Ground magnetometer networks show that very different drivers of geospace disturbance — interplanetary magnetic field (IMF) reorientations that produce penetration electric fields, sudden solar wind dynamic pressure impulses, magnetospheric substorm onsets, and ultralow-frequency (ULF) waves — generate remarkably similar global ionospheric equivalent disturbance current systems that are completely re-established within about one minute (Huang, 2020). The source paper explicitly left open the central attribution and mechanism questions behind this phenomenology: (i) the relative quantitative contributions of ionospheric horizontal Hall/Pedersen currents, Region-1/Region-2 field-aligned currents, the ring current, and magnetopause currents to the observed equivalent disturbance currents and their latitude/magnetic-local-time dependence are undetermined; and (ii) the dominant transmission physics of the sub-minute global response — the zeroth-order transverse magnetic (TM0) mode in the Earth–ionosphere waveguide or fast magnetosonic waves — and the conditions under which each mode controls the low-latitude response are not established. Neither question has been resolved in the literature to date, and the work on the individual components remains active and partly conflicting. The global Pc5 wave-characteristics sub-issue flagged in the same paper was subsequently treated in detail by Huang (2021) and is not part of the open problem addressed here.

Background

Ionospheric disturbance currents have been inferred from ground magnetometers for more than a century. The standard construct is the equivalent current: the two-dimensional horizontal current system flowing at ionospheric height whose magnetic field at the ground reproduces the observed horizontal perturbation field; for a thin-sheet ionosphere it is obtained from the ground perturbation components by a 90-degree rotation, and it is an integral quantity that blends the effects of all current systems — horizontal ionospheric currents (Hall currents perpendicular to, and Pedersen currents parallel to, the ionospheric electric field), field-aligned currents (FACs) flowing along magnetic field lines between the magnetosphere and ionosphere (the Region-1 FAC system near the auroral oval poleward boundary and the Region-2 FAC system equatorward of it), the ring current of trapped energetic particles encircling Earth, and the currents on the magnetopause boundary of the magnetosphere. Key disturbance phenomena include: penetration (or prompt-penetration) electric fields, which transmit high-latitude convection electric fields to low latitudes almost immediately after an IMF southward turning, and shielding electric fields that oppose them after IMF northward turning; the associated two-vortex ionospheric current system is historically called the DP2 current system. Sudden commencements (SCs) are the global magnetic signatures of rapid solar wind dynamic pressure changes striking the magnetosphere, each with a preliminary impulse and a main impulse. ULF (ultralow frequency) waves in the Pc5 band (periods of roughly 150–600 s) are magnetohydrodynamic fluctuations excited at the magnetopause or by field-line resonances. At the dayside magnetic equator, the eastward equatorial electrojet is enhanced by the Cowling effect, the strong eastward current that arises when a vertical (downward) polarization electric field suppresses the vertical Pedersen current in the low-conductivity direction. Joule heating — dissipation of electric-field energy through Pedersen currents — is the dominant energy sink of the high-latitude ionosphere–thermosphere during disturbances. Huang (2020) derived the equivalent disturbance currents (net changes after removal of the quiet-time SqS_q daily variation and baselines) from global magnetometer networks for 11 events spanning these four driver classes and found, surprisingly, that the equivalent current systems are nearly independent of the driver: a double-vortex system (large dayside/evening vortex plus a small vortex near dawn) for IMF reorientations and substorm onsets, and a single-vortex system at middle and low latitudes for pressure impulses and ULF waves, with the whole global pattern re-established within about 1 minute. Two candidate mechanisms could explain such rapid global establishment: the zeroth-order transverse magnetic (TM0) mode in the Earth–ionosphere waveguide, in which electromagnetic disturbances propagate from high to low latitudes at essentially the speed of light (Kikuchi & Araki, 1979), and fast magnetosonic waves, the compressible magnetohydrodynamic fast mode, which simulations show can transmit energy and momentum from the polar cap to the equator with propagation times of a few seconds and a new steady state within about 20 s (Tu & Song, 2019); MHD-wave propagation has also been invoked for low-latitude preliminary-reverse-impulse signals (Chi et al., 2001). The paper stated explicitly that the relative importance of the different current sources in producing the equivalent currents is beyond its scope, that both transmission candidates can explain the observations without a determination of which dominates, and that the relation between the disturbance currents and the total current system, with its energetic consequences, is a different, unaddressed topic.

Problem Statement

For the global ionospheric disturbance current systems observed with ground magnetometer networks in response to the four driver classes identified by Huang (2020) — IMF reorientations (penetration and shielding electric fields), sudden changes in solar wind dynamic pressure, magnetospheric substorm onsets, and ULF waves — determine quantitatively: (1) the relative contributions of the ionospheric horizontal (Hall and Pedersen) currents, the Region-1 and Region-2 field-aligned currents, the ring current, and the magnetopause currents to the ground-inferred equivalent disturbance currents, as functions of magnetic latitude and magnetic local time, for each driver class; and (2) the transmission physics that dominates the establishment of the global current system within roughly one minute — specifically whether the zeroth-order transverse magnetic mode in the Earth–ionosphere waveguide, fast magnetosonic waves, or some combination of the two controls the response, and under what driver, ionospheric-conductance, and geomagnetic conditions each mode governs the low-latitude response. The question includes resolving the high-latitude structure of the small near-dawn current vortex and its relationship to the Region-1 versus Region-2 field-aligned current distribution, which sparse magnetometer coverage left undetermined in the source analysis.

References

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