278 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



of space and in the neighborhood of the target. If a pair of radars 

 operating at widely separated frequencies, such as 50 and 400 mega- 

 cycles, can measure the travel time to distant bodies to 1 part in 60,000, 

 and if the effect of the earth's ionosphere can be subtracted out, then 

 the density of free electrons in intei-planetary space could be deduced 

 from the excess time of flight obsei-ved with the low-frequency radar 

 over that of the high-frequency one. (Of course, this experiment 

 would tell us only the total number of electrons between us and the 

 planet, and in the absence of other data we could not say what fraction 

 was in space and what fraction was in the vicinity of the planet.) 

 Finally, knowing the total electron content, we could improve the 

 original range measurement. 



RADAR AND THE SUN 



The procedure that was used to detect echoes from the sun's ionized 

 corona is much like that employed for Venus, with two important 

 differences. First, the sun itself generates so much radio noise that 

 there is no particular point in working hard to minimize receiver 

 noise. Second, the operation is at much lower frequencies, 20 to 50 

 megacycles being required. If higher frequencies are used, the signal 

 penetrates so far into the corona before reflection that absorption losses 

 become severe. At still lower frequencies, the signal is apt to be 

 blocked by our own ionosphere. 



In April 1959, the first successful solar radar experiment was car- 

 ried out by researchers at Stanford University. (See p. 281 of the 

 March 1960 issue of Slcy and Telescope.) The strength of the echoes 

 turned out to be in very close agreement with theoretical predictions 

 published by the Australian radio astronomer F. J. Kerr in 1952. 

 One important difference was that the returns appeared to come more 

 or less uniformly from a wide range of depths in the corona. This 

 might be expected if the coronal region had large irregularities. 



The Stanford experiment used a high-power communications trans- 

 mitter operating at 26 megacycles, feeding the array of eight rhombic 

 antennas already partly shown in figure 5. The transmission con- 

 sisted of a series of alternate 15-second on-and-off periods lasting for 

 15 minutes, approximately the time of flight to the sun and back. 

 Again, a digital computer processed the received signal, so that the 

 combined energy of all the individual returns could be used to enhance 

 the final signal-to-noise ratio. 



With the rapid progress of radar techniques, we may look forward 

 to even more revealing radar studies of the sun over the next few years. 

 Range-frequency maps of the corona, analogous to those already made 

 for the moon, might unlock many secrets about the dynamics of the 

 sun's outer envelope. 



