M-I Coupling: Summer-Winter Asymmetry of Nightside Field-Aligned Currents

场向电流(FAC)和极光加速度是相互关联的，在磁层-电离层(M-I)耦合中是最重要的。最近的研究发现，在夜间，冬季(黑暗)半球的极光发射和电子降水往往比夏季(阳光充足)半球更强烈，观测到的极光发射的不对称性表明，尽管两个半球的极光区域由磁力线连接，但冬半球的背景电离层电导较低，但冬半球的场向电流更强。这个拟议的项目试图通过观察测试三种可能的解释来研究这个自相矛盾的问题。这些解释是：(1)冬半球向上的FAC在纬度上更受限或更有结构，造成局部更密集的电流；(2)冬半球增强的电子降水过度补偿了太阳照明造成的电离层电导的不对称；(3)夏季和冬半球的区域1电流在磁尾有不同的来源，M-I耦合分别在两半球发生。来自DMSP-F7、F12和F13卫星的磁强计数据和GeoTail数据将用于研究电离层上方的FAC。将对这些数据集应用自动识别FAC结构的程序，并将定量确定FAC的特征，如电流强度、电流密度、FAC薄片的纬向厚度和空间不规则性。该项目将比较夏季(有阳光的)和冬季(黑暗)半球的这些特征。拟议的项目将是首次全面研究夜间FAC的夏季-冬季不对称性，并将为电离层电导在M-I耦合中的作用提供新的线索。

The field-aligned current (FAC) and the auroral acceleration are related to each other and are most important in the magnetosphere-ionosphere (M-I) coupling. The recent studies have found that on the nightside, auroral emission and electron precipitation tend to be more intense in the winter (dark) hemisphere than in the summer (sunlit) hemisphere.The observed asymmetry of auroral emission, suggests that the field-aligned current is more intense in the winter hemisphere, even though auroral regions in two hemispheres are connected by the magnetic field line, and the background ionospheric conductance is lower in the winter hemisphere. The proposed project seeks to study this paradoxical problem by observationally testing three possible explanations. Those explanations are (1) The upward FAC is more confined or structured in latitude in the winter hemisphere creating locally more dense currents; (2) The enhanced electron precipitation in the winter hemisphere overcompensates for the asymmetry of ionospheric conductance due to solar illumination; and (3) The region 1 currents in the summer and winter hemispheres have different sources in the magnetotail, and the M-I coupling takes place separately in each hemisphere.Magnetometer data from the DMSP-F7, F12, and F13 satellites and Geotail data will be used to examine FACs above the ionosphere. An automatic procedure to identify FAC structures will be applied to those data sets, and the characteristics of FACs such as current intensity, current density, latitudinal thickness of FAC sheets, and spatial irregularity will be quantitatively determined. The project will compare those characteristics between the summer (sunlit) and winter (dark) hemispheres.The proposed project will be the first comprehensive study of the summer-winter asymmetry of nightside FACs and will shed new light on the role of the ionospheric conductance in the M-I coupling.