gg INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



disturbances peculiar to both the superior and inferior mirage. The 

 name Fata Morgana has become generic for all such multiple images, 

 wherever they occur. 



AURORA POLARIS 



The aurora polaris is a well-known but imperfectly understood 

 luminous phenomenon of the upper atmosphere. 



While no two auroras are exactly alike, several types have been 

 recognized, such as arcs, bands, rays, curtains or draperies, coronas, 

 luminous patches, and diffuse glows. The arcs normal to the mag- 

 netic meridian, often, but not always, reach the horizon. Their under 

 edge is rather sharply defined, so that by contrast the adjacent por- 

 tion of the sky appears exceptionally dark. The rays, sometimes 

 extending upward from an arch, at other times isolated, are parallel 

 to the lines of magnetic force. Many auroras are quiescent, others 

 exceedingly changeable, flitting from side to side like wandering 

 searchlights, and in some cases even waving like giant tongues of 

 flame. 



The aurora of the Northern Hemisj^here occurs most frequently, 

 about 100 per year, at the latitudes 60° (over the North Atlantic 

 and North America) to 70° (off the coast of Siberia). Its frequency 

 appears to be less within this boundary, while with decrease of lati- 

 tude it falls off so rapidly that even in southern Europe it is a rare 

 phenomenon. At the same latitude it is distinctly more frequent in 

 North America than in either Europe or Asia. 



The distribution of auroras in the Southern Hemisphere is not so 

 well known, but it appears to be similar, in general, to that of the 

 Northern. 



It is well established that on the average auroras are more numer- 

 ous during years of sun-spot maxima than during years of spot 

 minima. They also appear to be more numei'ous before midnight 

 than after. Relations of frequency to phase of the moon, seasons, 

 etc., have also been discussed, but with no conclusive results. 



Color. — Many auroras are practically white. Red, yellow, and 

 green are also common auroral colors. Some streaks and bands 

 are reddish through their lower (northern), portion, then yellowish, 

 and finally greenish through the higher portions. Much of the light 

 is due to nitrogen bands, but the source of the prominent green line 

 of the auroral spectrum is oxygen. 



There is good evidence that this green light, the light that produces 

 the ''auroral line,"' is always present in the sky, though whether 

 wholly of auroral oi-igin, or due in part to bombardment by meteoric 

 dust, or to some other cause, is not known. 



Height. — The problem of the height of auroras has often been in- 

 vestigated, but only recently solved. By simultaneously photograph- 

 ing the same aurora from two stations against a common background 

 of stars, many excellent height measurements have been secured. 

 The upper limits of the auroral light vary from about 100 kilometers 

 to over 500 kilometers; and the lower limits from ])ei'haps 85 kilo- 

 meters to 170 kilometers, with two well-defined maxima, one at 100 

 kilometers, the other at 106 kilometers. 



Cause. — The fact that brilliant shifting auroras are accompanied 

 by magnetic storms renders it practically certain that they, and pre- 



