<i>Swarm</i> field-aligned currents during a severe magnetic storm of September 2017

Abstract

Abstract. Swarm satellites observations are used to characterize the extreme behavior of large- and small-scale field-aligned 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 in both hemispheres. The equatorward boundaries of FACs mainly follow the dynamics of ring current (as monitored in terms of the SYM-H index). The minimum latitude of the FAC boundaries is limited to 50° MLat, below which saturation occurs. The FAC densities are very variable and may increase dramatically, especially in the nightside ionosphere during the storm-time substorms. At the peak of substorm, the average FAC densities reach 3 μA/m2, while the quite level is below 0.1 μA/m2. The dawn–dusk asymmetry is manifested in the enhanced dusk-side R2 FACs in both hemispheres. Filamentary high-density structures are always observed confirming that a substantial fraction of R1/R2 FACs is composed of many small-scale 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/m2) 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.

Response to Referee 3

“…Here, these structures may represent a signature of the cusp plasma injections which are accompanied by pairs of FACs generated due to flux transfer event (FTE) formation (Southwood, 1987) or localised reconnection at the magnetopause. Magnetic topologies associated with FTEs were previously observed by the MEO satellites (Marchaudon et al, 2004;2006;Pu et al, 2013) but not by the LEO satellites. Neubert and Christiansen (2003) reported small-scale currents primarily found in the cusp and pre-noon region with densities 1-2 orders of magnitude larger than R1 and R2 currents.…”

“…Reply to Comment 3.2 A brief description of the results obtained by the authors cited has been added to the forth para (former 2nd) as follows. "Utilizing the magnetic field measurements by CHAMP satellite Wang et al (2006) "The mission has a multi-instrument payload. The main module is the high-sensitivity vector (fluxgate type) and scalar magnetometers for determining the magnitude and direction of the total vector and variations of the geomagnetic field with an accuracy of more than 0.5 nT (Merayo et al, 2008).…”