Polar Cap Precursor of Nightside Meso-scale Auroral Enhancements

Abstract

Author(s): Zou, Ying | Advisor(s): Lyons, Lawrence R | Abstract: Although the large-scale auroral oval has been well examined, what controls the occurrence of meso-scale enhanced auroras remains ambiguous. The answer is critical for forecasting ionospheric scintillation and remote-sensing magnetospheric disturbances. One important meso-scale auroral enhancement is poleward boundary intensifications (PBIs), which are intensifications along the poleward boundary of the nightside auroral oval and are produced by magnetic reconnection, a fundamental energy conversion process in the magnetosphere. This thesis is dedicated to understand when and where PBIs and the associated magnetotail reconnection occur, spontaneously or driven by external forcing, based on coordinated observations of all-sky imagers (ASIs) and radars. We found that the occurrence of PBIs is consistently accompanied with narrow and fast flows near the auroral poleward boundary in the polar cap. They occur simultaneously with or 1–2 min before PBIs near the PBI longitude, and direct equatorward toward the auroral poleward boundary. They have a duration and width comparable to those of PBIs. Although the prior flow evolution could not be obtained based on the limited radar field-of-view, a polar cap ASI can substantially expand our observing area by enabling flow tracing using airglow patches and polar cap arcs. It shows that deep in the polar cap, localized fast flows typically propagate at ~600 m/s, persist tens of minutes to hours, and are of a few hundred km width. They appear as channels of flow enhancements that are elongated in the noon-midnight meridian, and significantly contribute to magnetic flux convection across the polar cap. The mosaic ASI images further show that as these fast flow propagate equatorward from the magnetic pole and impinge on the nightside auroral poleward boundary, they are followed by intensifications within the auroral oval that are spatially connected to them and occur within a few minutes and l~ 10? longitudes. Such intensifications are major disturbances that do not occur until the impingement of polar cap flows, suggesting the intensifications to be triggered by these flows. Our results suggest that locally enhanced nightside auroras, and the associated magnetic reconnection, can be preceded by, and developed around, localized flow enhancements arriving at the auroral poleward boundary from the polar cap. This preceding signature in the polar cap is essential to understand the development of magnetotail reconnection, and gives the potential of forecasting the specific time and location of disturbances in the magnetosphere and ionosphere. The existence and importance of such polar cap precursor were not recognized in the past due to the lack of coordinated radar and ASI observations, as well as broadly covered mosaic from an ASI array.