282 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



region where the thermonuclear energy-producing processes are taking 

 place. However, its surface temperature as observed from earth 

 through ordinary optical instruments is about 6,000°. Systematic 

 observation of this solar surface reveals a number of variable features. 

 The most striking is the apparition of sunspots, whose appearance on 

 the disk varies in an 11 -year cycle. Although sunspots were observed 

 by Galileo — who was involved in a bitter dispute with Father Scheiner 

 over the priority of discovery — their origin and nature are still not 

 fully understood. Occasionally, when there is a rapidly changing 

 group of sunspots, a solar flare occurs, accompanied by a violent 

 ejection of hydrogenous material. The study of sunspots and solar 

 flares with spectrohelioscopes and other optical instruments has in 

 recent years been supplemented by radio astronomical studies. One 

 of the earliest discoveries made during the rapid development of radio 

 astronomy after the war was that sunspots, and particularly on the 

 occasions when they associated to such an extent that a big flare 

 occurred, generated powerful radio waves. Many types of intense 

 and sporadic radio wave emissions from the sun are now recognized, 

 in association with disturbances on the solar surface. The corona 

 or atmosphere of the sun also generates radio waves which, although 

 much weaker than the irregular outbursts, are present all the time. 

 When the sun is eclipsed the vast gaseous layer of the corona can be 

 seen streaming out to a few solar radii. This coronal gas is in a state 

 of turbulent motion and the conditions are such that at half a solar 

 radius above the visible disk there are about 30 million atoms per c.c. 

 and the effective temperature is a few million degrees absolute. These 

 conditions create a most interesting situation and recent calculations 

 indicate that there is a resultant outward pressure which causes the 

 material of the solar corona to expand outward continually with a 

 speed of between 500 and 1,500 km./sec. This streaming material is 

 known as the solar wind. The experimental evidence for the existence 

 of this solar wind has, until recently, been rather scarce, but in the 

 spring of 1961 the Americans launched a space probe equipped with 

 instruments specifically designed to detect the existence and measure 

 the constitution of this material streaming from the sun. Although 

 the probe stayed up for only about 48 hours it succeeded in its task of 

 recording and measuring the existence of the solar wind in the inter- 

 planetary space. The situation which occurs when sunspots and flares 

 are seen on the solar disk is a violent modulation of this steady stream- 

 ing away from the sun, because on these occasions the material of the 

 corona and chromosphere is ejected at velocities several times that of 

 the normal streaming velocity of the coronal material. 



Another intriguing new concept concerns the behavior of magnetic 

 fields. Hitherto we have tended to visualize magnetic fields as entities 



