Sub-auroral flow shear observed by King Salmon HF radar and RapidMAG
Abstract
We examine in detail the evolution of ionospheric flow shears in the sub-auroral region associated with alternate northward/southward turnings of the IMF. The flow shear structures are often observed in the dusk sector by the SuperDARN King Salmon (KSR) HF radar. Interestingly, some of those show the eastward (westward) flow on the lower (higher) latitude side, respectively, opposite to the typical polarity of the dusk convection cell. In those flow shear events, the IMF has a weak but persistent southward component (~ -1 to -3 nT) before onset of flow shears and following decreases of the southward IMF or even northward turning appear to cause the flow shears. The ground magnetograms provided by the Russian Auroral and Polar Ionospheric Disturbance Magnetometers (RapidMAG) show gradual increases (abrupt declines) of the H-component in association with the increases (decreases) of the merging electric field, respectively, derived from the simultaneous solar wind-IMF observations. The fairly coherent increases (decreases) of the H-component over the wide range of local time (afternoon to evening) indicate development (decay) of the large-scale DP2 current system. A detailed analysis on the 2-D convection structure near the lower latitude edge of the dusk convection cell shows that the ionospheric plasma generally flows westward there and has a larger speed with increasing latitude particularly during increases of the merging electric field. However, once the southward IMF decreases or even shifts to northward and thereby the merging electric field goes down, the region of westward flow moves toward higher latitudes and instead an eastward flow emerges there, forming a flow shear of the counterclockwise sense. This indicates that a downward field-aligned current (FAC), which is the Region-2 (R2) sense on the dusk side, flows into the flow shear region. Subsequently the convection returns to a westward flow again upon increases of the merging electric field due to the southward turning of IMF. A likely interpretation of these observations would be as follows: The R2 convection cell associated with a downward FAC is overwhelmed by the DP2 convection under the southward IMF. As the DP2 cell weakens rapidly, however, the R2 cell is still driven for a while by the partial ring current with its slowly decaying nature, resulting in the emergence of the R2 sense flow shear in the ionosphere. In other words, the ionospheric convection structure is controlled by the dynamic balance between that driven by the Region-1 FAC (the dusk DP2 cell) and that by the R2 FAC. The radar observation also reveals that those changes of convection structure take place rather quickly (~several min), very sensitive even to small variations of the merging electric field.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMSA41A1712H
- Keywords:
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- 2411 IONOSPHERE / Electric fields;
- 2431 IONOSPHERE / Ionosphere/magnetosphere interactions;
- 2463 IONOSPHERE / Plasma convection;
- 2790 MAGNETOSPHERIC PHYSICS / Substorms