252 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



research in the ultraviolet and X-ray regions of the spectrum. Very 

 soft X-rays (10 to 100 A.) cannot penetrate to less than 100 kni. above 

 the ground. Harder X-rays (1 to 10 A.) reach progressively deeper 

 levels down to the bottommost fringe of the ionosphere. At still 

 shorter wavelengths, X-rays are sufficiently penetrating to be observed 

 with balloon-borne apparatus. 



The history of solar X-ray measurements begins with the use of 

 German V-2 rockets brought to White Sands inmaediately after World 

 War II. One of the primary objectives of the rocket astronomy pro- 

 gram initiated at that time was the study of the solar spectrum beyond 

 the atmospheric barrier in the near ultraviolet, beginning at about 

 3,000 A. It soon became evident that the spectral energy distribution 

 declined so rapidly toward shorter wavelengths that photographic 

 spectroscopy would experience great difficulty. X-ray measurements 

 were therefore attempted with sensitive detectors such as Geiger 

 counters and ionization chambers coupled with filters that provided 

 spectral resolution in comparatively narrow wavelength intervals, for 

 example, 2 to 8 A., 8 to 18 A., and 44 to 60 A. 



Although spectroscopy of the ultraviolet range has made tremen- 

 dous strides in the past dozen years, our knowledge of the X-ray region 

 is as yet confined to only the broad features. High-resolution spectra 

 still remain to be achieved, but the goal no longer appears very far 

 off and may be well within the reach of the first orbiting solar obser- 

 vatories soon to be launched. 



X-ray photometry from rockets has been carried on by the author 

 and his colleagues at the Naval Research Laboratory for more than 

 a full sunspot cycle, beginning in 1949. In a typical experiment, the 

 detector is mounted against an aperture in the skin of the rocket 

 looking outward. Its view of space during the course of the flight 

 depends entirely on the spin and yaw motion of the rocket. As the 

 rocket traverses the upper atmosphere, signals are telemetered con- 

 tinuously via a radio transmitter in the rocket to a receiA^er on the 

 ground. When the spinning rocket reaches altitudes to which solar 

 X-rays can penetrate, modulated signals appear in the record with a 

 roll frequency maximizing whenever the detector looks closest to the 

 direction to the sun. Essential to such an experiment is a visible 

 photocell measurement, which permits the calculation of the aspect of 

 the rocket at all times during the flight and therefrom the appro- 

 priate correction for the dependence of X-ray signals on the angle of 

 incidence of the radiation. 



Near sunspot minimum, in 1953 and 1954, the rocket measurements 

 indicated a marked reduction in X-ray emission below 20 A. In 

 some experiments no emission at all was detected below 10 A. With 

 the approach to solar maximum, the over-all X-ray flux increased, but 



