Fehruabv, 1902.] 



KNOWLEDGE, 



33 



THE POLAR RAYS OF THE CORONA. 



By Mits. Walter Malndku. 

 The corona of lOdl was of tbe most luonouiiceil iniuimuui 

 t^-pe, its form was simpler tlian any of which astrouonu-rs 

 have had experience since 188;i. This circumstance, 

 though it may seem to detract from its beauty and 

 interest in some respects, is not without its advantages, 

 as the relations of many of its details can be more 

 distinctly followed and their significance better appreciated. 

 Es])ecially is this the ease with regard to the structures 

 immediately round the two poles of tiie sun's axis. At 

 times of maximum, the great synclinal rays are found, 

 not merely in the iieighliourhood of the sun's ecpiator, but 

 all round the limb, and the polar rays arc obscured by 

 the more conspicuous type of formation. But in an 

 eclipse Uke that of last May, the polar regions are left 

 absolutely free except for the beautiful and regular tufts 

 of light which have earned for themselves the appropriate 

 name of " plumes " or " panaches." These are then seen 

 there in utmost distinctness, and can be studied to 

 greatest advantage. No doubt structures similar in 

 general character may exist in every region of the corona, 

 both at minimum and maximum, but in the equatorial 

 regions the greater brightness of the great synclinal 

 curves, and the complexity shown by the regions of the 

 corona close to the sun's limb, prevent the " plumes " 

 from being easily recognised. 



Restricting our attention for the present to the rays 

 round the southern pole, thej' present something like the 

 following appearance. Starting from the great south-east 

 synclinal group, we tiud that the first polar rays are drawn 

 over into a curvature which nearly, though not precisely, 

 conforms to the southern outline of that formation. As 

 we approach the pole the rays are less and les3 curved, 

 till at the pole itself, and in its immediate neighbourhood, 

 we find them as straight as if they were ruled. And this 

 change of sha])e is not confined to the bright rays. 

 Between the bright rays there are dark streaks — whether 

 we regard them as mere rifts or intersi)aces, or as actually 

 dark rays, having an objective existence, as being, in other 

 words, an absorption and not a mere contrast effect — 

 showing precisely the same excessive curving near the 

 south-east synclinal group, and straightening gradually 

 as they approach the pole until they become there as truly 

 rectilinear as the bright rays amongst which they are 

 found. Proceeding from the pole towards the west, we 

 find in like manner that the curvature of the rays 

 increases the further we get from the pole, although we 

 do not get the same extreme bending noticed on the east. 

 But it is noteworthy that the south-west equatorial wing 

 is more diffused than the south-east ; its outline is not so 

 sharp and well defined, and the curve of that outline is 

 much gentler, except at one particular point with which 

 I am not at present concerned. 



The dark rifts or rays interspersed between the bright 

 p)lar rays deserve special attention. Many of these 

 ap])ear to be, and ]iroi)al)ly are, mere contrast effects, 

 interspaces between the bright rays. But others seem to 

 me to be of quite a different order. Thus on the east of 

 the pole we find two bright rays, the curvatures of which 

 lead them apart from each other the furtlicr they recede 

 from the sun. The darli space between them therefore 

 broadens the further we go from the limb. But within 

 this space we find an inten.sely black line; not a contrast 

 effect, for it borders neither of the bright rays, it does not 

 broaden as it recedes from the sun, but is equally narrow 

 throughout, and its curvature corresponds to that of 

 neither of the bright rays between which it runs. More 

 remarkable still it can be traced further from the sun 



than the bright rays. A similar remark a]>plies to 

 another ray even longer and darker than the first, that 

 springs almost exactly from the sun's pole. 



The polar rays towards the west appear to show some 

 evidence that they are not in the same plane. To the 

 extreme west two short bright rays of considerable curva- 

 ture appear to be distinctly nearer to the spectator than a 

 pair of straight rays wliich make a considerable angle with 

 the solar axis. The more westerly of the pair would 

 indeed seem to l)e partly hidden by the curved rays ; the 

 more southerly in its turn appears to conceal the lower 

 portion of three straight rays, which show a slighter 

 divergence from the solar axis. Elsewhere in the corona 

 of 1901 there were similar indications, none very strongly 

 marked, of a certain amount oi relief. The general effect 

 of this corona, as of all corona;, is of that of an essentially 

 plane phenomenon. The impression produced is that of 

 a superficies, not of a solid ; of a body having extension in 

 two dimensions, not in three. 



Yet we know for a certainty that the material composing 

 the corona must be distributed in all three dimensions ; 

 taking one eclipse with another it must on the average be 

 equally extended in all solar longitudes. We are looking 

 down upon the suu day by day through a depth of the 

 corona which on the average corresponds to its mean 

 depth as seen in the tangential ])lane. 



This consideration is so certain that it has led to the 

 system of polar rays being regarded as in fact a sub-polar 

 ring, the pole itself beiug bare. But at a distance from 

 it of about 20° or 25° a number of rays rise from the 

 surface and all of them bend over towards the sun's 

 equator. On this theory, those rays which lay very nearly 

 in the line of sight would necessarily appear to us to be 

 straight or nearly straight, through the effect of fore- 

 shortening, and would be seen by projection in the 

 apparent neighbourhood of the pole. It has been 

 further suggested that this ring of rays would be a 

 phenomenon essentially similar to our terrestrial aurorae, 

 the long coruscations of which have frequently impressed 

 themselves upon observers as resembling in appearance 

 these polar rays of the corona. But indeed this latter 

 suggestion is a very superficial one, and entirely ignores 

 the difference between the positions of the spectator m 

 the two cases. In the case of a terrestrial aurora we may 

 indeed see the rays shooting up from low down in the 

 north, high across our zenith, but we have no guide by 

 which we can tell as to whether those rays are really 

 rising or falling in our atmosphere, or as to whether, 

 which is far more probable, they are moving practically 

 at one given level ; their ajjparent rise or fall beiug simply 

 the effect of perspective. So far as our knowledge goes, 

 a terrestrial aurora, watched from the moon, would seem 

 to be essentially a surface phenomenon ; as entirely so as 

 the streaks which radiate from Tycho or Copernicus 

 appear to be to us. 



The difficulty which we have in regarding the polar rays 

 as situated in a ring at about 65° of solar latitude lies 

 here ; that in such a case they should obviously be much 

 more crowded together as we approach the greatest 

 apparent distance east and west from the pole, whilst near 

 the pole they should be considerably shorter and fainter, 

 and much more sparsely scattei'ed. In other words, not 

 only would there be a marked flattening of the corona 

 itself at the two poles, but the polar rays would show such 

 flattening among themselves. But in general these features 

 are just what are not seen. The straight rays close to the 

 pole are in general at least as long and as bright as those 

 nearest the synclinal groups, and they are nearly, if not 

 quite, as closely packed. Yet a careful examination of the 

 region before us does show a certain crowding towards the 



