Abstracts Astronomy & Space Science

Properties of Mesoscale Flows in the Nightside Auroral and Subauroral Ionosphere

by Lacourt Beatriz Gallardo

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…