SOLAR RADIO ASTRONOMY—MAXWELL 301 
BACKGROUND RADIATION FROM THE SUN 
The radio emissions from the sun, the only star from which radio 
waves have as yet been detected, consist of a background thermal 
emission from the solar atmosphere, and transitory disturbances, 
sometimes of great intensity, which originate in localized active 
areas. The first successful observations of the background radiation 
were carried out in 1942 by Southworth, who worked in the United 
States with equipment operating at wavelengths between 3 and 10 
cm. Since that time a considerable amount of effort and ingenuity 
has been devoted to determining the parameters of the background 
radiation, which gives us direct information about the distribution 
of temperature and electron density in the solar atmosphere. A great 
deal of this work was done with interferometers of variable spacing, 
and many of the observations were carried out during the recent 
sunspot minimum, from 1952 through 1955. The experimental work 
has substantially confirmed the radio models of the solar atmosphere 
proposed by Ginzburg, in Russia, in 1946, and independently by Mar- 
tyn, in Australia, in 1946. 
The early observations revealed that radio emissions of different 
wavelength originate from different levels in the solar atmosphere; 
the shorter wavelengths from the lower levels (the chromosphere), 
and the longer wavelengths from the outer regions (the corona). The 
intensities of these emissions not unexpectedly depend on the tem- 
perature of the appropriate emitting level, which is of the order of 
20,000 degrees in the chromosphere, and 1 million degrees in the outer 
corona. The observations also show that at the longer wavelengths 
the sun has a diameter of about 1 degree, which is twice that of its 
optical disk. 
RADIOHELIOGRAPHS 
Despite the small angular size of the sun, it has been possible to 
determine the precise distribution of brightness over the solar disk 
at various radio wavelengths. That is, pictures have been obtained 
showing how the sun would look if our eyes were tuned to radio waves 
instead of light. One of the most interesting of such experiments 
has been carried out by Christiansen, who has equipment operating 
at 21 cm. (not the radio spectral line of hydrogen). Christiansen 
uses 64 paraboloids, each having a diameter of 19 feet. These are 
arranged in two rows, north-south and east-west, in the form of a 
cross. The paraboloids are spaced at 40-foot intervals, so that the 
overall length of each arm of the cross is 1,240 feet (pl. 1, fig. 1). 
The signals from the two arms are combined, alternately in phase 
and out of phase, and the component of the output which alternates 
in synchronism is recorded. The system has a directional diagram 
which comprises a grid of points in the sky, and as the earth rotates, 
