Culgoora Radioheliograph Research Articles

Australian Journal of Physics Astrophysical Supplements

A description of nine moving type IV radio events is given, which completes the description of 24 such events observed with the 80 MHz Culgoora radioheliograph between September 1968 and November 1971. Radioheliograph data and radiospectrograms are presented together with related optical, X-ray, and microwave observations. Discussion is limited to the classification of the kind of moving source involved (advancing front, expanding arch, or isolated source) and a comparison with the characteristics of previously published events.

Radio evidence of twisted bi-polar magnetic fields in the solar corona

Observations of bi-polar type I storm centers with the Culgoora radioheliograph operating at 80 MHz show that in many cases they are not oriented as we should expect for emission in the ordinary mode and for the simplest magnetic field geometry. We interpret this as evidence for a twist in the magnetic field.

Multiple-frequency Operation of the Culgoora Radioheliograph

The advantages to be obtained from multifrequency operation of the Culgoora radioheliograph were appreciated during the early design phase of the original 80-MHz system and steps were taken then to facilitate the addition of other observing frequencies. The heliograph has already been converted to operate at both 80 and 160 MHz and work is well advanced to add a third frequency of 43.25 MHz. After describing the original 80-MHz system this paper outlines reasons for the choice of these frequencies and shows how the system was modified to incorporate them.

The Absorption by the Interstellar Medium of 80 MHz Radio Emission from Galactic Supernova Remnants

High resolution 80 MHz observations of 20 galactic supernova remnants have been made with the Culgoora radioheliograph. More than half of the sources have an unexpectedly low flux density at 80 MHz, probably as a result of free?free absorption taking place in the inner arms of the Galaxy. The observations imply an r.m.s. electron density of about 0�1?0�5 cm?3 along the line of sight. This large value for the electron density seems to require a greater ionizing flux than that which occurs in the solar vicinity.

A moving Type IV radio burst and its relation to the coronal magnetic field

A moving Type IV burst, observed with the Culgoora radioheliograph on 1970 April 29, moved out to about 3 R⊙ and attained high circular polarization before fading. The appearance of the moving Type IV source suggests an isolated, self-contained, synchrotron emitting plasmoid. Magnetic field maps of the corona derived from photospheric observations indicate that the plasmoid moved almost radially outward from the flare region along open field lines. To explain the observed source structure and high unipolar polarization, we suggest that a ring of electric current was ejected from the low corona and guided by coronal magnetic field lines; the radio emission was synchrotron radiation generated by mildly-relativistic electrons trapped in the poloidal magnetic field of the ring current.

Use of the Culgoora Radioheliograph for the Generation of Diffraction Patterns

A series of diffraction patterns produced with the electronic imaging system of the Culgoora radioheliograph is presented. These patterns serve both to illustrate the quality of this type of imaging system and to demonstrate the advantage, in special circumstances, of constructing a radio-frequency simulation of an optical imaging device.

A Coronal Disturbance Observed Simultaneously with a White-Light Coronameter and the 80 MHz Culgoora Radioheliograph

Observations on 1970 August 11 and 12 at Mauna Loa (Hawaii) of the white-light corona revealed a rare solar event in the form of a short-lived coronal brightening off the eastern limb of the Sun. The intensity of this region, as measured at a distance ρ = 1.5 (where ρ = R/R⊙), more than doubled over a period of five hours and then fell very abruptly (~10 min) to below its initial brightness. A complex radio event in the same region was observed simultaneously with the 80 MHz radioheliograph at Culgoora (Australia). Associations between the optical and radio events will be described and the physical implications briefly discussed.

Some Investigations of the Solar Corona: The First Two Years of Observation With the Culgoora Radioheliograph

The first meaningful observation with the 80 MHz Cul-goora radioheliograph was made on 2 September 1967, a few weeks before the instrument was officially commissioned. Systematic observations, normally for 4 to 6 hours per day, began late in February 1968 and the Sun has since been observed on 438 days. The remaining days and many nights have been used for investigations of radio sources and the interplanetary medium and for maintenance and instrumental extension. The standard of reliability thus achieved has been due to the sustained efforts of K. V. Sheridan, W. J. Payten, K. R. McAlister, M. Beard and their colleagues. These first two years have been a period mainly of exploration—a phase of great interest to those engaged in observing, though possibly bewildering to those not. The present review is a first attempt to collate and summarize what has so far been found and where possible to draw preliminary conclusions. The observations to be described and their discussion have been due to a joint effort on the part of a number of workers, particularly G. A. Dulk (on leave from the University of Colorado), K. Kai (on leave from the University of Tokyo), N. R. Labrum, D. J. McLean, A. C. Riddle, K. V. Sheridan, S. F. Smerd, R. T. Stewart, and the present writer. The period has also seen considerable development in the evolution of intricate computer programmes for analysing the data, particularly on the part of L. H. Heisler and D. J. McLean.

80 MHz Observations of the Movement of a Type IV Source out to 6 R®

A moving type IV radio source on 1970 March 21 was observed with the 80 MHz Culgoora radioheliograph to move out to a distance of p = 6 (where p = R/R®). Starting as a single source of complex shape it later developed strong circular polarization (> 90% R.H.) and separated into components which retained the same polarization. The source appeared soon after a prominence eruption (or flare spray) from which some of the ejected matter travelled in the same direction as, and with a velocity close to, that of the radio source.

Evidence of Type II and Moving Type IV Solar Bursts Excited by a Common Shock Wave

Travelling disturbances in the solar corona with velocities ~103 km/s manifest themselves at radio frequencies by two distinctive phenomena—the type II burst, the spectrum of which shows a slow frequency drift that corresponds to quasi-radial outward motion, and the moving type IV burst, for which positional observations show transverse motion directly. A close relation between the two phenomena has long been suspected, and each type has separately been ascribed to a shock wave disturbance. In this paper we summarize three events recorded by the Culgoora radioheliograph and spectograph (two in the course of publication and one unpublished) in each of which a type II and a moving type IV burst can be consistently attributed to the effects of a common shock wave.

Radio Observations of the Structure of the Solar Corona

In this preliminary study the first two-dimensional pictures showing detailed features of the quiet Sun and weak but moderately stable structure at metre wavelengths are presented. The observations were made at 80 MHz with the Culgoora radioheliograph.

Scatter in the Positions of Sources of Associated Type III Solar Bursts

For a number of years the occurrence of isolated groups of apparently related type III bursts has been recognized as a common feature of the dynamic spectral records of solar radio emission at metre wavelengths. Interferometer observations supported the simple hypothesis that bursts of a group were usually located in the same position. In this note we report 80 MHz observations made with the Culgoora radioheliograph which show that although the sources of the bursts in a group tend to overlap one another, their centroids may show marked scatter and that their distribution tends to be along narrow lanes across the solar disk. Since the 80 MHz sources occur at heights (≳ 0.6 R⊙) near or above the plasma level, these lanes doubtless reflect some structural feature of the outer corona.

Observations of Centaurus A at 80 MHz

We present observations made at 80 MHz with the Culgoora radioheliograph of the compact central components of the radio source Centaurus A, which is associated with the peculiar galaxy NGC 5128.

The structure, polarization, and spatial relationship of solar radio sources of spectral types I and III

A description is given of recent high-resolution observations of the sun made at 80 MHz with the Culgoora radioheliograph. In many cases the sources of type I storms show bi-polar structure, the two components being spatially separated by about 3′ arc and strongly circularly polarized in opposite senses. Similar behaviour has been found for stationary type IV bursts which appear indistinguishable from type I storms on the heliographic records. On the other hand the sources of the weakly polarized type III bursts show uni-polar structure. The latter avoid the precise locations of type I sources even when they are associated with the same active centre. It is suggested that the type I storms originate in strong magnetic fields above sunspots while the type III bursts originate in weak magnetic fields outside the strong-field regions, possibly in neutral sheets.

Expanding arch structure of a solar radio outburst

A flare-associated complex outburst was observed on 1968, October 23–24 with the 80 MHz Culgoora radioheliograph. Two harmonic type II bursts were followed by two successive extended sources with arch structure which appeared further beyond the optical limb than the preceding sources. The second arch showed a remarkable expansion with a projected velocity of 1200 km/sec. At its maximum the arch extended to a height of 2R⊙. The height-time plots derived from both the radioheliograph and spectrum observations suggest that two shock waves of different propagation velocities were initiated at the flash phase of the flare: the faster one was responsible for the first type II burst and the first radio-emitting arch; the slower one for the second type II burst and the second arch whose expansion advanced with the shock front.

Observation of the magnetic structure of a type IV solar radio outburst

A continuous record of the 80 MHz image and polarization of a type IV solar outburst has been made with the Culgoora radioheliograph from which the magnetic structure of the event can be directly inferred. The first (‘moving’) part of the event appears beyond the limb as an expanding magnetic arch along which three concentrated sources develop: one unpolarized source near the peak, attributed to synchrotron radiation; and two polarized sources of opposite polarity near the feet, attributed to plasma radiation. The radio-emitting arch appears to lie above an eruptive prominence seen in Hα. The second (‘stationary’) part is seen later as a separate highly polarized source on the disk above the projected position of the flare that had previously triggered the prominence activity.

Observations of Motions in a Solar Storm Centre

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.

Use of atomic frequency standards for phase calibration of large aerial arrays

The use of two extremely stable radio-frequency oscillators, one stationary and the other mobile, is described for measuring relative phase difference between widely separated elements of large aerial arrays. Results of measurements made at 80MHz on the 96-aerial Culgoora radioheliograph, using an atomic frequency standard for the mobile unit, demonstrate that the method is suitable for phase-calibration purposes.

80 MHz Heliograph Observations of Two Type IV Events on 4 and 6 May 1968

Radio pictures of the Sun from the Culgoora radioheliograph have already shown instances in which flares have initiated radio bursts in parts of the Sun remote from the flare position. In this paper we discuss two such events on 1968 May 4 and May 6, in each of which it appears that shock-waves arising from a flare produced distant prominence activity which led to the generation of metre-wave continuum radiation.

Radio Evidence of a Coronal Instability Before the Onset of a Solar Flare (9 June 1968)

The outburst and storm of more than four hours’ duration that accompanied the importance 2 flare beginning at 0h25m U.T. on 9 June 1968 were observed at 80 MHz with the Culgoora radioheliograph. More than 104 heliograms in both senses of circular polarization were recorded at one-second intervals both as cathode-ray-tube photographs and as digital characters on magnetic tape. It is the purpose of this preliminary account to summarize these observations and bring attention to a new phenomenon observed before the start of the flare.