Focusing of HF radio-waves by ionospheric ducts

Abstract

This paper presents the first direct observations of HF focusing induced by natural and artificial ionospheric ducts along with a simple theoretical model. The experiments were conducted by injecting HF radio-waves using the Ionospheric Research Instrument of the High Frequency Active Auroral Research Program located in Gakona, Alaska and detecting them with instruments on the overflying French micro-satellite DEMETER. The latter observed a multiple frequency band structure, which is characteristic of a strong HF signal exceeding the detector's saturation level. Analysis of the O + density measured by DEMETER along its orbit shows that the strong radio signal coincides with the presence of a “negative” duct in the ionosphere. “Negative” refers to the presence of a plasma density depletion with the peak depletion located near the center of the duct. Such ducts induce changes in the index of refraction leading to the focusing of HF waves in a manner equivalent to a “thick” plasma lens. Examination of the data along with a simple plasma lens model indicates the presence of focal node(s) in the vicinity of the overflying satellite. Two examples, one corresponding to focusing by a natural duct and one by an artificial one are presented.