IAU day 5: Viewing the Sun with radar

Posted by Korey Haynes
on Monday, August 10, 2015

The Sun is a strong source of radio waves, evidenced here by the Very Large Array. // Stephen White (Univ. of MD)/NRAO/AUI
Following my post from yesterday about radar, Monday at the International Astronomical Union (IAU) meeting revealed yet more radar tales. Miller Goss from the National Radio Astronomy Observatory (NRAO) shared the story of how the first original records of solar radio astronomy were lost for decades and then found only last year.

Joseph L. Pawsey (1908-1962) was not the first to detect the Sun in radio emission, but earlier records have also been lost. To Goss' knowledge, the newly discovered documents, found in the files of Sally Atkinson, Pawsey's personal assistant for many years, are the earliest original records in the field of solar radio astronomy.

Pawsey was not an astronomer by training, and in fact neither was anyone else working on his project just after the close of World War II in a suburb outside Sydney, Australia. But they were physicists and had technical training, and Pawsey had previously researched Earth's ionosphere. Intrigued by new work on cosmic and solar radio noise, he and his team simply took an old radar station, left over from the war, and turned off the transmitter, thus converting it into a radio receiver telescope. Because the structure had been built as a radar device, it pointed only straight toward the horizon, making it useful exactly twice a day for roughly a half-hour around sunrise and sunset. But by happenstance, the Sun went through a particularly active phase in the years just after WWII, giving the team a rich data set to investigate.

The team's newly converted instrument allowed them to discover that the Sun has "bursting" and "quiet" phases, and by studying the baseline quiet phase, they determined the brightness temperature of the Sun for the first time. They also studied and confirmed the existence of the hot corona of the Sun's outermost layer, which had until then been within reach of astronomers only during solar eclipses. And on a very basic level, they confirmed that the Sun is an active radio source.

According to Goss, Pawsey coined the term "radio astronomy" and is considered one of the founding fathers of the field, especially in his home country of Australia. While his science was carried on in journal articles and textbooks, his original records are an important piece of astronomy history, and it is nice to see them recovered after so many years. 

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