Abstract
Field-aligned currents (FACs) flowing in the auroral ionosphere are a complex system of upward and downward currents, which play a fundamental role in the magnetosphere–ionosphere coupling and in the ionospheric heating. Here, using data from the ESA-Swarm multi-satellite mission, we studied the complex structure of FACs by investigating sign-singularity scaling features for two different conditions of a high-latitude substorm activity level as monitored by the AE index. The results clearly showed the sign-singular character of FACs supporting the complex and filamentary nature of these currents. Furthermore, we found evidence of the occurrence of a topological change of these current systems, which was accompanied by a change of the scaling features at spatial scales larger than 30 km. This change was interpreted in terms of a sort of symmetry-breaking phenomenon due to a dynamical topological transition of the FAC structure as a consequence of FACs and substorm current wedge intensification during substorms.
Highlights
This was justified by the fact that there is much evidence for a north–south asymmetry in the processes taking place in the polar ionospheric region, which could be a consequence of the different topology of the geomagnetic field and the different insolation/seasonal conditions [44,45,46,47]
Field-aligned currents (FACs) is the same over at least two orders of magnitude (τ ∈ [1, 100] s), and it is characterized by a cancellation exponent κ 0 ∼ 0.63; (iii) during periods of disturbed geomagnetic conditions, i.e., substorm times, we observed the occurrence of a symmetry-breaking in the sign-singularity features of the FACs, which were characterized by the same cancellation exponent of quiet conditions at the shortest timescales (τ < 4 s), but not at long timescales (τ > 4 s), where the cancellation was κ 00 ∼ 0.33
These results suggested that the FACs undergo a topological change during geomagnetic substorms, which is characterized by an increase of the fractal dimension D⊥
Summary
Iijima and Potemra [4] (see [5,6]) were the first to study the detailed structure of the Birkeland currents in the high-latitude ionosphere and to investigate their link with the occurrence of geomagnetic substorms. They identified two main FAC regions, Region 1 (R1) and Region (R2), which connect the high-latitude ionosphere to different magnetospheric regions, the dayside and flanks of the magnetopause, and the tail current sheet, respectively
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