Spatial and temporal variations of the atmospheric sodium layer observed with a steerable laser radar
LASER radar observations of resonance scattering from atmospheric sodium during the night have, on occasions, shown evidence of short term changes in the height distribution1–3. It has been suggested that these changes arise from the movement of a horizontally structured sodium layer, but the measurements have all been limited to the vertical direction, and have provided no direct evidence of a horizontal variation. Evidence of horizontal structure at mesospheric heights has been provided by observations of hydroxyl emissions in the airglow, both from the ground4 and from an aircraft5,6. In addition, the observed fluctuations of intensities and infrared rotational temperatures of the hydroxyl bands have been associated with the passage of atmospheric gravity waves7,8. We report here observations intended to examine the short term and small scale horizontal variations of the sodium layer which might also be associated with such waves.
Lidar observations of a horizontal variation in the atmospheric sodium layer
Abstract A steerable laser radar system has been used to measure the distributions of sodium between 80 and 105 km by observations of resonance scattering in three directions from Winkfield (51.4°N, 0.7°W). Simultaneous observations of Rayleigh scattering from heights of 20 to 35 km were used to normalise the sodium concentration. Sequences of measurement at zenith angles of 30° have permitted a comparison to be made of the sodium concentrations at three locations separated horizontally by distances of about 80 km. The results for two nights in August 1976 showed clear evidence of a horizontal variation near the peak of the layer, the concentration changing by about 50% between locations. The results for a third night did not show such a variation near the peak of the layer, the major differences being observed at greater heights.
On the possibility of ice cloud formation at the mesopause
As a contribution to the discussion about the nature of noctilucent clouds some aspects of the ice particle hypothesis are considered. A model of an ice cloud is computed, assuming a temperature profile and wave motions in agreement with observations. Some of the observed characteristics of noctilucent clouds are well reproduced by the model. Similar computations for stratiform clouds show that a diffusion coefficient of the order 10 7 cm 2 /sec seems to be required to keep particle sizes within the observed range. It is concluded that the ice particle theory and the cosmic dust theory should be considered as equally important in future studies of noctilucent clouds. DOI: 10.1111/j.2153-3490.1962.tb01339.x