The nature of the mesoscale field-aligned currents in the auroral oval for positive IMF BZ: More frequent occurrence in the dawnside sector than in the duskside sector

Abstract

To understand the similarities and differences between the duskside mesoscale quasistatic field-aligned currents (FACs) and the dawnside mesoscale FACs, we examined the magnetic field data obtained through the constellation measurements of Swarm satellites for a four-year period. The automatic event identification method developed in the previous study (Yokoyama Y et al., 2021) identified events of quasistatic mesoscale FACs, which are embedded in the diminished dawnside and duskside Region 1/2 current systems, in 774 passes out of 4001 passes on the dawnside, and 443 out of 3755 passes on the duskside, respectively. The dawnside and duskside mesoscale FACs have similar occurrence tendencies such that both have a relatively high occurrence ratio for positive IMF BZ, occur regardless of sunlight conditions, and have the current density increasing with the solar wind proton density. On the other hand, one notable difference was found; the occurrence ratio of the dawnside mesoscale FACs is approximately 1.7 times higher than that of the duskside mesoscale FACs. This difference is discussed in terms of the dawn–dusk asymmetry of the occurrence of Kelvin Helmholtz instabilities on the magnetopause.