Resistive global-MHD simulations with the MagPIE magnetosphere–ionosphere model reproduce the morphology of ionospheric field-aligned current (FAC) signatures produced when flux transfer events (FTEs) reconnect with cusp field lines, but the peak FAC amplitudes (∼±1 μA m−2\sim\pm1\ \mu\mathrm{A\,m^{-2}}) fall roughly 40×40\times below a compared SWARM observation, and it is undetermined whether the FAC patches are locally generated by cusp–FTE reconnection or are pre-existing currents channelled through newly reconnected flux tubes. The open problem is to establish, at the mechanism level, how cusp–FTE reconnection produces the observed ionospheric FAC signatures: identify the FAC generation-versus-channelling mechanism, quantify how collisionless reconnection and kinetic-scale physics modify FAC amplitudes, reconnection rates, and propagation signatures relative to resistive MHD, and determine whether the morphological correspondence with observed FAC pairs generalises beyond the single compared event.

Background

Magnetic reconnection between the solar-wind interplanetary magnetic field (IMF) and Earth's dipole field at the dayside magnetopause episodically tears off rope-like magnetic structures called flux transfer events (FTEs). Southwood (1987) predicted that the reconnection-associated FACs — currents flowing along magnetic field lines (Birkeland currents) — should travel to the ionosphere as shear Alfvén waves (magnetohydrodynamic waves propagating along the field at the Alfvén speed), depositing a characteristic bipolar up–down current pair that drives a twin-vortex ionospheric flow pattern. Glassmeier & Stellmacher (1996) and Crooker & Siscoe (1990) developed the mapping of FTEs to ionospheric footpoints near the polar cusp (the funnel of open field lines converging on the dayside high-latitude ionosphere), and observational work has long associated FTEs with poleward-moving auroral and flow features, enhanced flow channels, and travelling convection vortices (TCVs). A long-standing puzzle for this framework is whether the strong ionospheric currents seen during FTE passage are generated by the reconnection itself or are pre-existing currents merely redirected (channelled) along newly opened field lines; no definitive explanation of TCV generation had been provided.

Paul, Strugarek & Vaidya (2023) addressed this with MagPIE, a two-way coupled global magnetosphere–ionosphere model in which resistive magnetohydrodynamics (MHD, using uniform or specified resistivity rather than collisionless kinetics) is coupled to a height-integrated thin-shell ionosphere with prescribed Pedersen and Hall conductances (the height-integrated ionospheric conductivities that close field-aligned currents through horizontal ionospheric currents). Their simulations showed that reconnection between an FTE flux rope and cusp field lines produces ionospheric FAC patches with a distinct morphology — an 'I'-shaped patch surrounded by a 'U'-shaped patch of opposite sign — extending well equatorward onto closed field lines, resembling discrete dayside auroral arcs and generating vortex flows, and morphologically matching one specific FAC pair observed by the SWARM low-Earth-orbit satellites on 7 October 2015. However, the simulated peak FAC magnitudes (consistently near ±1 μA m−2\pm1\ \mu\mathrm{A\,m^{-2}}) are approximately 4040 times lower than that observed event, which the authors attribute to collisionless reconnection rates being much higher than the resistive-MHD approximation allows; the simulated angular propagation speed (about 4.9×10−3 rad s−14.9\times10^{-3}\ \mathrm{rad\,s^{-1}}) exceeds the selected observation by about an order of magnitude, with the causes (model physics, resistivity choice, field-line mapping approximations, or sampling assumptions) not isolated. The comparison rested on a single SWARM event, so generality to other observed FAC-pair events is undemonstrated. The magnetosheath and high-altitude cusps operate in a collisionless regime accessible only to kinetic methods — hybrid codes (kinetic ions, fluid electrons), such as the hybrid-Vlasov Vlasiator model — which evolves the ion velocity distribution function directly in six-dimensional position–velocity phase space — recently coupled to an ionosphere (Ganse et al. 2025), or embedded particle-in-cell (PIC) regions — which quantitatively modify reconnection rates and current generation. Distinguishing local FAC generation from channelling of pre-existing FACs requires diagnostics such as field-line topology tracing or Alfvén-wave-packet tracing, as demonstrated for the large-scale Region 1 current system (the innermost, permanent Birkeland-current circuit of the polar ionosphere) by Ebihara & Tanaka (2024).

Problem Statement

Establish how cusp–flux-transfer-event (FTE) reconnection produces the ionospheric field-aligned-current (FAC; current flowing along magnetic field lines) signatures attributed to it, by answering three linked questions:

(1) Mechanism: are the dominant FAC patches observed at the ionosphere during FTE passage generated locally by reconnection between the FTE flux rope and cusp field lines, or do they arise mainly from channelling of pre-existing field-aligned currents through newly reconnected flux tubes — and with what diagnostic evidence (e.g. field-line topology or current-source tracing, controlled simulation experiments) that discriminates the two?

(2) Magnitude and kinetic physics: quantitatively, how do collisionless reconnection and kinetic-scale physics (e.g. via hybrid-Vlasov, hybrid, or embedded particle-in-cell (PIC) simulations) modify the FAC amplitudes, reconnection rates, and ionospheric signatures relative to the resistive magnetohydrodynamic (MHD) MagPIE results — in particular, does kinetic physics remove the roughly 40×40\times deficit between simulated peak FACs (∼±1 μA m−2\sim\pm1\ \mu\mathrm{A\,m^{-2}}) and observed event amplitudes, and what accounts for the order-of-magnitude discrepancy in angular propagation speed (model physics, resistivity choice, mapping approximations, or sampling assumptions)?

(3) Generality: is the morphological correspondence between cusp–FTE reconnection signatures and observed FAC pairs robust across other observed events in which FAC pairs are seen, or specific to the single compared SWARM (ESA's low-Earth-orbit magnetic-field satellite constellation) event?

The three questions are linked but independent: resolving one does not by itself resolve the others, and whether (1) supports or refutes cusp–FTE reconnection as the general cause of these signatures is distinct from the kinetic-physics account of the magnitudes, rates, and propagation signatures in (2) and from the robustness across further observed events in (3).

References

  1. Paul, Arghyadeep; Strugarek, Antoine; Vaidya, Bhargav (2023). Global-MHD Simulations Using MagPIE: Impact of Flux Transfer Events on the Ionosphere. Journal of Geophysical Research: Space Physics 128(11), e2023JA031718. DOI 10.1029/2023JA031718; arXiv:2311.16134. https://doi.org/10.1029/2023JA031718
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