Aspect sensitivity of stratospheric VHF radio wave scatterers, particularly above 15‐km altitude

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Abstract
The aspect sensitivity of VHF radar scatter in the stratosphere is investigated with particular emphasis being placed on the region above 15‐km altitude. It is found that the scatter is highly aspect sensitive below about 18 km in the stratosphere as is well known, but above this height the scatter becomes more isotropic again. The highest degree of aspect sensitivity occurs around 18‐km altitude, where the backscatter polar diagram has a 1/e half‐width θs of less than 1.5°, and the horizontal correlation length can exceed 30 m. At heights above 20 km, however, θs is in excess of 4° and increases with increasing height. At any one height there is a range of scatterer shapes, from scatterers with large length‐to‐depth ratios up to almost isotropic scatterers. These results are confirmed using studies of backscattered power as a function of angle, comparisons of radial velocities as a function of beam bore‐tilt angle, and studies of the spectral width compared to the expected beam‐broadened spectral width. The results complement and support the only other published study of scatterer anisotropy in the region of 15‐ to 30‐km altitude (Hocking et al., 1986).