John Paul Wild - Solar Burst Discoveries

Solar Burst Discoveries

The new breed of electronic astronomers that Paul Wild joined were applying their wartime skills to radiophysics research, the Radiophysics Laboratory having achieved a number of successes since it was established, early in the war, to bring radar to Australia. In 1948, groups at the laboratory were studying several fields in addition to solar. Wild's work arose from the phenomenon of embryonic radar technology sometimes being jammed by mysterious interference, later discovered, in England, to be radio noise coming from the Sun.

When Wild joined the solar group there were two teams from which to choose. He chose to work for Lindsay McCready in building a radiospectrograph, at the suggestion of Pawsey. As he later said, "I knew if I joined McCready I would be able to do my own thing … That's why I became a solar man".

The spectrograph – the first ever built – looked at the spectrum of bursts of radiations from the Sun over a wide spectral range for frequencies from 40 to 70 megahertz. It produced some spectacular results, demonstrating the great complexity of burst and storm phenomena. At Penrith, 50 kilometres west of Sydney in the foothills of the Blue Mountains, a fairly primitive wooden aerial was pulled around with ropes, and every twenty minutes it was changed so that it pointed towards the Sun. The data were analysed after four months of observations. In the first paper, published in 1950, he wrote: "We have identified three distinct spectral types of burst and … we shall call them Type I, Type II and Type III."

Wild's team now needed a site for a new, better engineered and more powerful radiospectrograph and a large swept-frequency interferometer with which to observe the radio source. In September 1950, he and three colleagues borrowed a decrepit ex-military ambulance and with a spectrum analyser assessed potential sites on the outskirts of Sydney and down the New South Wales south coast that would be least affected by interference from radio transmissions. They chose a grazing property outside Dapto, 15 kilometres south of Wollongong, shielded by a 1500-foot mountain. Here the Radiophysics Solar Group went from strength to strength, to the extent that Wild later said "there was no question that we were the world champions". Professor Marcel Minnaert, the eminent Belgian astronomer, wrote in 1963:

The history of solar radio-spectroscopy is mainly the history of Australian work on this subject. ... At each meeting of the International Astronomical Union, highly competent specialists such as Wild … were able to announce spectacular progress.

This work was done in primitive buildings and facilities. The equipment hut comprised a minute workshop at one end, an office in the middle and another minute room at the other end, used as a kitchen and after-work gathering place while dinner was prepared: after returning from the pub, conversation would be animated as one of them hammered veal then cooked wiener schnitzel in an atmosphere "thick with fug". Staff members, who spent several days per week there, slept and ate in an adjacent single-roomed weatherboard hut with a table down the middle and camp stretchers around the sides. But among these achievers, morale and excitement was very high and their social life was lively. Colleagues from the days at Dapto, describing Wild – as many did – as "colleague, mentor and good friend" said " He could light up a room with his wit, intelligence and charm. He loved a party and a few beers."

The bursts were distinguished by the way the frequency drifted with time. The team deduced that the type II bursts were associated with shock waves coming out through the solar atmosphere at 1000 km/sec and were associated, 30 hours later, with aurora in the Earth's night sky. They had discovered the answer to a century-old riddle: what was the agency that conveyed the disturbance from the solar flare to the Earth? Type II bursts continue to be closely monitored by spectrographs on the ground and in satellites for "space weather" reporting, since their disruption of the geomagnetic field and ionosphere can cause blackouts to radio communication and the systems of navigational and other satellites.

Wild's team associated type III bursts with streams of electrons being ejected at a third the speed of light and taking less than half an hour to reach the Earth. There remained a few sceptics about this interpretation until, a decade or so later, American physicists using satellite data regularly detected bursts of electrons 25 minutes or so after solar flares. This was just one component, but to Wild and his colleagues a very exciting one, of a much wider world-leading research program.

The mechanisms proved to be correct and their naming of the phenomena became the international standard. Wild likened this research to the study of taxonomy that preceded Darwin's Origin of species. His analysis of the anatomy of the solar flares and his development of the physical interpretation culminated in a unified model that integrated the apparently complex radio flare phenomena in the solar chromosphere, solar corona, and in the interplanetary space.

In the course of this solar work, Wild became interested in the radio spectrum of hydrogen and wrote up an internal report related to the potential for spectral lines in the solar bursts. When Ewen and Purcell in the US first observed the 1420 MHz transition in 1951, he went back to his report, generalised it to include the interstellar medium, and six months later published the first detailed theoretical paper on the hydrogen lines – a classic in the field.