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At the XVIth General Assembly, Commission 40 decided to curtail its triennial reports, confining them to aspects and developments which do not naturally find a place in the report of another commission. In letters to Presidents of eighteen commissions our new policy was explained and Commission 40 members willing to serve as radio astronomy contact person with one of these commissions were introduced. All Commission 40 members received notice of our intentions, with the names of the contact persons and an outline of this report in September 1978. We trust that from now on research at radio observatories will be integrally reported in the thematically appropriate reports of relevant commissions.
This paper describes a preliminary series of observations of the Sun made at a frequency of 80 MHz with the 3 km radioheliograph of the Culgoora Observatory. The instrument records, at one-second intervals, pictures of the solar image in the form of 60 (E-W) × 48 (N-S) points, each separated in angle by half the Rayleigh limit (2’ arc in the zenith). At the time of the present observations the instrument was incomplete in three main respects : (a) the facilities for recording opposite senses of circular polarization were not available; (b) the automatic image compensation for zenith-angle foreshortening was not available—hence the optical disk of the Sun appears elliptical; and (c) the phase and amplitude calibration procedures had not been fully established, resulting in a higher sidelobe level than that specified in the design—the effects are sometimes evident in the pictures as spoke-like brightenings.
We describe a method for reducing displacements in radio source positions caused by ionospheric refraction. Our method is an improvement on that used by Wild et al. (1959), who assumed all frequencies in the 40-70 MHz band came from the same position above the Sun and applied an f-2 correction for ionospheric refraction. Our method retains the dispersion of source position with frequency, which is inherent in the radio source, but allows for the f-2 ionospheric effect. We also discuss ways of estimating the absolute source positions from a knowledge of ionospheric density gradients. A typical example of ionospheric variations on solar records is shown in Figure 1(a). Here we have plotted the observed source positions on an f-2 scale. The measurements refer to a solar storm continuum burst which occurred on 1981 May 9 following an importance 2B flare at 08°N., 38°E., heliographic coordinates. From previous observations of such events we think it highly likely that the true source positions are stationary and displaced with decreasing frequency outwards along a line close to the radial direction above the flare. Figure 1(a) shows that: (a) the 160, 80 and 43 MHz source displacements vary as f-2; (b) there is a systematic slow drift of the source positions towards the south and east which increases with increasing hour angle; (c) superimposed on this steady drift is a quasi-periodic variation in source position with a period ~20 min.
The original design of the Culgoora radioheliograph (Wild, 1967; Sheridan et al., 1973) incorporated a number of features, such as high time resolution and limited declination coverage, which were particularly suited to its primary function of observing the Sun. Although some of these features are disadvantageous for cosmic work, the instrument has nevertheless proved useful for a variety of non-solar studies as well.
Our success or failure in understanding solar bursts is largely determined by the kinds of instrument with which we observe them. For example, although the basic feature of Type II bursts, a slow drift from high to low frequencies, was recognized and correctly interpreted from radiometer measurements at a few frequencies (Payne-Scott et al., 1947) other properties of Type II bursts such as fundamental-harmonic structure, split bands and herringbone structure can only be recognized on dynamic spectrograms. For this review I have chosen to group together the observations made with a particular type of instrument. I have also tended to emphasize what remains to be done rather than what has already been done. Unfortunately, with so many topics to choose from my treatment can only be cursory; and in attempting to select the interesting topics I have inevitably been biased toward the work I know best - that is, the work of the group to which I belong.