NATURAL PHENOMENA 



perature inversion may exist. If the motion of the ship permits use 

 of a bubble sextant, it can be quickly determined whether the appar- 

 ent horizon is appreciably displaced. 



AURORAS 



Auroras are caused by positively charged atoms, mainly hydro- 

 gen atoms, together with electrons emanating from the sun and 

 entering the earth's atmosphere at speeds exceeding those of 

 meteors. Auroras differ in color, form, motion, time of occurrence, 

 and geographical distribution. They are associated with other 

 phenomena, such as sunspots, magnetic storms, and ionospheric 

 storms. 



The auroras commonly occur only in high latitudes, both northern 

 and southern. Occasionally they are visible from middle and low 

 latitudes far beyond normal limits. They present a rare spectacle 

 of great beauty and mystery to inhabitants of non-polar areas. 

 Observations of these phenomena have been made in India (latitudes 

 19°N. to 36°N), in the West Indies, and Aden (latitudes 10°N. to 

 24°N.), as well as regions closer to the poles. 



AURORAL FORMS (see Fig. 2).— The Homogeneous Arc, as the 

 name indicates, appears as a segment of a circle across the sky, 

 usually nearly at right angles to the meridian. The lower edge is 

 more sharply defined than the upper, but there is no apparent fine 

 structure. It may be high or low in the sky. The sky below the arc 

 is sometimes very dark; this is called the dark segment. When 

 several arcs appear they are closely parallel. The lower border, in 

 most instances, becomes more luminous before the appearance of 

 rays. Sometimes only short lengths of arcs appear. In this case, 

 definite east-west motion may be apparent. Single narrow arcs, 1° 

 to 2° in width, are sometimes about twice the usual height. 



A more irregular form of arc with folds has been called a 

 Homogeneous Band. It may move east-west with an undulating 

 motion, or it may oscillate in a north-south direction. The arc may 



become very bright or have a red lower edge and may change sud- 

 denly into ray forms. 



The Rayed Arc has the same elongated shape as the homogeneous 

 arc, but it is composed of rays arranged side by side. Sometimes the 

 homogeneous characteristic is retained; in others only the rays are 

 present. When the rays are short and curves are present, they are 

 commonly called Rayed Bands. Sometimes the rays become long 

 and the rayed arc shows folds moving as though blown by wind; 

 these have been called Draperies. They sometimes move as a whole 

 and finally break up into groups of long rays. Whenever ray forms 

 approach the magnetic zenith, they appear to converge to form a 

 radiant crown or Corona. When a ray form moves over the mag- 

 netic zenith it appears as a thin band of light containing many bright 

 points (rays seen end on). Rayed arcs are characterized by rays 

 standing side by side in a pattern elongated at right angles to the 

 individual rays. 



A Ray is the term applied to isolated individual rays or narrow 

 masses or bundles of rays. They may be present with other forms, 

 but the rays do not appear connected. Observation of rays is im- 

 portant because they are usually high above the earth. Rays appear 

 quite straight, like searchlight beams, and may be from 2° or 3° to 

 over 60° in length. The width varies from a few minutes to many 

 degrees of arc. The vertical edges are quite sharply defined. The 

 brightness usually varies from a low value at the top to a maximum 

 value about one third of the way from the bottom. Sometimes the 

 rays are very bright at the bottom and decrease in intensity gradually 

 upward. 



The Pulsating Arc consists of a homogeneous arc or a part of a 

 homogeneous arc which changes in light intensity, fading and 

 brightening in a rhythmic fashion. Each pulsation may last from a 

 few seconds to a few minutes. This form often drifts along slowly. 



The Pulsating Patches or Spots are any diffuse patch or spot, 

 not in arc form, which pulsates in brightness. They may be red in 

 color. 



1 1 1 



PULSATING ARC 



PULSATING SPOT 



HOMOGENEOUS ARC 



Figure 1' 



Auroral Forms 



46 



