Abstract
The next major stage in Payne-Scott’s career had been planned in September 1947 before Pawsey left for his year-long trip to the US and Europe in late September.1 This involved a new instrument, constructed at RPL, and a new site, Potts Hill. The observations made there had a lasting impact on the progress of solar physics in the latter half of the twentieth century. The new instrument was the first “vertical interferometer”2 (a Michelson) at RPL; it was also the first swept-lobe interferometer in radio astronomy. The solar radio astronomers in Sydney had recognised the awkward nature of the sea-cliff interferometer: observations were only possible at dawn and the large errors produced by uncertain refraction corrections to the phase of the interferometer fringes made for substantial uncertainty. Also the use of eclipses (Hey 1955) of the sun by the moon to study small scale solar radio structures had proved to be an awkward technique (an example was the eclipse of 1 November 1948 observed in eastern Australia); the times of observation were infrequent and the results were often ambiguous, leading to uncertain interpretations. Thus the ability to make high resolution observations at most times during daytime represented a major step forward. As Wild (1968) has pointed out: “… another Pawsey-inspired experiment was put in to operation and brilliantly performed by Payne-Scott and Little. The idea was to locate … the instantaneous position of the dominant source on the sun at any one time.”
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Notes
- 1.
The PC meeting of 23 September 1947 (2 days before Pawsey’s departure for San Francisco) contained a thorough discussion of a new solar interferometer for a new field station at Bankstown. R.F. Treharne and Alec Little were in charge of this 100 MHz instrument. The site location was moved to Potts Hill Reservoir in April 1948, at which time Payne-Scott was in charge of the project. Already on 6 June 1947 the PC minutes contained a brief reference to an “improved interferometer,” to be designed by R. Treharne. No location was specified and the instrument was to be “capable of yielding interference patterns in a fraction of a second with a view to extending this technique to ‘bursts’.”
- 2.
See footnote 44 Chap. 7. As Barry Clark has suggested (private communication, April 2009), we might think that a “sea cliff interferometer” would have been called the “vertical interferometer”!
- 3.
The “imaging” was indeed crude; the technique only involved rapid scans in a one dimensional manner of the fringes across the sun. With a single major solar event, it was then possible to determine this one dimensional position at the rate of 25 times per second. With modern high resolution radio solar telescopes, a true two dimensional image of the sun can be determined at a rate of many times per second.
- 4.
NAA: B2/2, Part 2. This new instrument (to become the Potts Hill interferometer) had been discussed initially at the 6 June 1947 meeting of the Solar Noise Group. R. Treharne had been given the job to design “...[100 MHz] equipment capable of yielding interference patterns in a fraction of a second with a view to extending this to bursts. Initial ideas are to use manual phase variation to aerials connected by a transmission line”. At this time no specific site for the instrument was suggested.
- 5.
Freeman (1991) and Sullivan (2009) have provided short summaries of the remarkable career of Little, who became an Associate Professor of Physics at the University of Sydney; he played a major role in the development of the MOST (Molonglo Observatory Synthesis Telescope) in the 1980s, before his untimely death in 1985.
- 6.
NAA: C4659, 8.
- 7.
The PC Minutes of 14 November 1947 also reported on the initial successful tests (and the completion of the motor driven local oscillator phase shifter), followed by the minutes of 20 February 1948 reporting on problems due to the mains voltage stability (the commercial power at 240 V AC).
- 8.
Pawsey had left Sydney for San Francisco about 3 weeks earlier.
- 9.
NAA: C4659, 8. The history of the site selection and some aspects of the scientific research programs at the Potts Hill site were described by Frank L. Kerr (1918–2000) in the January 1953 issue of the Sydney Water Board Journal (Sullivan archive). Although by this date the swept-lobe interferometer was no longer in use, the determination of solar noise positions from active regions was described in detail. The article began: “In 1948, the Chief of the Division of Radiophysics at C.S.I.R.O., Dr. E.G. Bowen, sought and the Board was pleased to grant, permission for the Division to install certain equipment on the Board’s land at Potts Hill for the investigation of solar radio activity … The Board has been very happy to have been able to provide the Division with the space and other activities needed.” An important factor in the success of RPL in the utilisation of the site was the role played by H.A. Stowe. He was the Chief of Electrical Engineering of the Water Board and a keen radio “ham”. In 1953, Kerr described the new 36 ft antenna being constructed for HI use (Wendt 2009).
- 10.
The “Solar Noise Research Report” (at a meeting of “Dr. Pawsey’s group”) on 30 April 1948 reported: “A search for a site to replace Bankstown has resulted in the choice of Pott’s [sic] Hill reservoir (which should also provide a suitable stretch of water for K-band [must have meant L band or 20 cm; this was to be the solar grating array of Christiansen] interferometry; negotiations with the [Sydney] Water Board have begun. Meanwhile equipment is being made to replace that stolen, and some obvious faults in the system are being remedied.” The acquisition of the new site was trying for McCready.
- 11.
Also Alec Little was finishing off the X-ray experiments at RPL; this project then moved to Melbourne.
- 12.
Treharne resigned from RPL in the middle of 1948 to join Philips Electrical Industries of Australia. In a letter from Bowen to Pawsey of 5 May 1948 (containing a general report on activities of the laboratory since the previous year, during Pawsey’s trip), he wrote: “[Treharne] unfortunately now has other interests and the work has not been going at all well recently. Ruby is there taking it over and, and due to difficulties at Bankstown, is acquiring a new site for the work at Potts Hill.” In the same report, he is quite positive about the significance of Payne-Scott’s determination of the time of arrival of bursts at Hornsby (Chap. 8). NAA: C3830, F1/4/PAW/1, Part 2.
- 13.
Also on this date, Yabsley reported that Bolton had just returned from the radio astronomy expedition to New Zealand. Bolton and Stanley were getting ready at this time to join John Murray and Don Yabsley in Tasmania for the 1 November 1948 solar eclipse (John Murray, letter, April 2009).
- 14.
In the 6 June 1948 letter, McCready also seemed to blame himself for the conflicts which he had been unable to resolve. The excuses for the imbroglio were (1) Payne-Scott was herself very keen to start the new Potts Hill project and (2) the work at Hornsby was going nowhere. Even Bowen had suggested that she “fold it up and get into something more profitable”. But as we have seen, the pessimism about the Hornsby endeavours was not justified.
- 15.
With the final instrument, this observation was difficult due to limited sensitivity. The quiet sun has a total flux density of about 2 × 104 Jy at 100 MHz; the flux density sensitivity of the interferometer was a few thousand Jy for observations of a few hours and only a few hundred thousand Jy for the subsecond observations. In addition, the fringe size of about 40 arc min was comparable to the size of the radio quiet sun of 40–50 arc min.
- 16.
This was never carried out; the project was partially fulfilled by Wild, Sheridan and Trent (1959) and Wild, Sheridan and Nylan (1959) at the Dapto field station for the frequency range of 40–70 MHz (Type I, II and III bursts).
- 17.
This sophisticated use of the instrument was never fulfilled in practice.
- 18.
Other groups were: Bolton, Stanley, Slee (Dover Heights Program No. 1); Payne-Scott and Marie Clark (Dover Heights Program No. 2); Lehany, Yabsley and Harragon (Georges Heights at 200–3,000 MHz); McCready (spectrum analysis of solar noise with no mention of Wild!). Smerd and Westfold (theory) also presented long range plans.
- 19.
NAA: C4659, 8. As was often the case with letters from Ryle, he apologised to Pawsey for the late reply. The end of term at Cambridge and especially “four parties of visitors” were the reasons for the lateness. On the same day (NAA: C3830, A1/1/1, Part 3), Ryle wrote Bolton (the report by Smith was enclosed) with a summary of the discussions that had been held with Pawsey about the large discrepancies. A repeat of the observation was suggested. Motion of the source was not precluded: “… if you find it has moved – then the astrophysicists must think again!” A flurry of letters resulted to and from Bowen and Pawsey.
- 20.
Sullivan (2009) has a fascinating description of the time evolution of the determination of the Cygnus A position. Baade and Minkowski (1954) identified the object with a high redshift galaxy, based on the accurate position obtained by Smith (1951) with the Cambridge instrument. The accuracies of the radio coordinates determined by Smith were about 0.2 arc min in right ascension and 1 arc min in declination.
- 21.
NAA; C3830, A1/1/1, Part 4.
- 22.
In this report, there seems to be no appreciation of the fact that atmospheric refraction can be neglected (to first order) for a spaced interferometer, based on the assumption of a plane parallel atmosphere. The almost equality of the phase change in both arms of the interferometer leads to a cancellation of the effect. As far as we can ascertain, the first published recognitions of this important fact in Australia were made first by Little and Payne-Scott (1951)” – “A uniform plane sheet of refractive material does not introduce any differential path difference between parallel rays, and hence the refraction due to a uniformly stratified region above a plane Earth can be ignored …”, and then by Mills (1952) – “It can be shown that for an interferometer with a plane atmosphere the refraction correction is automatically applied if the free-space velocity of radio waves is used in the declination calculation”.
- 23.
NAA: C3830, A1/1/,1 Part 4.
- 24.
Ryle concluded his letter to Pawsey by stating that a joint Jodrell Bank-Cambridge experiment had shown that radio star fluctuations were mainly due to a cause induced “by some relatively local effect”. Thus the ionosphere was the likely culprit, not the intrinsic behaviour of the source. Ryle pointed out that their conclusion agreed with the results of the “spaced receiver” data obtained in 1948 from New Zealand to Australia. The Cambridge–Jodrell results (with no mention of the RPL results) appeared in back to back papers in Nature (1950) by Smith (Cambridge) and Little and Lovell (Jodrell Bank).
- 25.
Wendt (2009) has provided a detailed description of the various projects carried out at Potts Hill in the years 1948–1962.
- 26.
In her letter to Ryle on 9 December 1948, Payne-Scott described the end of the Hornsby campaign (Chap. 8) and her plans for the new interferometer. “I have now transferred my interests to the type of interferometer that you use, but provided with a phase-changing system so that we can sweep through the complete lobe in 1/25 s instead of relying on the motion of the source, and hence providing a means of measuring the position on the sun of short-period variations of either type [we assume she meant Type I and possibly Type III bursts]. I should like to hear your views on the stability of the calibration of this type of interferometer …” She then described the new Wild and McCready swept-frequency spectrograph (called a “spectrum analyzer”) to scan the solar spectrum from 50 to 100 MHz. “It should solve many problems that spot-frequency observation leaves open, particularly that of the distribution of intensity with frequency in a burst.”
- 27.
The change of name has been described in Chap. 8. As discussed by Sullivan (2009), the first use of the term “Radio Astronomy” was by Pawsey in a letter in January 1948 and by Ryle in April 1948 at a meeting of the Royal Astronomical Society (he did, however, use the term inside inverted commas). Ryle wrote: “More refined observations in this new ‘radio astronomy’ should provide us with much new information on such processes.” The new term caught on quickly.
- 28.
Also at this meeting, Wild gave his first report on the Penrith swept frequency instrument, after only three days of data collection! He attempted to compare the results of different types of bursts with this new instrument (70–120 MHz) with the fixed frequency instruments previously used by Payne-Scott. At this time, the clear spectral distinction of the various types of solar bursts was not yet evident.
- 29.
Already at the 8 February 1949 meeting he had expressed his doubts about the sensitivity: “It appears that a larger aerial is desirable” to detect more than a few cosmic radio sources. The swept-lobe interferometer had, after all, been planned to detect large solar bursts and outburst.
- 30.
At this meeting, Mills and Thomas announced the results of their first Cygnus A position determinations. Discrepancies of about one degree were found with both Bolton’s and Ryle’s previously determined positions. At the next meeting on 7 July 1949, Mills announced that he was giving up on the Potts Hill interferometer for cosmic source positional determinations due to the limited sensitivity. The total flux density of Cygnus A at 100 MHZ was about 15,000 Jy with noise about 3,000 Jy. In 1951, he and Thomas published the Cygnus A position, followed by a refinement by Mills (1952) with an error 0.5 (RA) by 1.5 (Dec) arc min; even with this small error rectangle the optical object eluded detection. See footnote 20, this chapter, for the optical identification that followed in 1954 from Smith’s (1951) improved radio position.
- 31.
At the 1 September 1949 meeting, preparations were summarised for a second series of RPL solar eclipse observations for the 22 October 1949 eclipse in Sydney, Tasmania and Victoria.
- 32.
We do not know Payne-Scott’s reaction as the name of the “unpolarised” bursts evolved to “isolated” bursts, then α bursts, and finally the accepted name, Type III bursts.
- 33.
A number of personnel-McCready, Yabsley, Marie Clark, and Norm Labrum-left the Radio Astronomy Group at this time.
- 34.
Her preferred term was “non-polarised”.
- 35.
The frequency of the meetings of the full committee had decreased in the second half of 1950; Bolton gave a presentation on 12 December 1950, after his return from an overseas trip, reporting on US and European radio astronomy. In 1951 there were five meetings before Payne-Scott’s resignation on 20 July, including a meeting on 18 July.
- 36.
One reason was given for working more closely: “One interesting development is that there are some US astronomers working there. These groups previously at Johannesburg.”
- 37.
RPP 135. The first draft was prepared on 10 August 1950 (close to the cessation of data taking) and the complete manuscript was sent to the Australian Journal of Scientific Research, Series A – Physical Sciences, on 28 June 1951. RPP 136 was Paper II in the series and prepared and submitted at the same time. The papers appeared back to back in the December 1951 issue on pages 489–525.
- 38.
This derivation was probably one of the first derivations in radio astronomy of the response of an interferometer to a polarised signal. In a letter to Pawsey (in London) on 11 June 1948 (NAA: C4659, 8), Payne-Scott derived the response of linearly polarised antennas to a signal of arbitrary polarisation.
- 39.
Alec Little, as reported to Sullivan in 1978 (Sullivan 2009).
- 40.
These results had been summarised in detail at the 27 July 1950 Radio Astronomy Committee meeting.
- 41.
Based on a discussion with Mills (2007), it seems likely that the attribution is incorrect. The report may well have originated with Little.
- 42.
Sullivan (2009) in his Chap. 13 (“The Radio Sun”) discusses the thermal theory proposed by Ryle to explain solar bursts. The problem was that a fatal flaw was incorporated into the theory; he had assumed that the collision cross sections and rates of emission for the high temperatures of the solar corona were the same as the earth’s ionosphere at a modest temperature of only 300 K.
- 43.
Modern evidence indicates that the detection of linear polarisation may not have been significant; the Faraday depth of the corona is large at 97 MHz (Bastian, private communication, April 2009) leading to large de-polarisation of linearly polarised radiation.
- 44.
These results were presented initially at the 27 July 1950 Radio Astronomy meeting in which three outbursts were described. Two of the radio events started near an optical flare and the radio emission then moved outwards towards the solar limb.
- 45.
Paper III in the series was RPP 137 and was published in March 1952. A comparison of the results published in Papers II and III with the ANCORS publication of January 1950 (see Appendix M) shows that a thorough understanding of the behaviour of Type I bursts was achieved shortly after the observations began in May 1949; a clearer elucidation of the Type IVM outbursts was arrived at only after more extensive observations in the period February–August 1950.
- 46.
Stewart (1985) has pointed out that in the years 1966–1980, the Culgoora radioheliograph only recorded 56 Type IVM bursts, in comparison to 560 Type II events. The stationary component of Type IV events are associated with non-storm, flare-related continuum emission (Robinson 1985). Indeed the Type IV class is confusing!
- 47.
In at least one case studied by Stephen White, Tim Bastian and colleagues, motions of Type II bursts had in fact been detected near the solar limb with velocities of about 900 km/s (Bastian, email 19 April 2009; data from the Green Bank solar radio burst spectrometer and the Nançay Radioheliograph).
- 48.
The radiation had the characteristics of a smooth continuum, consistent with the gyro-synchrotron theory. However, a number of problems existed with the model. Stewart (1985) has summarised the pros and cons of the gyro-synchrotron model; a major problem was the presence of substantial circular polarisation. He suggested a hybrid model consisting of second-harmonic plasma emission followed by gyro-synchrotron emission. On the other hand, Smerd and Dulk (1971) made a strong case for gyro-synchrotron emission.
- 49.
None of these Type II/IVM events was associated with a major sunspot; for the 1949 event, and 17 February and 5 August 1950, the sunspot areas were only in the range 1,100 millionths, much less than the giant sunspots of 1946 and 1947.
- 50.
His advisor was Ratcliffe working on a Ph.D. (“The Propagation of Very Long Radio Waves in the Ionosphere,” submitted 1 September 1949). Ronald N. Bracewell (1921–2007) was on the scientific staff of RPL from 1944 to 1955. From late 1955 to 1991 he was a member of the faculty of the Electrical Engineering Department at Stanford University in California; after his retirement in 1991 he was professor emeritus. At Stanford Bracewell developed a 10 cm spectroheliograph, a radio telescope for rapid imaging of the sun. In 1998, Bracewell became an Officer of the Order of Australia. Portions of Bracewell’s archive are in the NRAO Archives in Charlottesville, Virginia.
- 51.
Subsequent articles described the controversy about the nature of the new compact radio sources (“Radio Stars or Radio Nebula”) after the large angular extent of some of the new sources (Sgr A and Vela) had been determined at RPL by Mills. The new discovery of Cygnus X (the HII region complex in the Milky Way only a few degrees from Cygnus A) was also reported (Piddington and Minnett). The third papers in the series described the new proto-Mills cross at Potts Hill. The final papers in the series concerned a possible association of meteor showers and rain (the work of Bowen) and the use of low frequency galactic radiation at 18 MHz by Shain to detect sudden ionospheric disturbances (SID) caused by solar flares.
- 52.
This publication by Bracewell elicited a response from Charles Seeger III (1913–2002), the son of the famous musicologist, Charles Seeger Jr. (1886–1979) and brother of the well known folk singer, Pete Seeger (1919–). Seeger had recently left Cornell and was for a short period in late 1950 at Chalmers University of Technology in Gothenburg, Sweden. Seeger wrote to Payne-Scott on 16 December 1950 with extensive questions about the new swept-lobe interferometer (NAA: C3830, A/1/1, Parts 5 and 6); apparently Seeger was waiting for the winter weather in Sweden to pass before undertaking the design of a similar instrument. Payne-Scott wrote on 1 February 1951 with extensive details (and block diagrams), with emphasis on the new 25 Hz phase changer at the local oscillator frequency. She urged the group in Sweden to build an instrument at frequencies other than 100 MHz (the swept-lobe Potts Hill instrument) or 1,200 MHz (the planned Christiansen cross which was to begin construction in 1951). Apparently, no solar interferometer was constructed in Sweden; shortly, Seeger was to move to Leiden where he remained until the early 1960s.
- 53.
This figure was reproduced by both Pawsey and Smerd (in The Sun, 1953),Wild (1968) and Kundu (1965). Kundu wrote: “The distinctive features of this observation were an initial outburst followed by a rather smooth and less intense continuum … Although part of the initial motion was perhaps due to the associated type II source... the prolonged movement and late return near the flare position was definitely due to the type IV source.”
- 54.
Pawsey Lecture presented by J.P. Wild in Brisbane, Australia, 30 April 1968.
- 55.
RPP 125. The first draft was dated 30 June 1950 and the paper was submitted to the Australian Journal of Scientific Research on 28 September 1950.
- 56.
See Fig. 9-12 for the motion of the radio event in an eastward direction to a position 10 arc min off the solar limb; the time scale for the motion was about 30 minutes. The origin of the 97 MHz emission began near the position of the optical solar flare.
- 57.
Provided by Tim Bastian (NRAO), August 2006.
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(2010). Payne-Scott at Potts Hill, 1949–1951: Movies of the Outward Motions of Solar Outbursts with the Swept-Lobe Interferometer. In: Goss, W.M., McGee, R.X. (eds) Under the Radar. Astrophysics and Space Science Library, vol 363. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-03141-0_9
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