Implications of statistics of near-range Doppler velocity observed with the Syowa East HF radar

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A large data-set of line-of-sight Doppler velocity obtained with the Antarctic Syowa East HF radar from February to December 1997 is analyzed to discuss the statistical characteristics of Doppler velocity (V_D) at ranges of 180-1200 km and their implications. Syowa Station K-indices during the observation period were between 0 and 7 with a maximum occurrence at K = 1. On average V_D has a minimum of about 100 m/s at 180-225 km ranges. With increasing range it increases monotonically to attain a maximum of 300-350 m/s at 400-500 km, decreases gradually to reach 250-300 m/s at about 700km, and again increases slowly at farther ranges. These values of V_D and the range vary depending on both local time and radar beam direction. In the light of recent knowledge of plasma instabilities in the ionosphere we suggest that such range profile of V_D is mainly caused by the combined effects of altitude-dependent phase velocities of ionospheric plasma waves, HF wave refraction due to enhanced E region electron density, and latitude-dependent electric field. We infer that the low V_D (〜 100 m/s) at ranges of 180-225 km may originate in part from neutral winds and/or turbulence of the neutral atmosphere.
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