Abstract
A statistical study using a large dataset 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 depending on the combination of the polarity of SI and interplanetary magnetic field (IMF)-By. Detailed statistics of the SuperDARN observations reveal that the dawn-dusk asymmetry of flow vortices due to IMF-By appears during negative SIs, while such asymmetric characteristics are not seen during positive SIs. 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.
Highlights
Sudden impulse (SI) is a rapid increase or decrease in intensity of a geomagnetic field recorded almost simultaneously by the world-wide network of magnetic observatories and typically lasts for a few tens of minutes (Araki 1977)
The flow perturbation associated with positive SIs and negative SIs is deduced as described in the previous section and shown in the top two panels and bottom two panels, respectively, of Fig. 3
In addition to the SI polarity, we have divided the dataset into the positive interplanetary magnetic field (IMF)-By and negative IMF-By cases to examine dependence of the flow perturbation on the IMF-By polarity
Summary
Sudden impulse (SI) is a rapid increase or decrease in intensity of a geomagnetic field recorded almost simultaneously by the world-wide network of magnetic observatories and typically lasts for a few tens of minutes (Araki 1977) Such geomagnetic variations are caused mainly by interplanetary shocks and tangential discontinuities propagating in the solar wind (Chao and Lepping 1974). Both stepwise increases and decreases of the geomagnetic field are often observed globally (Nishida and Jacobs 1962a; Nishida and Jacobs 1962b) and are referred to as positive sudden impulses (positive SIs) and negative sudden impulses (negative SIs), respectively. In addition to the spatial structure, a two-phase perturbation with a preliminary impulse (PI) and the subsequent main part of an SI, referred to as main impulse (MI), are interpreted as an alternate evolution of the twin vortex current systems of opposite polarity
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