Acceleration mechanism of high-speed neutral wind observed in the polar lower thermosphere

摘要

We analyzed data obtained with the European incoherent scatter (EISCAT) Svalbard Radar (ESR) at Longyearbyen (78.2°N, 16.0°E in geographic coordinates, 75.2°A in invariant latitude) to advance our understanding of the acceleration mechanism of the lower thermospheric neutral wind when the ionospheric convection becomes enhanced. The Advanced Composition Explorer satellite observed a southward turning in the interplanetary magnetic field at 0843 UT on 16 June 2005. At 0900 UT, the F region ion velocity and the lower thermospheric neutral wind (at 118-km altitude) observed with the ESR began to accelerate significantly in the westward and northwestward directions, respectively. The neutral wind was remarkably accelerated within 1 hour from 0900 UT, and its speed became a value of —500 m s -1 at 1000 UT. The wind speed was significantly higher than a typical wind speed at 118 km. We evaluated the ion drag contribution on the generation of the high-speed neutral wind. Our evaluation verifies that the ion drag force could not generate the high-speed neutral wind within such a short time (-1 hour). This result implies that the major driving force was the horizontal pressure gradient force induced by the Joule heating. We deduced the contribution of the pressure gradient force based on a quantitative estimation of the Joule-heating-induced pressure gradient from the ESR and the Super Dual Auroral Radar Network data. We concluded that the pressure gradient force was the most probable force in this event.