THE EDGE OF THE SUN — MENZEL 225 



at a certain height and break up into knots or streamers, as it flows 

 toward the solar surface. Such an observation implies that the hot, 

 coronal matter suddenly became cooler or at least denser, so that the 

 hydrogen ions could recombine with the electrons and thus radiate the 

 characteristic spectrum of neutral hydrogen. In the hot, tenuous 

 corona, recombination goes on at a negligible rate. 



The so-called loop prominences, which constitute a distinctive class, 

 present a very special problem. Employing motion-picture photog- 

 raphy, we have frequently recorded the life history of the loops. Al- 

 though some variations occur, they generally start as an exceptionally 

 brilliant point well above the solar surface and then grow downward 

 on both sides, often attaining the dazzling brilliance of a solar flare. 

 On occasion, several loops may form and grow almost simultaneously. 

 The total development is generally rapid, of the order of two or three 

 minutes. As the loop fades in brightness, knots or lumps form and 

 tend to flow downward along both sides. Although the loop maintains 

 its shape, the lumps continue to form mysteriously and descend in 

 regular procession. The downward flow may last for an hour or 

 longer. The surprising thing is that we do not see material flowing 

 into the loop. 



I have devised an ad hoc explanation of the phenomenon. Suppose 

 that convection, turbulence, or some associated activity has blown into 

 the corona not only material from the rim of a sunspot but also some 

 of the associated electric currents. We expect the pinch effect to 

 operate as previously described, sweeping material in toward the axis 

 of the current. Increased pressure causes ions and electrons to recom- 

 bine. The simple pinch is unstable, however. As it develops, the 

 central core tends to break into separate lumps, something like a 

 string of sausages. Gravitation probably has little or no effect. The 

 force of the pinch squeezes the material downward toward the sun, 

 where the pinch is weakest. The pinch continues imtil finally all the 

 current in the region has united, sweeping up the involved ionized 

 gas. 



It is significant, I think, that fuzzy loops show up in the hot corona, 

 associated with the sharp loops of the cooler hydrogen emissions. 



A magnetic field can also act as a mechanism for ejecting matter 

 under certain conditions. It is well known that any conductor, such 

 as aluminum or copper, introduced into a magnetic field, experiences 

 a resistance. The force originates because the field does not pene- 

 trate the conductor, wherein surface currents must be induced. If the 

 piece of metal, so introduced, is later released, it will fly out. In cer- 

 tain regions of the sun's atmosphere, perhaps near the poles or in the 

 borders of spots, where a weak field cannot completely inhibit convec- 

 tion, lumps of ionized gas may become separated from the field, whose 



