258 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



cause both types of radiation require million-degree sources and vary 

 in intensity with the square of the electron density. To compare the 

 radio map with the X-ray picture, it was photographed while rotated 

 about its center to match the motion of the X-ray camera during the 

 rocket flight. The resulting smeared radio image is shown in the 

 lower left corner of figure 3. Its major features closely resemble the 

 smeared features of the X-ray photograph. In order to enhance the 

 similarity, the contrast of the radioheliograph was heightened by 

 eliminating the two lowest isophote intervals of the original map. 

 In fact, one of the more important differences is the much greater 

 contrast between active regions and background in the X-ray picture 

 than in the radio picture. The bright, nearly central, region of the 

 X-ray image is about 80 times as intense as the quiet background when 

 allowance is made for the effect of smearing, and at least four-fifths 

 of the emission is concentrated in the active areas. In comparison, 

 the integrated radio emission from active areas is roughly equal to 

 the background emission. The X-ray photograph also matches fairly 

 well with a 21-cm. radioheliograph, but the detailed correspondence 

 is not as clear as the 9.1-cm. map. Studies of the relationship between 

 E-layer ionization and the solar decimeter wave flux show that a good 

 correlation exists between 3 cm. and 30 cm. On a short time scale, the 

 best correlation seems to occur in the range 10 to 15 cm. 



The X-ray emission and the microwave emission are both asso- 

 ciated with regions of greater than normal density in the corona above 

 sunspot groups. These coronal condensations are optically brighter 

 in proportion to the electron density. They appear to have semi- 

 spherical or elliptical forms without any resolvable internal structure. 

 So-called permanent condensations measure 1 to 2 minutes in arc, 

 range in density from 10^ to 10^° particles per cc, and persist for 

 several days. Sporadic condensations may form out of the permanent 

 condensations. The diameter of a sporadic condensation is typically 

 about 0.5 minute of arc ; its lifetime may be minutes to hours ; and it 

 is accompanied by the formation of loop prominences and the emis- 

 sion of bursts of centimeter wave emission and solar flares. 



Originally, the condensations were thought to be at very elevated 

 temperatures, as high as 6 or 7X10^ degrees K., but they are now 

 believed to be at near normal coronal temperatures in the range 

 1.6X10^ to 0.06X10^ degrees K. The association of X-ray emission 

 with the coronal condensations implies an upper limit of the order of 

 2 X 10^ degrees K. for the temperature of a condensation. It has been 

 argued that thermal conductivity in a condensation is so high that 

 it cannot maintain a high temperature relative to its surroundings. 

 If a condensation were at a temperature of 6 X 10^ degrees, as origi- 

 nally proposed, it would lose all its energy to the neighboring corona 



