NASA NTRS1983
The difference in character between mesospheric returns from about 70 and about 80 km was noted. The 69-km echo is characterized by a single return with about .1 Hz width, while the 82.5 km return extends over more than 3 Hz bandwidth; this difference is also perceptible, but to a lesser degree, on the fading curves. The conclusion seems inescapable that internal random velocities of a few m/s are present within the scattering volume for the 82.5-km echo. The most likely source for these rather large velocities is convective instability arising from deformations of the temperature profile by breaking gravity waves. The distinction between the two types of scatter at these altitudes probably accounts for the behavior with frequency of the nightime fading period at low and very low frequencies. The fading period of D-region reflections at night was constant at about 7 min from 16 to 43 kHz, but that at frequencies of 70 kHz and above, the fading period decreased in such a way as to indicate the presence of irregularities smaller than about 1 km in size. This suggests that frequencies of 48 kHz and above, the fading period decreased in such a way as to indicate the presence of irregularities smaller than about 1 km in size. This suggests that frequencies of 48 kHz and below were reflected primarily from the region below 80 km where the narrow spectral irregularities dominate.