report of these observations has been given in IAU Circular No. 2067.

The occultation of a star by a planet of similar brightness allows accurate determination of the planet’s radius R, mean density ρ and of the ratio T/μ, where T and μ are the planetary atmospheric temperature and mean molecular weight. These quantities are important for the formation theory of the planet and of its atmosphere (see e.g. Öpik).

The Narrabri Stellar Interferometer has been used to observe the binary stars α Virginis (April 1966) and γ2 Velorum (January 1968).

Theoretical Hα profiles have been used by Rodgers and Bell (hereafter denoted by RB) to obtain the effective temperatures of several cepheid variable stars. They find that the profiles are sufficiently sensitive to temperature to enable them to determine temperatures which are accurate to within about 300°, while being insensitive to other factors such as gravity or the form of the temperature distribution in the model atmosphere.