Cluster Multipoint Observations of the Spatial Structure and Time Development of Auroral Acceleration Region Field-aligned Current Systems, Potentials, and Plasma

Abstract:

The auroral acceleration region is an integral part of the magnetosphere-ionosphere electrodynamic system, and plays a key role in the transport of plasma and energy between Earth and space. This region is embedded with field-aligned currents that couple the magnetosphere to the ionosphere and is where parallel electric fields form that accelerate plasma to and from these regions. Though considerable progress has been made, the complex interplay between field-aligned current system formation, the development of parallel electric fields, changes in the plasma constituents, and auroral emissions consequences are not fully understood. The Cluster mission is well suited for studying the structure and dynamics of the auroral acceleration region. Over its lifetime, Cluster has sampled much of this region with closely spaced probes enabling the distinction between temporal effects from spatial variations. Moreover, this data when combined with auroral images from IMAGE or THEMIS GBO-ASI enable an assessment of the auroral emission response to spatial morphology and temporal development of structures seeded in the auroral acceleration region. In this study we present a survey of Cluster multi-point traversals within and just above the auroral acceleration region (≤ 3 Re altitude). In particular we highlight the spatial morphology and developmental sequence of auroral acceleration current systems, potentials and plasma constituents, with the aim of identifying controlling factors, and assessing ionospheric consequences under different conditions. Our results suggest that the "Alfvénic" activity may be an important precursor and perhaps may be playing an essential role in the development of "quasi-static" current systems during quiet and substorm active times. Such events are generally the result of an injection mediated process at or near the plasma sheet boundary layer, resulting in the local expansion of the plasma sheet. Key features of the conversion from Alfvén wave dominated to "inverted-V" current systems, such as transition times, spatial scales, and relationship to electric field, plasma density, and distribution features, and other properties will be discussed. Causal linkages between such processes and auroral emission features will also be addressed.