ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-27-3065-2009 HF echo types revisited: aspect angle attenuation effects Makarevich R. A. 1 Carter B. A. 1 Department of Physics, La Trobe University, Melbourne, Victoria 3086, Australia 06 08 2009 27 8 3065 3075 Copyright: © 2009 R. A. Makarevich 2009 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://angeo.copernicus.org/articles/27/3065/2009/angeo-27-3065-2009.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/27/3065/2009/angeo-27-3065-2009.pdf

The high-spatial-resolution observations of the auroral E-region echoes by the Super Dual Auroral Radar Network (SuperDARN) HF radar at Pykkvibaer, Iceland considered in a previous study are re-examined. Both the spectral power and Doppler velocity of the E-region HF echoes exhibit strong dependence on the slant range and expected off-perpendicular magnetic aspect angle (aspect angle attenuation). We consider the aspect angle attenuation effects in identification of spectral HF echo types. It is argued that echoes with Doppler velocities close to the nominal value of the ion-acoustic speed <I>C<sub>s</sub></I> (~350 m/s) and echoes with velocities greatly exceeding the <I>C<sub>s</sub></I> (up to 600 m/s) are more related in their generation mechanisms than previously thought. It is proposed to treat the echoes near the <I>C<sub>s</sub></I> as aspect-angle-attenuated counterparts of the high-velocity echoes and that both types are generated directly by the modified two-stream instability with aspect angle attenuation resulting in the preferential phase velocity saturation at the <I>C<sub>s</sub></I>.

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