346 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 64 



cannot be heated by it. There simply must be another kind of radia- 

 tion in the sun that varies in more or less the same way as does the 

 radio flux and that influences the orbit of the satellite. 



It was reasonable to assume that extreme ultraviolet radiation, 

 including soft X-rays, emitted from sunspots varies in a manner 

 similar to that of the 10-cm. radiation ; that is, the same primary cause 

 underlies the two phenomena and therefore they are in unison. If this 

 is true, then the 10-cm. flux serves as a fairly accurate indicator of 

 variations of the emission of the extreme ultraviolet. The latter can- 

 not, of course, be observed from earth because it is completely shielded 

 by the atmosphere. Its existence, however, had earlier been confirmed 

 and measured by rockets lofted before the IGY. Since sunspots have 

 a tendency to concentrate in a few long-lasting active areas, the radio 

 flux and the extreme ultraviolet flux will show a maximum every 27 

 days when the rotation of the sun brings them near the center of the 

 visible disk. And since the number of sunspots greatly varies with the 

 11-year solar cycle, there is a corresponding variation in the two fluxes, 

 which is reflected in the heating of the atmosphere. Actually, this 

 variation with the 11-year cycle is by far the largest observed in the 

 atmosphere. 



This, then, was the first major discovery concerning variations of 

 upper atmospheric density made from the tracking of satellites. The 

 importance of this finding has frequently been compared to that of 

 the Van Allen radiation belt. 



Shortly after the discovery of the 27-day variations, another kind 

 was found from observations of the rocket of Sputnik III (1958 81). 

 Jacchia noted that one two occasions during the lifetime of the object 

 the drag increased much more suddenly than it did during the 27-day 

 fluctuation. Each of these increases occurred within a matter of two 

 days, one during which density rose, and the other when it fell. He 

 then searched for any unusual happening on those days. They proved 

 to be the dates of the only two large magnetic storms during the life- 

 time of the satellite. The maximum of each storm coincided with 

 the maximum of disturbance in the drag to within a fraction of a day. 

 Once he computed the acceleration curves and compared them with 

 the magnetic indices, he found that they were almost identical. 



Such magnetic storms are caused by solar flares. In both cases, 

 in July and in September, the magnetic storms started approximately 

 24 hours after the appearance of a + 3 flare on the sun. 



The agent that caused the storm was corpuscular radiation. Its 

 role in heating the atmosphere was completely unknown before the 

 space age. Violent flares on the sun emit charged particles. Wlien 

 they are in the vicinity of our planet, tliey interact with its magnetic 

 field and cause perturbations of the magnetic needle. The same par- 

 ticles indirectly also cause the temperature of the atmosphere to in- 



