Upper atmospheric turbulence determined by means of rockets
Résumé
The motion field of the upper atmosphere was studied with the aid of sodium trails brought to the 100-km level by means of a Véronique rocket. Below 102 km the trail shows globular distortions in the form of elements having an average diameter of 0.5 km; above that height the trail is completely smooth, though highly curved, showing no small irregularities. The turbulence state of the upper atmosphere was studied by investigating diffusion of the sodium atoms. Above 102 km the broadening of the trail can be completely accounted for by molecular diffusion; below that height the observed rate of increase of the globule diameters can be explained only by assuming a combination of molecular and turbulent diffusion. This information and the increasing chemical differentiation above 102 km strongly indicate that the 100-km level marks the difference between the turbulent and the nonturbulent parts of the upper atmosphere. The absence of turbulence in the upper atmosphere is bound to be due to the increasing relative importance of the friction forces there. The Reynolds number rapidly decreases with increasing atmospheric height. The absence or occurrence of turbulence cannot be due to shear effects: the Richardson number is too great in the region studied by us, which shows that shear turbulence is not to be expected in this part of the earth's atmosphere. Since the energy fed into the turbulence field is rather small, turbulence cannot be fully developed; in fact, it is predicted that the turbulent element can only have diameters smaller than about 2 km, which is not in disagreement with the observed element diameter of 0.5 km. The smallest visible wavelength of the motion field is of the order 50 to 100 meters, in agreement with ionospheric investigations (20 to 60 meters) and theoretical predictions (60 meters).
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