RADIO ASTRONOMY — HAWKINS 283 



source forms an indistinct pattern. The depth of the minima gives 

 a measure of the diameter of the radio object. In specialized work, 

 following the rapid movements of gas jets across the sun for example, 

 the interferometer beam has been made to scan at a fast rate but the 

 method presents practical difficulties and is not often used. The 

 scanning is performed electrically by introducing a variable phase 

 lag in the cable from one of the antennas. 



SIGNALS FROM THE SUN 



There are remarkable differences in the appearance of the sun at 

 different radio wavelengths. Optically we see down through the solar 

 atmosphere to the incandescent layer of gas called the photosphere. 

 This layer is at an average temperature of 6200° C., but occasionally 

 large areas become cooled to about 5000 °C. and a dark sunspot ap- 

 pears. Sunspot regions are greatly disturbed and have been likened 

 to storms. Cine films show that part of the interior of the sun is dis- 

 gorged to rain down incessantly as streams of white-hot gas. The 

 whole area is pierced by an intense magnetic field which probably has 

 its origin in whirlpool motions below the photosphere. Sometimes a 

 bright flare of light appears near a spot, as shown in figure 3, and this 

 is thought to mark the ejection of a stream of charged particles which 

 impinge on the atmosphere of the earth a day or so later, causing 

 beautiful displays of the Aurora Borealis. Above the photosphere 

 we find the chromosphere, which is a red-colored layer about 10,000 

 km. thick, visible during a total eclipse of the sun. During an eclipse 

 a white halo is also seen extending outward for about a solar radius. 

 This is the solar corona, an envelope of ionized gas shining with 

 scattered sunlight. It has recently been shown that the outer edge 

 corona is at a temperature of a million degrees; this is a helpful 

 clue in explaining some of the peculiar radio effects that have been 

 observed at long wavelengths. 



At centimetric wavelengths the sun looks very much the same as it 

 does in the optical band, except that the steady light is now able to 

 pass freely through heavy cloud, rain, or fog. At wavelengths of 20 

 cm. the sun ceases to be uniformly bright but develops a ringlike 

 halo. Viewed with radio eyes it would appear as a brilliant circle 

 with a dusky center. This is caused by the temperature inversion in 

 the corona where the temperature increases as we move out from the 

 sun. Looking at the center we see the cooler layers below, and looking 

 at the limb we see the hotter layers edge-on. In addition to the limb 

 brightening, starlike points appear on the disc of the sun and con- 

 tribute to the general radiation. It has been shown that these points 

 occur near the visual sunspots, so at 20 cm. the radio astronomer has 

 a completely reversed image, a dark sun with bright sunspots. 



