220 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



cyclones. On earth, a cyclone is a region of low pressure into which 

 the surrounding gases are expanding, subject to adiabatic cooling. 

 Also, cyclones are vortices, which derive their motion from the Coriolis 

 force of the earth's rotation. Thus, astronomers were in effect going 

 back to the "trade-wind" hypothesis for sunspot origin, despite the 

 lack of evidence for differential temperature between pole and equator 

 to serve as a driving mechanism. There was, perhaps, the pious hope 

 that the sun's well-known equatorial acceleration might in some way 

 achieve the desired result. 



The vortex hypothesis ran into a good many obstacles and Hale 

 himself was well aware of them. The discovery that sunspot groups 

 in the northern and southern hemispheres switched their polarities 

 from one cycle to the next was a serious complication. On earth, 

 cyclonic motion is counterclockwise in the Northern Hemisphere and 

 clockwise in the Southern. Since the direction of the earth's rotation 

 fixes the sign of the Coriolis force we could not possibly get a reversal 

 of directions in the terrestrial atmosphere. Hence, if the same forces 

 were responsible for the solar vortices, and if the vortex were respon- 

 sible in turn for the magnetism, why should we expect the reversal? 

 Hale sought to show that the directions of the whirls in the upper 

 atmosphere were invariant, but neither the observations nor the argu- 

 ment were convincing. 



Bjkernes proposed an ad hoc model, in which the sun contained 

 four doughnut-shaped vortices below the surface, two in each hemi- 

 sphere. Each pair of vortices rotated in a different direction and also 

 rotated around each other, the outermost vortex moving toward the 

 equator while the other sank and moved poleward through the hole 

 in the outer doughnut. This picture accounted satisfactorily for the 

 reversal if the direction of vortex rotation determined the polarity. 



Application of the relatively new science magnetohydrodynamics 

 casts new light on the nature of highly ionized gases in the presence 

 of a magnetic field. A gas, sufficiently hot so that the electrons have 

 been torn from most of its component atoms, is highly conductive to 

 electricity. The magnetic fields associated with any electric currents 

 that may be present impart to the gas a certain amount of rigidity, as 

 if the material were semisolid. Biermann has pointed out that con- 

 vection has a hard time getting started in such a gas. The atoms can 

 flow up and down, to a certain extent, parallel to the lines of force. 

 But we cannot expect the violent turbulence ordinarily associated with 

 strong convection. We must, therefore, completely revise our ideas 

 about sunspots. They are not storm areas. They are, in fact, quite 

 the reverse : islands of intense calm floating in the otherwise turbulent 

 sea of the sun's entire convective atmosphere. 



I discussed earlier the general nature of convective motion and 

 pointed out, in particular, that the presence of water vapor in the 



