In the upper atmosphere --above say 70 km --the density is very small and decreasing upwards and the matter is directly exposed to various solar and cosmical agencies and influences, which rapidly deminishes downwards towards the earth's surface. It is therefore to be expected that owing to these influences and the smale density, the composition and physical state of this upper region will differ essentially from the conditions we' observe'in the lower part toward the ground.
In order to aquire reliable knowledge of the physics of the upper atmosphere, we must observe and study such processes and effects which take place in these regions. The study of these processes may not only give us important results regarding the physics of the upper atmosphere, but the interpretation of the observed effects involves a knowledge regarding the solar and terrestrial influences. Therefore the study of processes taking place in the upper atmosphere will also contribute to our knowledge of the solar and terrestrial relationship.
The most important phenomena which takes place in the higher atmosphere and which may be observed from the ground ~ are:
- The aurora Boreales and magnete variations; 2. The night sky luminiscence; 3. the Zodiacal light; 4. Twilight luminiscence ; 5. Meteors; 6. The state of ionisation and electric conductivity and its influence on the propagation of electro-magnetic waves.
Among the luminiscence phenomena which can be studied spectroscopically, the aurora boreales no doubt call for the greatest interest and attention. Its hight and position in space can be measured very accurately. The long auroral streamers some times extending from