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.