Auroral post-secondary ions from the nightside ionosphere in the inner magnetosphere

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• 作者：George Sofko ; Robert Schwab ; Masakazu Watanabe ; Chaosong Huang ; John Foster ; Kathryn McWilliams
• 关键词：Secondary ions ; Bounce– ; drift motion ; Shielding ; SAPS
• 刊名：Journal of Atmospheric and Solar-Terrestrial Physics
• 出版年：2007
• 期刊代码：41_13646826
• 类别：ear
• 出版时间：July 2007
• 卷：69
• 期：10-11
• 页码：1213-1232
• 文件大小：1292 K

摘要

During magnetically disturbed conditions, the auroral zone is occupied by widespread aurora mostly created by precipitating primary auroral electrons associated with upward field-aligned currents (FACs) that are highly variable in space and time. The energy flux carried by these electrons is substantial, with each kilo-Rayleigh (kR) of optical emission being associated with a flux of about 1 erg cm⁻² s⁻¹. The production of secondary ions and electrons is large, because only about 35 eV of precipitating electron energy is needed to produce a secondary pair. The auroral ionosphere, as well as the region above it, is very dynamic. Parallel electric fields can occur there, as inferred from observations of beam-like secondary ion signatures. Ions also undergo transverse acceleration of ions (TAI), likely due to wave-particle interactions. As a result, these suprathermal “auroral post-secondary (APS) ions” acquire a markedly different energy spectrum from the secondary electrons, with APS ion energies in the range between thermal (1 eV) and auroral (1 keV). The APS ions acquire enough energy to escape gravity and then execute bounce motion along the magnetic field, while simultaneously undergoing drift motion dominated by convective drift, rather than the curvature-gradient drift of the higher energy auroral and ring-current ions. As a result, the ion drift trajectories follow magnetospheric equipotential surfaces that project to the ionosphere as flow streamlines. On the basis of their convection paths subsequent to their production and acceleration, they can be subdivided into three categories, namely Dusk APS (APS/D) and Subauroral APS (APS/S) ions in the evening convection cell, and Morning APS (APS/M) ions in the morning convection cell. Satellite observations are presented to show the unique energy spectra of the APS/D, APS/S, and APS/M ions, and their multi-energy band structure, with each band having a dispersive signature characterized by lower energy at lower latitude. Two possible effects of these ions are discussed. The first is the role of APS/S ions in the production of subauroral radar echo regions in the plasmatrough. The second is a decrease in the morning-side negative shielding charge because of the arrival of the positive APS/M ions that have bounced and convectively drifted from an auroral injection region near midnight.