STUDIES OF SEASONAL BEHAVIOR OF THE SHAPE OF MESOSPHERIC SCATTERERS USING A 1.98 MHZ RADAR

Seasonal behaviour of the aspect sensitivity of mesospheric scatterers at a frequency of 2 MHz in the region of 70-98 km is studied using three independent techniques. Two techniques show that there exists a definite seasonal variation in the aspect sensitivity, with minima occurring in the southern hemisphere winter, and maxima in the summer. These are both techniques which are sensitive to the most oblate scatterers. The third technique, which involves comparisons of a vertical and an off-vertical beam and is sensitive to more isotropic scatterers, does not show any seasonal behaviour. These results support the notion of at least two distinct types of scatterers co-existing in the mesosphere. Furthermore, it is shown that aspect sensitivity can be used to estimate the length-to-depth ratio of the oblate scatterers, with this also showing definite seasonal behaviour. We emphasize that these are statistical results, accumulated using large data sets, and the existence of occasional highly aspect-sensitive scatter (far more aspect sensitive than the mean results reported here) cannot be ruled out. However, the descriptions discussed here apply for the majority of the time.