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. We provide an explanation for a mysterious phenomenonthat has been recognized in recent years in European Incoherent Scatter(EISCAT) UHF incoherent scatter radar (ISR) measurements during many high-power high-frequency (HF) ionospheric pumping experiments. The phenomenon is an apparentincrease in electron density observed above the HF reflection altitude,extending up to the observable limits usually in the range 400–650 km, asshown in several publications in recent years. It was shown by Senior et al. (2013) that several examples of these enhanced backscatter could not beexplained by increases in electron density. A summary of characteristics ofthe backscatter enhancements is presented as well as the results of a surveyof events. We propose that medium- to large-scale HF-induced field-alignedirregularities (tens to hundreds of metres scale) act to refract the radarsignals along the magnetic field, thereby acting as a guide so that thefree-space r−2 spreading of the signals no longer applies. The nature ofthe irregularities and the physical mechanism of their production bypowerful HF waves is an exciting topic for future research since,surprisingly, they appear to be preferentially excited by X-mode waves. Theexplanation proposed here involving HF-induced irregularities may well applyto other ISR observations of the ionosphere in the presence of specificnatural irregularities.

Ducting of incoherent scatter radar waves by field-aligned irregularities