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

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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.