Observational astronomy extends, in terms of energy in the electromagnetic spectrum, from below 10-8eV to above 108eV. Studies of cosmic rays extend this range to the neighbourhood of 1019eV (about a Joule) and the aspects of high energy astrophysics discussed in the present paper are those concerned with cosmic rays of energy upwards of about 1015eV.

In the past decade, planetary nebulae have assumed considerable importance in elucidating our understanding of the final stages of stellar evolution at low mass. This began with the work of Shklovsky, O’Dell and Seaton, who showed not only that the nuclei of these nebulae were among the hottest stellar objects, but also that they evolved on a track in the Hertzsprung-Russell diagram (the Harman-Seaton sequence) on a time scale very rapid by stellar evolutionary standards (~ 20,000 years).

Geomagnetic storm sudden commencements (ssc) and sudden increases (si) can be caused by either a propagating discontinuity (i.e. a hydromagnetic shock or blast wave) in the interplanetary plasma or by the convection past the Earth of a discontinuity in the ambient solar plasma flow.

It is now well established that observations of interplanetary scintillation from a single station can yield useful information on the angular structure of radio sources, and on the propagation of large-scale disturbances through the interplanetary medium. However, when it is required to obtain detailed information on the plasma properties single station observations suffer from the disadvantage that, in general, it is not possible to measure the velocity and scale of the irregularities independently. For example, a change in the width of the observed temporal power spectrum of the scintillations might be caused by either a change in velocity, or a change in scale, or a combination of both.

A description was given in a previous paper of the first observations of the two-dimensional image of the Crab Nebula as it became broadened by the solar corona in June 1969. In this paper we describe further observations at 80 MHz during 1970 and 1971, again using the CSIRO Radioheliograph at Culgoora, N.S.W., and we discuss the derived values for radial and tangential broadening in relation to previous work at various phases of the solar cycle. Other methods of observing angular broadening have generally employed two or three interferometers at different position angles, and only the simplest model for the image could be assumed in interpreting the results. The radioheliograph, however, has the advantage of recording the complete two-dimensional image and also, simultaneously, the surrounding background.

In the preceding paper, observations of the coronal broadening of the Crab Nebula during 1969-71 were described. The basic parameters, radial and tangential broadening, and their relation to previous work were discussed. Whereas previous observations have utilized two or three interferometers only, so that the degree of broadening along any particular axis could only be obtained under the assumption of a particular form (e.g. Gaussian) for the angular power spectrum, the present work has enabled complete sampling of the two-dimensional brightness distribution of the broadened source. It is therefore possible, and of considerable interest, to compare the observed distributions to those computed on the basis of various theoretical models for the scattering process.

The structure of small diameter radio sources can be investigated by studying the scintillation of the source due to the interplanetary medium when the line of sight approaches the Sun. Observations of radio source scintillation are currently being undertaken with the separate arms of the 1 mile radio telescope at the Molonglo Observatory. The EW arm allows successive transit observations with three fan beams, 1′.4 EW by 4°.2 NS at 408 MHz, bandwidth 2.5 MHz. Sources transit the half-power points of each beam in 6 sec δ seconds of time (δ is the source declination). The NS arm gives eleven fan beams at neighbouring declination, 1′.5 sec Z NS by 4° EW (Z is the zenith angle). Complete transit of a NS beam takes 15 sec δ minutes.

A combined multi-station radar equipment of c.w. and pulse at 27 MHz was used at Adelaide (lat. 35°S) during the period December 1968 to June 1969 and again in October 1969 to measure radiants and velocities of individual meteors down to limiting radio magnitude MR = + 8. Reflections from 3 to 5 points typically separated by 2 to 6 km along each trail were used and corrections for the effects of meteor region winds and wind shears were included. To date 1667 orbits have been determined from 6 months of data.

Extensive air showers radiate because they are essentially sheets of charge moving through a medium at relativistic velocities. Of interest in the present experiment are their emissions in the optical and radio regions of the spectrum—the first long accepted as primarily of a Cerenkov origin, and the second very far from fully understood.

In a previous paper it was shown how one could improve upon the Böhm-Vitense model of the solar convection zone by the inclusion of four different length scales and by the determination of these length scales with the use of the quasi-Vitense model as developed by Unno. In this way the vertical wave number kz, associated with a characteristic eddy, can be determined by the integration of a second order differential equation. The integrations have to be started at a suitable depth and all model calculations depend critically on the assumed structure of the top layer.

An investigation into the influence of rotation on thermal convection has some applicability in the study of the solar convection zone. Of particular interest is the effect of rotation on the total heat transport and the cell size for maximum heat transport at high Rayleigh number, which is estimated to be as high as 1020 for the Sun.

It is well known that a magnetic field has an inhibiting effect on thermal convection. It is also likely that a magnetic field might have a marked effect on the preferred cell-size and it is therefore of some importance to investigate the effect of such a field on finite amplitude convection. We restrict ourselves to fluids in which the Boussinesq approximation holds. For astrophysical application, especially in a study of the convective layer of the Sun, one should consider the compressible case but unfortunately the full compressible equations are much more difficult to handle numerically. In any case, the integration of the equations for the incompressible case will yield at least an indication of the effect of a magnetic field.

The region of the solar atmosphere where the brightness temperature is less than 9 x 103 K. defines the photosphere and low chromosphere. In physical scale this covers the range from 300 km. below to 2 x 103 km. above the visible surface.

In the first four years of operation with the 80 MHz heliograph at Culgoora, 24 events classified as moving type IV were observed. Their observed characteristics can be interpreted in terms of gyro-synchrotron radiation from mildly relativistic electrons. Many of these events have been presented at previous ASA meetings and 12 have been discussed in the literature in detail.

Scattering of radiation from solar radio sources by in homogeneities in the electron density structure of the corona can have marked effects on the observed time profile and the brightness distribution. These effects are appreciable for any localized source emitting a brief burst of radiation and are most pronounced when the radiation propagates close to the plasma level; it is such sources, illustrated here by those of type III bursts, that will be considered in this paper.

Observations of the relative positions of fundamental and second harmonic bursts can be used to study refraction and scattering effects in the corona. Smerd, Wild and Sheridan have pointed out that the observed positions of type II bursts can be interpreted as favouring the backward emission of the second harmonic in a smooth corona. In this case the harmonic is seen after reflection from near the fundamental plasma level. Several other examples of type II and type III bursts have been reported which show this effect. We wish to present further evidence from a recent study of a large number of type III bursts observed with the 80 MHz heliograph and 158 MHz interferometer.

Although cosmic rays detected in interplanetary space have often been correlated with visible flares at the Sun, little is known about the transport of these particles through the corona. Lin demonstrated a good correlation between ≳20 keV electron events detected by spacecraft near the Earth and type III radio bursts at the Sun. In a detailed investigation of many of these electron events from one particular active region source, Lin proposed that the injection of electrons was characterized by a source region in the corona which extended over ~70° in longitude, such that in this region the electrons had direct access to an ‘open cone’ of propagation in interplanetary space. When the spacecraft was situated outside this open cone (by up to 15°), impulsive electron events were still recorded, but these were now modified by diffusion through the corona of the electrons from the 70° source region.

On 1971 January 24 a 3B flare at 18° N., 49° W. was associated with the acceleration of protons to relativistic energies; it was one of the rare events recorded by ground-level neutron monitors. Excellent radio coverage was obtained with single-frequency radiometers in the range 1000-9400 MHz, and at Culgoora with the 8-8000 MHz spectrograph and the 80 MHz radioheliograph. At the Earth relativistic protons and electrons arrived very promptly from the flare, whose site was near the foot of the nominal interplanetary field line which connects to the Earth.

In recent years there have been numerous investigations of the helium shell-burning evolution of low-mass stars, and it was in such studies that Schwarzschild and Härm and Weigert independently discovered the thermal instability phenomenon. In the case of stars with hydrogen-rich envelopes, its reality has been amply confirmed. On the other hand, studies have also been made of the shell-burning in pure helium stars (many for comparison with the nuclei of planetary nebulae), and here the situation is far less clear. Some investigators have found the instability, while others have not. Paczyński has drawn attention to the fact that in all cases where thermal pulses have been reported for pure helium stars, the helium shell-source was treated as an abundance discontinuity, while in all cases where a detailed abundance profile was used, there was no evidence of pulses. He suggests therefore that the shells in pure helium stars are stable. We wish to report a calculation for a 0.8 ɱ⊙ pure helium star, with a detailed shell abundance profile, in which a single thermal pulse was encountered at the end of the shell-burning evolution.

Recently E. M. Burbidge and P. A. Strittmatter1 have found that the DB-like star G61-29 has broad He emission lines suggesting a rapid rotation. The purpose of this paper is to suggest a further white dwarf candidate for rapid rotation.