The radio installations at Culgoora Observatory evolved from the work carried out at Dapto field station between 1952 and 1965—which in turn was based on earlier observations. The basic instrument at Dapto was a radiospectrograph which produced two solar spectra per second over a frequency range originally of 40-210 MHz and finally of 5-2000 MHz. Until 1957 the Dapto radio spectrograph was the only one operating in the world and it fell upon this instrument to reveal many of the spectral phenomena which are now well known. The spectrograph observations referred to the total flux from the Sun observations with high directivity began at Dapto in 1958 with the introduction of a swept-frequency interferometer which measured the one-dimensional (east-west) positions of bursts and their approximate angular size over a continuous range of frequencies between 40 and 70 MHz. The results obtained from this combination of spectrograph and interferometer indicated that great advances would be made in our knowledge and understanding of the phenomena if two-dimensional metre-wavelength pictures of the Sun could somehow be recorded at short time intervals of about Is—again in combination with spectrographic observations. This requirement led to the start of the radioheliograph project. One requirement for this instrument was a site with linear dimensions of the order of 3x3 km. This was far too large for the Dapto site and a new site was selected at Culgoora in the north-west plains of New South Wales. The virtues of this site are its size, flatness, freedom from flooding, low radio noise level and accessibility from Sydney by air transport. Its sunshine and optical-seeing properties also made it a highly desirable site for optical observations, and developments assumed a new significance when Dr. Giovanelli and his optical colleagues decided to join us at the same observatory.

The Sun being a highly ionized gas, the basic physical quantities most needed for understanding solar phenomena are temperature, pressure (or density), velocity and magnetic field, together with their variations in space and time. The general aim at Culgoora is to secure observations from which these quantities may be derived (insofar as is practicable) simultaneously over extended solar regions. To do this has involved the development and use of filters of high spectral and spatial resolution, the study of atmospheric seeing and ways of securing high-resolution observations, and the study of methods of analysing observations to yield the physical quantities needed.

It is known that in the radio spertrum the limb of the quiet sun is brighter in the equatorial regions than near the pole. But most of the available theoretical calculations of the brightness distribution over the quiet sun have been made with the assumption of spherical symmetry. We have therefore calculated two-dimensional distributions at several decimetre and metre wavelengths, taking account of the observed asymmetry in the north-south direction. Newkirk’s method of ray-tracing was used, the calculations being made with a CDC 3600 computer. Some of the preliminary results (particularly for a sunspot minimum period) are presented here; they indicate that the electron temperature of the solar corona has a value of about 1 to 1.5 x 106 °K.

Transit observations of the Sun can be made with the Molonglo radio telescope which is now in full operation at 408 MHz. Thus the Sun can be scanned once per day by eleven separate beams evenly spaced in declination. Because of foreshortening of the north-south arm with increasing zenith angle, the total coverage varies and in winter about 28′ arc can be covered by the eleven beams, whereas only 15′ is covered in mid-summer. This also means that the beamwidth in declination changes from 2′.9 at the zenith to about 5′.8 for the mid-winter sun. The beam is 2′.8 wide in right ascension. A further complication is the fact that the positions of the eleven beams cannot be chosen arbitrarily but can only be selected from a predetermined set of beam positions. These latter have been fixed by the phasing connections built into the telescope and cannot be changed easily. Hence for all the above reasons the strip of Sun which can be scanned will vary from day to day.

This year the School of Electrical Engineering began observations with the new Fleurs compound interferometer. This instrument operates at 21 cm and has a fan-beam response 35″ arc wide in an east-west direction.

During the period 1966 July 12 to August 5 observations were made of the Sun at three radio observatories. The instruments used were the east-west arm of the Mills cross at Molonglo (408 MHz) and the Christiansen cross at Fleurs (696 MHz and 1424 MHz). The aim of these observations was to study the discrete sources of the slowly varying component of solar radio emission, while activity was comparatively quiet. The three frequencies enabled the variation of source structure with height of solar atmosphere to be studied. It has been pointed out by Swarup et al., and Christiansen et al. that the determination of the frequency dependence of these discrete sources is important for defining the physical mechanisms causing the radio emission.

Of the metre-wavelength solar radio bursts which have been recognized, those of type II are characterized by the most complex set of spectral features. Apparently acceptable explanations have now been put forward for most of these features. However, not all these explanations can be considered to be established. In particular the phenomenon of band splitting has been explained in different ways by Sturrock, by Tidman et al., by Zheleznyakov and Zaitsev. However, the theories of Sturrock and Tidman apparently require magnetic fields so strong as to preclude the formation of a magnetohydrodynamic shock wave by a disturbance moving at the velocity (<~ 1000 km/s) attributed to type II bursts. The same problem is encountered in other earlier theories of band splitting involving magnetic effects. The other theory does not involve the magnetic field strength. However, the details of this theory do not appear to have been properly worked out as yet.

Large outbursts from the Sun which are widely extended over centimetre and decimetre ranges and show smooth time variations in intensity have been called microwave type IV bursts (μ-IV). Those bursts generally consist of more than two successive components, the spectra of which move progressively towards the lower frequencies. Furthermore there is a tendency for the decay times, which are defined as the times occupied by the decreasing parts of the bursts, to become successively longer. It seems worth while to make a quantitative study of the time variation of the decay times and flux densities of μ-IV burst spectra and to infer how the physical conditions in the source change with time.

Solar X-ray bursts have been observed by a xenon-methane filled proportional counter on the satellite IMP-F. The counter is one inch deep and has a 14 mg/cm2 beryllium window. A slat collimator restricts the field of view to a full width of 5° in the direction of the satellite spin while allowing the counter to see ±60° in the plane containing the spin axis. The effective window area of the counter is 2.2 cm2, after allowing for the collimator.

Recent high resolution Ha photographs by Bray and Loughhead (unpublished) have confirmed that much of the chromosphere is quite non-uniform. Rather, it appears to consist of many distinct structures, which have a typical transverse section of several seconds of arc, i.e. about 2000 km.

The aim of the research reported in this contribution is to attempt to analyse the non-uniform chromosphere by first investigating a single structure. A simple model of a structure was chosen. From the model, line profiles of Hα and the H and K lines of Can were computed assuming a variety of physical conditions to exist in the feature.

It has recently been shown that the planet Mercury exerts a small control on relative sunspot numbers, the degree of control depending on the positions of the other planets.

This paper describes the results of extending the work to include all the planets.

As a working hypothesis we have assumed that gravitational tidal forces induced on the Sun by the planets may modulate solar activity and have accordingly calculated relative equilibrium ‘high tide’ displacements of the solar surface for each day from the positions, masses and distances of the first six planets. Although the tides are very slight, the mass of displaced material is appreciable and varies over the very wide range of nearly 5 to 1.

The first daily maps showing the two-dimensional radio brightness distribution over the Sun were produced at Fleurs ten years ago when the 64-antenna grating cross was completed. The maps had a resolution of 3′ arc at λ = 21 cm.

When the Fleurs field station was given to the University by CSIRO in 1963 it was decided to use the antennas of the grating cross and add to them four or more larger antennas (45 ft diameter) to produce a pair of high resolving-power compound interferometers.

The compound interferometer at Fleurs, N.S.W., consists of the separate arms of the Christiansen grating interferometer with the addition of two 45-ft dishes to each to give maximum spacings of 3710λ at 1415 MHz. The closest physical spacing is 60 ft and the spacings progress at 40-ft intervals to a maximum of 2580 ft as shown in Figure 1.

During the last solar cycle a number of observations of solar radio emissions were made in a wide frequency range from which an enormous amount of information has been obtained. However the results obtained so far are limited by rather poor angular resolution. Observations with much higher resolution (of the order of 1′ arc) have been required for further studies of solar radio emissions. At the present stage such observations have proceeded in the microwave range; also the radioheliograph at 80 MHz has just started at the Culgoora Observatory. At the Tokyo Astronomical Observatory a high-resolution study of radio bursts in the metre-wave range has been planned since 1960, and the construction of a new compound interferometer operating at 160.3 MHz was started in April 1967 at a new site, Nobeyama. This site, located about 150 km north-west of Tokyo, is surrounded by mountains and quite free from man-made interference. We shall give a brief description of this equipment; details will be published later.

From 1951 to December 1964 the Radiophysics Division of CSIRO regularly operated a solar radiospectrograph at Dapto, N.S.W. This instrument finally spanned the frequency range from 5-2000 MHz with 8 separate receivers, 6 of which used mechanical tuning. Simple rhombic aerials were used on all bands except the 200-2000 MHz band which used a parabolic reflector. The quiet sun could be detected on the latter band only.

During 1965 a section of this spectrograph covering the frequency bands from 10-210 MHz was moved to the CSIRO Solar Observatory, Culgoora, N.S.W., and operated as an interim measure to maintain continuity of observations during the installation of the 80 MHz radioheliograph.

The function of the 158 MHz interferometer operating at the CSIRO Solar Observatory, Culgoora, N.S.W., is the measurement of solar radio burst positions at a frequency close to the second harmonic frequency of the radioheliograph. The interferometer is designed to measure the north-south and east-west position co-ordinates of an isolated active region on the Sun with an accuracy of ~1′ arc, at rates up to 16 s−1.

The study of the sources of the slowly varying component at decimetre and centimetre wavelengths has been a major preoccupation of solar radio astronomers. Grating interferometers and other high-resolution aerial systems have enabled the separation of the S-component from the Sun’s total emission, and the study of individual sources. Nevertheless much of value remains to be derived from whole-sun intensity measurements—the most basic solar radio observation.

The accuracy of the absolute value of solar flux depends on the temperature calibration techniques and the knowledge of the gain of the aerial used for the measurements. Recent advances in low-noise receivers, microwave components, and amplifier stability have reduced considerably the errors due to temperature calibration techniques. At present the knowledge of the gain of the aerial used for calibrations (usually a pyramidal horn) presents the largest source of doubt of an absolute calibration. Investigations of this problem suggest that a ‘standard gain horn’ should be employed at all observatories in absolute solar calibrations.

This note describes a method of image formation which i applicable to radio astronomy. AH the information simultaneously reaching an aerial array from an extended region is utilized to produce the image so that the maximum sensitivity is realized. The method is simple in principle and involves relatively few components. One stage of the data processing presents some technical difficulty, but it is believed that particularly for observations below about.

In solar physics a need exists for filters which have very narrow passbands, ~0.01 nm or less. While Lyot-Öhman birefringent filters have been used particularly for investigations at the Hα line, the limited availability of the raw materials, especially calcite, have usually restricted these filters to passbands of ~ 0.025 nm or more. A possible alternative type of filter consists of a number of Fabry-Perot interferometers mounted one behind the other.