Archive | 2019
Swarm field-aligned currents during a severe magnetic storm of September 2017
Abstract
Swarm satellites observations are used to characterize the extreme behavior of largeand small-scale fieldaligned currents (FACs) during the severe magnetic storm of September 2017. Evolution of the current intensities and the equatorward displacement of FACs are analyzed while the satellites cross the pre-midnight, pre-noon, dusk and dawn sectors on both hemispheres. The equatorward boundaries of FACs mainly follow the dynamics of ring current 10 (as monitored in terms of the SYM-H index). The minimum latitude of the FAC boundaries is limited to 50° MLat. The FAC densities are very variable and may increase dramatically, especially on the nightside ionosphere during the storm-time substorms. At the peak of substorm, the average FAC densities reach >3 μA/m. The dawn–dusk asymmetry is manifested in the enhanced dusk-side R2 FACs on both hemispheres. Filamentary high-density structures are always observed confirming that a substantial fraction of R1/R2 FACs is composed of many small-scale 15 currents. In the pre-noon sector, the bipolar structures (7.5 km width FACs of opposite polarities adjacent to each other) dominate, while in the post-midnight sector the upward and downward FACs tend to form more latitudinally extended structures of a certain polarity. The most intense small-scale FACs (up to ~80 μA/m) is observed just in the post-midnight sector. Simultaneous magnetic and plasma perturbations indicate that this structure is likely a current system of a mesoscale auroral arc. 20