Dynamic subauroral ionospheric electric fields observed by the Falkland Islands radar during the course of a geomagnetic storm

[1] We present an analysis of ionospheric electric field data observed during a geomagnetic storm by the recently deployed HF radar located on the Falkland Islands. On 3 August 2010 at ∼1800 UT evidence of the onset of a geomagnetic storm was observed in ground magnetometer data in the form of a decrease in the Sym-H index of ∼100 nT. The main phase of the storm was observed to last ∼24 hours before a gradual recovery lasting ∼3 days. On 4 August, during the peak magnetic disturbance of the storm, a high velocity (>1000 m s−1) channel of ionospheric plasma flow, which we interpret as a subauroral ion drift (SAID), located between 53° and 58° magnetic south and lasting ∼6.5 hours, was observed by the Falkland Islands radar in the pre-midnight sector. Coincident flow data from the DMSP satellites and the magnetically near-conjugate northern hemisphere Blackstone HF radar reveal that the SAID was embedded within the broader subauroral polarization streams (SAPS). DMSP particle data indicate that the SAID location closely followed the equatorward edge of the auroral electron precipitation boundary, while remaining generally poleward of the equatorward boundary of the ion precipitation. The latitude of the SAID varied throughout the interval on similar timescales to variations in the interplanetary magnetic field and auroral activity, while variations in its velocity were more closely related to ring current dynamics. These results are consistent with SAID electric fields being generated by localized charge separation in the partial ring current, but suggest that their location is more strongly governed by solar wind driving and associated large-scale magnetospheric dynamics.