Evolution of negative SI‐induced ionospheric flows observed by SuperDARN King Salmon HF radar

Abstract

The spatial evolution of vortex‐like flow structures induced by a negative sudden impulse (SI−) is studied on the basis of SuperDARN King Salmon HF radar (KSR) with other ground and satellite data. A large dip in the solar wind density induced a fairly large SI− with a SYM‐H amplitude of ∼40 nT. The SI‐induced ionospheric flow signatures in the evening sector (MLT ∼ 19 h) were observed by KSR as a westward flow associated with the preliminary impulse (PI) followed by a more intense eastward flow with the main impulse (MI) in the sub‐auroral region of the magnetic latitude ∼60–70 deg, consistent with the local ground magnetic field observations. Following the first PI‐MI flow sequence, KSR saw a second and possibly third sequence of flow variation which were much smaller in flow amplitude than the first pair but showed qualitatively very similar flow variations and latitudinal/longitudinal propagation characteristics. These observations can be interpreted as aftershocks of the first PI‐MI; the same sequence of vortices and field‐aligned currents were generated and then drifted anti‐sunward with the same mechanism, namely the pumping motion of the dayside magnetosphere. These results are qualitatively consistent with predictions suggested by recent numerical simulations.