TUKE SKY PHENOMENA 



13 



Aurorae are sometimes classified into seven classes, namely: 

 Arcs, bands, curtains, rays, coronae, patches, and diffuse form. 

 Is this not a needlessly complex division? The bands appear to 

 me to be similar to the arcs; the rays and curtains to be similar 

 except for intensity; coronae tb be made of curtains and rays, 

 and patches either parts of bands or local diffuse areas. In 

 southern Ontario we rarely see anything but the rays; in the 

 north the rays usually rest on a foundation, if such a term may 

 be applied to anything so unsubstantial, of arc or curtains. Do 

 the rays seen farther south also connect with arcs too low to be 

 seen above the horizon, or do these manifestations in the higher 

 atmospheric strata occur farther south than the other forms? 



When we have a really brilliant display, the forms usually 

 appear in a regular sequence. First an arc will appear low in 

 the northern sky. This gradually rises, that is, moves south, 

 becoming apparently larger and more clearly outlined as it ap- 

 proaches. The space below the arc appears darker than the 

 rest of the sky, probably by contrast. Other arcs appear and 

 follow the first one. Portions of the arcs widen and become 

 brighter, and these areas spread rapidly. A distinct pulsing 

 movement, usually from West to East, appears. The arcs then 

 curve and loop and break in a great variety of forms, forming 

 great horseshoe loops, and circles, or rather cones. The broader 

 areas become striated in appearance; long, dancing, shafts shoot 

 up to the zenith, advancing, receding, flashing brightly, fading 

 quickly, justifying the habitants appellation "marionettes." 

 The arcs are usually devoid of color except pale yellow or g:een, 

 but the curtains frequently show other hues, most often rose, 

 occasionally crimson or violet. The color is not a stable feature. 

 So far as I have observed, it does not appear except when there 

 are rapid lateral, that is east and west, looping movements, and 

 the color appears on the angles of the curves, suggesting that it 

 may be due to some such cause as interference when the light 

 from one part is seen through the dense lattice-work formed by 

 another part in front of it. Gradually the movements subside, 

 the colors fade, the brilliancy decreases, and the sky settles into 

 a diffuse glow, brightened now and again by a patch or a solitary 

 ray. 



In March 1920 the greater part of the continent was treated to 

 an exhibition of what we see frequently, and our display was 



