SOLAR RADIO ASTRONOMY—MAXWELL 303 
thought that the radar equipment was being jammed by the enemy, 
but J. S. Hey, of the British Army Operational Research group, 
traced the source of the disturbance to the sun. He also noted that 
the radio emissions from the sun were associated with the presence of 
sunspots on the solar disk. At the same time, a similar effect was 
noted on long-range radar systems in operation at Norfolk Island 
in the South Pacific. The increased noise level in these radars was 
also traced to solar disturbances. 
After World War II these bursts of radio emission from the sun 
were investigated in considerable detail with fixed-frequency receivers 
operating at numerous points throughout the whole radio spectrum. 
It was found that the radio emissions usually came in the form of 
a storm of bursts, whose intensity level was often 1,000 times the back- 
ground radiation from the sun. These radio noise storms frequently 
lasted for several days. At times, generally coincident with the ap- 
pearance of a great flare on the solar disk, there were outbursts of 
radio noise in which the intensity of the emissions could increase by a 
factor of a further thousand or more. Comparison of the records ob- 
tained with receivers operating at different wavelengths at the time of 
such outbursts revealed an interesting effect. The solar radio out- 
burst was first observed with the equipment operating at the shorter 
wavelengths, while at the longer wavelengths it did not appear until 
several minutes later. Payne-Scott, Yabsley, and Bolton suggested 
in 1947 that this striking phenomenon might be caused by the out- 
ward passage of a disturbance, traveling at about 1,000 km. per sec- 
ond through the solar atmosphere. As the disturbance traversed the 
chromosphere it would send out a burst of radio waves in the centi- 
meter band; then, as it continued to travel outward through the solar 
atmosphere, it would send out radio bursts of successively increasing 
wavelength. 
SWEEP FREQUENCY RECEIVERS 
The most convenient manner of studying such solar bursts, which 
traverse a large part of the radio spectrum, is to use receivers which 
record emissions from the sun over a wide range of radio wavelengths. 
The first such instrument was put into operation in Sydney, in 1949, 
and it covered the band 70-130 Mc./s. It was subsequently extended 
to cover the range 40-240 Mc./s. This instrument marked a notable 
advance in solar radio techniques, and the resulting observations 
greatly simplified the existing complex classification of solar radio 
phenomena. 
In 1956 a similar experiment was put into operation at the Harvard 
Radio Astronomy Station, in Fort Davis, Tex. The equipment at this 
station covered a different part of the radio spectrum, was of greater 
sensitivity, and used a larger antenna. It originally operated over 
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