Ionospheric Irregularities and High-multiple Reflections

Abstract

The occurrence of high· multiple reflections (10 hops and more) from the F. layer of the ionosphere at night is considered. Sunspot-cycle, annual, and diurnal variations are presented. The sunspot. cycle and annual variations are similar to those for the upper-atmosphere neutral particle density. Periodicities in occurrence of around 60 min are found. Associations are found between high-multiple trace occurrence and sunset and sunrise times at the 90 km level. Ionospheric irregularities which are present at the time of high.multiple reflections indicate that these reflec· tions are probably not due to focusing effects. Evidence is presented to support a mechanism involving the reduction of non·deviative absorption to explain these reflections. This reduction may be caused by acoustic waves propagating in the high atmosphere.