198 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



itself that showers us with corpuscles, or some other member of the 

 retinue, remains to be determined, and for the present we should 

 perhaps just attribute the resulting magnetic storms to the active 

 regions. 



More mysterious are the periodic magnetic storms that are clearly 

 solar in origin, but seem to have nothing to do with any distinguishable 

 solar feature. They come in series, starting with a small magnetic 

 disturbance. Twenty-seven days later comes a larger disturbance, 

 and others at 27-day intervals. Successive storms build up in inten- 

 sity and then gradually die out. The whole series will include per- 

 haps a dozen recurrences at 27-day intervals. Yet the most refined 

 observational techniques have revealed nothing on the sun that 

 appears to accompany these storms. The evidence for their solar 

 origin is difficult to avoid, however. The 27-day period is exactly the 

 rotation period of the sun. It appears, therefore, that every time a 

 certain patch on the solar surface rotates into a position facing the 

 earth, we are treated to a shower of corpuscles. Just to have a name, 

 we call such a patch an M-region. If the equality of the solar-rotation 

 period and the recurrence of M-region storms were the only evidence 

 we might put it down to a remarkable coincidence. But this is not 

 all. The frequency of M-region storm sequences unmistakably shares 

 in the sunspot cycle. The odd thing about it is that we have most of 

 these magnetic storms around sunspot minima. It appears that the 

 M-regions on the sun do not get along well with sunspots or active 

 regions. We have specific confirmation of this. If we arbitrarily as- 

 sume that the M-region is the j)atch of solar surface that was squarely 

 at the center of the solar disk when the corpuscular shower started on 

 its trip from sun to earth, we find that the M-regions avoid the neigh- 

 borhoods of sunspots like the plague. Furthermore if a sunspot de- 

 v'elops in the M-region, the corresponding series of magnetic storms 

 abruptly ends. So we have learned that an M-region is a portion of 

 the solar surface that appears perfectly normal in every other respect 

 to our definitely limited perceptions. One of our efforts at Sacra- 

 mento Peak is to sharpen our perception enough to identify the 

 M-regions. 



I have mentioned prominences in active regions. Of all the objects 

 of astronomy the solar prominences are to me the most beautiful. In 

 variety and form they resemble the clouds of our own atmosphere, 

 but they are, of course, a very different animal indeed. We see them 

 best at the edge of the sun, where they stand up like scarlet flames 

 above the solar surface in beautiful contrast against the dark sky. 

 Aside from their esthetic appeal, they are fascinating objects from 

 the scientific point of view, largely because they are so hard to 

 understand. 



