X-RAYS FROM THE SUN — FRIEDMAN 259 



ill less than 20 minutes. On the other hand, if a permanent condensa- 

 tion were actually slightly cooler than the normal corona, the excess 

 energy radiated because of its higher density could readily be replaced 

 by heat conduction from the surrounding corona. To help us under- 

 stand such details of the structure of the corona, we may look for- 

 ward to the achievement of X-ray photographs of much higher 

 resolution. Satellites will offer the possibility of mapping such fine 

 detail because of the longer observing times available. 



Superposed on the slowly varying X-ray emission associated with 

 plages are short-lived, transient outbursts synchronized with flare 

 activity. Flares have only very rarely been observed in white light. 

 "When viewed in the red light of hydrogen H-a, a flare appears to de- 

 velop with great speed. In a matter of minutes an area of the order of 

 one-thousandth of the solar disk may increase tenfold in brightness. 

 Intense radio noise is generated and shortwave radio communications 

 are instantaneously blacked out until the flare disappears. Flares 

 cover a tremendous spectrmn in size from those just barely detectable, 

 so-called microflares, to the most catastrophic explosions. These latter 

 are accompanied by streams of particles of cosmic-ray energies which 

 arrive within a matter of minutes at the earth and streams of slower 

 moving plasma that may require a day or two to reach the earth where 

 they are manifested by magnetic storms and auroral displays. 



The earliest attempts to detect flare X-rays were made in the summer 

 of 1956 with the Kockoon, a combination of a small, solid-propellant 

 rocket, carried aloft on a Skyhook balloon. The procedure was to 

 launch a Rockoon in the morning from a ship at sea and permit it 

 to float at 80,000 feet. When a flare was detected optically or indi- 

 rectly indicated by a shortwave f adeout, the rocket was fired by radio 

 command. It was unfortunately necessary to fire the rocket at the 

 end of the day even if a flare did not occur. Although this approach 

 to the problem was not efficient, it succeeded in measuring the emission 

 of one small flare during the course of the expedition and clearly re- 

 vealed the importance of the accompanying X-ray flux. The result of 

 that particular measurement is included in figure 1 and identified as the 

 portion of the X-ray spectrum associated with a Class 1 flare. 



In 1957 two-stage, rail-launched, solid-propellant rockets capable 

 of transporting substantial payloads to ionospheric altitudes became 

 available. Experiments were conducted with the Nike-Deacon and 

 the Nike- Asp during the IGY. The latter rocket had the capability 

 of carrying a 50-pound payload to about 150 miles. Instrumented 

 rockets could be kept in constant readiness, requiring only the push 

 of a button to launch them when a flare was observed. With this ap- 

 proach, a number of measurements of X-ray and ultraviolet emission 

 were obtained during solar flares. At the peak of a moderately large 



