Properties of Mesoscale Flows in the Nightside Auroral and Subauroral Ionosphere
In addition to the traditional large-scale convection, nightside plasma sheet transport involves a significant amount of meso-scale fast flows that carry a large amount of magnetic flux. Those flows are coupled to the ionosphere, and auroral manifestations of such fast flows include auroral streamers and substorm onset. Sub-auroral polarization streams (SAPS) are another fast-flow phenomenon occurring just equatorward of the electron auroral oval. While in-situ measurements can only detect these mesoscale transient phenomena at a limited number of points or can characterize them statistically, measuring flows and aurora in the ionosphere allows us to evaluate the instantaneous two-dimensional evolution. In this thesis we first investigate the structure of these flow bursts using two-dimensional line-of-sight flow observations from the SuperDARN radars and auroral images from the THEMIS ground-based all-sky imager (ASI) array. Radar echoes captured at horizontal distances <~500 km from the radars were mainly used to detect small-scale flow structures that would otherwise be missed or poorly resolved in long-range radar echoes. After identifying 135 auroral streamers in the ASI images near radar echoes, we examined the flow evolution and properties of flow channels. Flow bursts and streamers are invariably correlated in all events. The flow bursts are often directed equatorward and appear simultaneously with the streamers. Equatorward flows are located just to the east of the streamers. Less frequently (~10% of the time), a poleward flow enhancement was detected even when a streamer propagated equatorward, the poleward flow enhancement being located to the west of the auroral streamer, or to the east of the equatorward flow enhancement, consistent with the expected spatial relationship between flow shear and upward field-aligned currents in plasma sheet flow bursts. The azimuthal width of the flow channel is on average ~75 km, and the azimuthal offset of the equatorward flow channel relative to the auroral streamer is ~57 km eastward. This study demonstrates the capability of radar-imager pairs for identifying the 2-D structure of localized flows associated with plasma sheet flow bursts.
In the second part of this thesis, we investigated ionospheric flow patterns associated with auroral onset beads using line-of-sight flow observations from the SuperDARN and auroral images from the THEMIS ground-based ASI array. We selected events that occurred when the SuperDARN radars operated in a high temporal resolution THEMIS mode (6 seconds) along northward looking beams, a time resolution comparable to that of the imagers, providing a unique tool to detect properties of flows associated with auroral onset beads. We have found very fast oscillating flows (~1000 m/s) that are correlated with the onset beads propagating across the THEMIS-mode beam meridian. 2-d radar measurements also show a wavy pattern in the azimuthal direction with a wavelength of ~78 km, which is close to the azimuthal separation of individual beads. We also used an imager and SuperDARN in Iceland and identified weak but significant azimuthal flow modulations associated with beads. These strong correlations (in time and space) between auroral beading and the fast ionospheric flows suggest that substorm onset occurs via an instability in the inner plasma sheet and is associated with intense flow shears. The flow shear is clockwise around auroral beads, consistent with converging electric fields associated with upward field-aligned currents in the shear center.
Finally, we present simultaneous measurements from the THEMIS ASI array and the 2-d SuperDARN radar measurements that share fields of view with the imagers to investigate the association between SAPS flow enhancements and auroral streamers. We first identified auroral streamers in the ASI images to analyze the ionospheric flow variations at subauroral latitudes. We also performed a reverse study starting with flow variations and then analyzed the auroral condition from the ASI. In this study we analyzed a total of 104 events. For the case of streamers observed near the equatorward boundary of the auroral oval, we find average westward flow enhancements of ~700 m/s slightly equatorward of the streamers. Our statistical results show that for the first part of this study (forward study, starting from streamers) 98% of the streamers that reach close to the equatorward boundary lead to SAPS westward flow enhancements. For the reverse study (starting from flow enhancements), we have found that streamers and enhanced convection nearly equally contribute to SAPS flow enhancements. We also characterize the SAPS flow channel width and timing relative to streamers reaching radar echo meridians. The strong influence of auroral streamers on the rapid evolution of SAPS flows suggests that transient fast earthward plasma sheet flows can lead to westward SAPS flow enhancements in the subauroral region, and that such enhancements are far more commonly occurring than during substorms alone, based on the frequent occurrences of streamers under various geomagnetic conditions.