The resistive tearing mode instability is a mechanism that in some cases will render unstable a magnetohydrodynamic equilibrium of a plasma that is ideally stable, i.e. stable if no dissipative oiesses are taken into account. There is much experimental evidence that this instability is the cause of the current disruptions observed in laboratory plasma devices (von Goeler et al. 1974). In the astrophysical context, the instability has been invoked in connection with the solar flare energy release mechanism (Coppi and Friedland 1971) and the problem of the disconnection of the protostar matter from the interstellar magnetic field during star formation (Mestel 1966). In the latter problem the tearing instability gives rise to a much smaller timescale for magnetic reconnection than does ordinary resistive diffusion.

The basic model for the precipitation of trapped energetic particles from a magnetic flux tube is Kennel and Petschek’s (1966) model. Their model is symmetric, implying equal precipitation rates at the two feet of the flux tube. We have developed a model for precipitation in an asymmetric flux tube (Melrose and White 1979). Here we explore some of the consequences for the precipitation model of Melrose and Brown (1976) for solar hard X-ray bursts. In Melrose and Brown’s model roughly half the X-rays arise from precipitating electrons. With present instruments it is not possible to resolve the two feet of the flux tube. However, if the feet can be resolved, either directly by future X-ray telescopes, or indirectly through secondary optical, UV or radio observations, then, as we shall show, the additional information obtained could be used to derive information on processes in the magnetic trap.

The Culgoora radioheliograph was designed in the early 1960s and commissioned in 1967. Since then there have been dramatic increases in the speed and versatility of digital integrated-circuit devices, and also a marked fall in their cost. It is now possible to replace the original signal processing electronics with equipment, based on modern digital technology, which will significantly enhance the performance of this radio telescope for solar and cosmic radio observations at metre wavelengths.

In this paper we review the evidence on the structure of the open magnetic field lines that emerge from solar active regions into interplanetary space. The evidence comes mainly from the measured sizes, positions and polarization of Type III and Type V bursts, and from electron streams observed from space. We find that the observations are best interpreted in terms of a strongly-diverging field topology, with the open field lines filling a cone of angle ~60°.

Reverse drift pairs were first described by Roberts (1958) and later by Ellis (1969) and de la Nòe and Moller-Pedersen (1971). Figure 1 shows a 10-min section of the spectrograph record and the corresponding section of the polarimeter record. The figure contains several reverse drift pairs and illustrates the main features of RDPs, namely: (1) frequency drift rate of about 4 MHz s-1 (i.e. a factor of ~50 faster than Type lis and a factor of ~2 slower than type Ills); (2) drift to higher frequencies (i.e. opposite to that of Type II and III bursts, hence the name ‘reverse’); and (3) an apparent ‘echo’ of the burst some 1 to 2 s later to form a pair. On 1979 February 17/18 a noise storm consisting of Type 1 bursts, Type III bursts and an underlying continuum was observed with the Culgoora spectropolarimeter, spectrograph, and radioheliograph. During this noise storm, the spectrograph record showed several hundred RDP bursts, and about fifty FDPs. Here we report new results on the polarization-of drift pair bursts, present further data on RDP positional and frequency characteristics, and then comment on existing theories concerning RDPs.

Most if not all metre-wave Type III bursts appear to be associated with solar active regions (e.g. Duncan 1977). There is a well-known association of Type III bursts and flares within a few minutes of the onset of the brightenings seen in Hα (Wild et al. 1954). Early descriptions of these observations were given by Malville (1961) (or see Kundu (1965) for review). More recently Jackson et al. (1978) have reported the occurrence of a broad maximum of Type III bursts 10 to 5 h prior to mass ejection transients seen by the white light coronagraph on Skylab.

We compare observations of a large coronal streamer made with the 80 MHz Culgoora radioheliograph, the Kcoronameter on Mauna Loa and the white light coronagraph on Skylab. The various measurements are not in agreement, and this raises fundamental questions about the nature of the corona and wave propagation through a streamer structure. The comparisons also indicate how precise data from the heliograph may contribute to a better understanding of these processes.

The past and present existence of smaller bodies in our solar system manifests itself in various ways through the appearance of asteroids, minor moons, planetary rings, surface cratering, etc. The occurrence of these planetesimals appears to be an integral intermediary step in the formation of the larger bodies from the initial grain state. In order to account for the discreteness and order of the planetary orbits and regular satellite systems, Prentice (1977, 1978) has suggested that the accumulation of this planetesimal material took place in a series of well defined orbits corresponding to the positions of a system of gaseous Laplacian rings that were shed by the primitive contracting parent cloud. Alfvén and Arrhenius (1975) have also drawn attention to the fact that planetesimal formation occurs far more effectively if the accumulation takes place along well defined ‘jet streams’. Hourigan (1977) has previously demonstrated how such concentrated rock/ice streams or ‘jet-streams’ naturally form on the mean Keplerian orbit of gaseous rings that are present in the Prentice-Laplace model.

Crannell et al. (1978) have reported an observed correlation between the time profiles and flux densities of impulsive hard X-ray and microwave solar bursts. We report here on a significant correlation between the flux density of extended bursts of hard X-rays and micowaves. These extended events follow after impulsive bursts and last much longer (see e.g. Fig. 1, Frost and Dennis 1971). However, as extended bursts only occur during very large flares the number of cases available for study is small. The significance of our observations follows from the suggestion of Wild et al. (1963) that the extended bursts are evidence for a second-phase acceleration process in the corona. We show that the observed characteristics of these extended microwave bursts (viz. a rather flat spectrum below a turnover frequency which is independent of intensity) can be explained by gyro-synchrotron radiation from the same population of energetic (E ≈ 100 keV) electrons as those emitting (thin-target) X-ray bremsstrahlung. A detailed source model is discussed in a companion paper (Nelson and Stewart 1979 — Paper B).

In a previous paper of this issue (Stewart and Nelson 1979 — Paper A) we showed that for extended bursts a good correlation exists between the observed 100 keV X-ray slux density and the 3.75 or 9.4 GHz microwave flux density. We also showed that the microwave spectrum of these bursts was much flatter (S ∝ f1.0 on the average) than the optically thick (self-absorbed) spectrum observed for impulsive microwave bursts (Crannell et al. 1978; Dulk et al. 1978). Furthermore, the microwave turnover frequency was > 10 GHz in eight of the nine events studied and <20 GHz in four of these cases. The remaining event, which was severely occulted by the solar limb, had a turnover frequency of ∽ 1 GHz.

In a technical sense radio astronomers do not use the radio spectrum; they are listeners only, and when the science began they had to listen in the quiet spaces between transmitting services. Radio astronomy was first recognized as a service by the International Telecommunications Union (ITU) in 1959, and the first frequency allocations were then made for it as a ‘passive’ service. In 1963 and 1971 further allocations and frequency protection were obtained at specialized World Administrative Radio Conferences (WARC). The allocations were required for two purposes: (i) to protect the frequencies of the most important spectral lines of atoms and molecules, and (ii) to provide a series of bands for continuum observations. The general protection so far provided has been vital to the growth of the science, and its continuation is of the highest priority to the future life and development of radio astronomy.

Radio position measurements with an error of <2” arc rms allow reliable optical identifications of compact radio sources to be made solely on the basis of radio-optical position coincidence. In this way neutral or red stellar objects, faint compact galaxies and faint QSOs can be reliably identified. Such identifications are of particular interest because they are rich in BL Lac objects, high-redshift QSOs, QSOs with unusual optical emission or absorption spectra and galaxies with active nuclei (see Jauncey et al. 1978).

About 1 km to the east of the township of Windsor, NSW and only 50 ft above mean tide level, stands the remains of one of the first observatories in Australia. Here John Tebbutt established his private observatory and for over 60 years continued a remarkable career as a gentleman astronomer in the best tradition of 19th century Europe.