UPPER ATMOSPHERE — MASSEY 181 



effect of atmospheric tidal motions on the E-region ionization, and 

 they demonstrate the periodicity of these tides. 



Many of the most interesting atmospheric light-emission phenomena 

 arise from levels in the neighborhood of 100 km. The night air-glow 

 occurs throughout the year and shows little variation with latitude. 

 It consists largely of the green and red line emission from atomic oxy- 

 gen, and band emission from molecular nitrogen and from hydroxyl. 

 The height of the emitting regions is probably not far from 100 km. 



A more spectacular optical phenomenon is the aurora. This is much 

 more intense than the night air-glow, and is concentrated in the so- 

 called auroral latitudes — there is a maximum frequency of occurrence 

 for the aurora borealis in a magnetic latitude of 67°, with a corre- 

 sponding maximum in the southern hemisphere for the aurora 

 australis. The auroral light includes strong emission at the wave- 

 lengths of the green and red oxygen lines, as in the air-glow, but the 

 strong band emission is from ionized, not neutral, molecular nitrogen. 

 In the latitudes of most frequent occurrence the main light emission 

 is from altitudes close to 100 km. At lower latitudes the much less 

 frequent aurorae often occur at higher altitudes, some as high as 600 

 km. Magnetic storms are usually associated with auroral displays, 

 and radio propagation is disturbed. 



As pointed out earlier, the 100-km. region is also the crematorium of 

 most meteors. So many of them are volatilized near the same level 

 that they produce a considerable concentration of ionization, often re- 

 ferred to as the sporadic, as distinct from the normal, E layer, which 

 is at a slightly lower altitude. 



There is still sufficient atmosphere well above 100 km. to be of im- 

 portance in many ways. Above 120 km. the concentration of electrons 

 and ions forming the E layer falls off, quite rapidly at first, but then 

 begins to increase again, and by day reaches a new and larger maxi- 

 mum at about 160-170 km. This corresponds to the Fi layer. 



After passing above this altitude, the ionization soon ceases to de- 

 crease, and then increases again to form the very broad Fg layer in 

 which the electron concentration is ten times as great as the maximum 

 in the E layer. At night the Fi layer merges with the Fg, and there is 

 an upward movement of the ionization to form the F layer which plays 

 a vital part in long-distance radio propagation. At 160 km. the atmos- 

 pheric pressure is about 3 X 10"^ of that at ground level, while the tem- 

 perature is perhaps 750° K. This upward trend in tem.perature almost 

 certainly continues, so that well within the Fg layer it probably ex- 

 ceeds 750° K. 



Our knowledge about the upper atmosphere has been gained by a 

 combination of direct and indirect observation. Until the last few 

 years no direct methods were possible, but the recent remarkable de- 



