Dawn-dusk asymmetry of SI-induced transient ionospheric convection

Abstract:

A statistical study using a large data set of Super Dual Auroral Radar Network (SuperDARN) observations is conducted for transient ionospheric plasma flows associated with sudden impulses (SI) recorded on ground magnetic field. The global structure of twin vortex-like ionospheric flows is found to be consistent with the twin vortices of ionospheric Hall current deduced by the past geomagnetic field observations. An interesting feature, which is focused on in this study, is that the flow structures show a dawn-dusk asymmetry. One is the difference in flow magnitude of the lower-latitude portion of the flow vortices, which is much clearer for positive SIs. The sunward flow perturbation on the dusk side is significantly larger than that on dawn side. A set of global MHD simulation runs performed by the present study have successfully reproduced the similar asymmetry of the lower-latitude transient flows. The result suggests that ionospheric conductance plays an important role in reproducing this dawn-dusk asymmetry. Another dawn-dusk asymmetry is found for the higher-latitude portion of flow vortices in which a larger flow appears either of dawn or dusk depending on the combination of the polarity of SI and IMF-By. Detailed statistics of the SuperDARN observations reveals that the dawn-dusk asymmetry of flow vortices due to IMF-By appear more clearly during negative SIs, while the flow vortices are more or less symmetric during positive SIs, regardless of IMF-By polarity. On the basis of the upstream observations, we suggest that this particular dawn-dusk asymmetry is caused by the interaction between the pre-existing round convection cell and a pair of the transient convection vortices associated with SIs. The combination of the two convection cell systems has also been reproduced by our MHD simulations.