Artificial periodic irregularities from the Tromsø heating facility

Abstract The Artificial Periodic Irregularity technique uses a standing wave set up by an upward propagating powerful high frequency wave and its reflection from the ionospheric F region to create a weak pattern of electron density irregularities which can cause Bragg scatter of other probing waves. This technique, which was pioneered in Russia, has been implemented at Tromso since early 1993 to investigate mainly the lower ionosphere from 50 to about 120 km. The decay times of the echoes can be used to test models of ion-chemistry in the lowest part of the ionosphere, and from the phase change one can deduce a vertical wind velocity. Some F region data is presented which show an interval of at least 10 km with enhanced backscatter just below the reflection height and, in the case of O-mode waves, also above the reflection height which decays within about 200 ms after heater switch off. These observations are repeatable. Tentative explanations for some of the backscatter features are offered in terms of a change in group delay caused by the periodic grating in refractive index as predicted by Fejer (1983).