A space hurricane over the Earth's polar ionosphere

The study investigates whether hurricane-like disturbances can occur in Earth’s upper atmosphere and, if so, how they form and transfer energy and momentum from the solar wind–magnetosphere system into the ionosphere. While hurricanes are well known in the troposphere, analogous features had not been reported in planetary upper atmospheres. Prior observations include auroral spirals in the nightside oval and high-latitude dayside auroral (HiLDA) spots during northward IMF, but these lacked key hurricane characteristics (large-scale spiral structure, strong flow shears and an eye). Energy coupling to the magnetosphere-ionosphere typically occurs via magnetic reconnection (dominant for southward IMF) or Kelvin–Helmholtz instability (under northward IMF with higher solar wind speed/density). During extremely quiet conditions (northward IMF, low solar wind density/speed), coupling is generally thought to be weak. The purpose of this work is to present and explain a large, long-lived upper-atmospheric “space hurricane” observed over the polar cap during such quiet conditions, and to identify the mechanisms responsible for its formation and geospace impacts.

- Auroral spirals (~50 km scale) occur in the nightside auroral oval but are not unusually intense and do not resemble full hurricane structures (Davis & Hallinan, 1976; Partamies et al., 2001). - High-latitude dayside auroral (HiLDA) spots occur during northward IMF with strong positive By and are associated with electron precipitation and NBZ field-aligned currents but lack hurricane-like spiral morphology (Frey et al., 2003, 2004; Korth et al., 2004; Carter et al., 2018; Frey, 2007). - Solar wind–magnetosphere coupling via magnetic reconnection (Dungey, 1961; Fairfield & Cahill, 1966; Perreault & Akasofu, 1978; Vasyliunas et al., 1982; Nishida, 1983) is efficient for southward IMF at the low-latitude magnetopause; Kelvin–Helmholtz instability facilitates transport under northward IMF with sufficiently high solar wind speed/density (Hasegawa et al., 2004; Guo et al., 2010; Li et al., 2013; Wang et al., 2013). - Under northward IMF with dominant By, high-latitude lobe reconnection tailward of the cusp governs polar cap convection and FAC patterns (Lockwood & Moen, 1999; Tanaka, 1999, 2007; Zhang et al., 2016). Knight (1973) and Lyons (1981) describe current–voltage relations enabling field-aligned acceleration to maintain current continuity. - Prior HiLDA observations lacked spatial resolution and coincident ionospheric drift to diagnose circular sheared flow and an eye, precluding identification of hurricane-like structures.

Observational event and conditions: - Date: 20 Aug 2014. Prolonged northward IMF (Bz > 0 for >8 h) with strong duskward By (~13 nT). Low solar wind speed (~340 km/s) and number density (~2 cm^-3), dynamic pressure ~0.5 nPa. THEMIS-B provided IMF/solar wind data, lagged by 9.5 min to the dayside magnetopause. Geomagnetic indices (SYM-H, AU/AL) indicated non-storm, quiet conditions. Space- and ground-based measurements: - DMSP F16 SSUSI auroral imaging (LBHS 140–150 nm) over Northern Hemisphere captured a cyclone-like auroral spot (>1000 km diameter) with multiple arms and anticlockwise trend around the magnetic pole. Four DMSP satellites observed consistent circular fast flows across MLT sectors. - DMSP F16 SSIES provided cross-track ion drift (near north-south) showing zero horizontal flow at the center (eye) and strong shears: sunward duskside up to ~2100 m/s and antisunward dawnside up to ~800 m/s. SSIES also measured ion upflows; electron temperature enhanced by ~1000 K. - DMSP F16 SSM magnetometer data were used to compute field-aligned current (FAC) along track, showing strong upward FAC associated with the hurricane and a negative-to-positive bipolar magnetic perturbation (circular B-field signature). - AMPERE (Iridium constellation) global FAC maps showed a co-located spot-like strong upward FAC (>1.5 µA/m^2) embedded within a negative potential cell, closed by downward cusp FAC equatorward, consistent with circular/vortical ionospheric convection. - DMSP F16 SSJ4 precipitating particle spectrometers measured electron energy flux and spectra. Within the hurricane, total time-integrated electron energy flux ΣJE = 2.48×10^14 eV/(cm^2·sr) over 110 s (16:16:58–16:18:51 UT), 91.49% of the entire polar pass ΣJE = 2.71×10^14 eV/(cm^2·sr). Average electron energy flux EnF_avg ≈ 2.25×10^12 eV/(cm^2·s·sr). Electron energies exhibited inverted-V acceleration: ~10 keV near center, ~1 keV at edges. Ion precipitation was negligible in the hurricane region. - Comparative passes (quiet without hurricane, typical southward IMF non-storm, superstorm) were compiled (Tables 1–2) to contextualize energy flux and spectra. Modeling: - Data-driven, high-resolution 3-D global MHD simulation using PPMLR-MHD with measured interplanetary inputs for the event. The model captures solar wind–magnetosphere–ionosphere coupling with low numerical dissipation and high shock resolution. - Coordinates: Cartesian (GSM-like): X sunward, Y dawn-dusk, Z north. Domain: X from +30 to −100 RE; Y,Z from −50 to +50 RE; grid 320×320×320; minimum grid spacing ~0.15 RE. - Inner magnetospheric boundary at 3 RE to avoid plasmasphere complexities; electrostatic ionosphere shell with height-integrated conductance couples to magnetosphere. Earth’s field approximated as a dipole of 8.06×10^22 A/m^2. - Simulation outputs: 3-D and 2-D FAC distributions, plasma velocity vectors, and field-line topology. Identified steady high-latitude lobe reconnection sites around cusp boundaries; reconnected open field lines draped dawnward then tailward in the lobe. A strong upward-FAC funnel connecting to the ionosphere reproduced the observed spot with multiple arms and anti-clockwise trend; downward FAC sheets on duskside matched observed arcs.

- First report of a long-lasting space hurricane in Earth’s polar ionosphere under extremely quiet geomagnetic conditions (northward IMF, low solar wind density and speed). Duration ~8 hours; diameter >1000 km; multiple-arm, cyclone-shaped aurora with anticlockwise trend near the north magnetic pole; no conjugate counterpart in the Southern Hemisphere under strong By conditions. - Ionospheric plasma flow exhibited a nearly zero-flow center (eye) and strong circular shears: sunward duskside flow up to ~2100 m/s and antisunward dawnside up to ~800 m/s; clockwise circulation in ion drift despite anticlockwise auroral arm trend. - Strong, localized upward FAC spot (>1.5 µA/m^2) co-located with a negative potential cell; surrounded and closed by downward cusp FACs, ensuring current continuity. Along-track magnetic perturbations showed a negative-to-positive bipolar structure consistent with circular currents. - Significant particle and thermal signatures: electron temperature enhanced by ~1000 K; pronounced inverted-V electron acceleration with energies up to ~10 keV near center and ~1 keV at edges; negligible ion precipitation in the hurricane area, indicating origin along open lobe field lines. - Energetics: Within the hurricane, ΣJE = 2.48×10^14 eV/(cm^2·sr) over 110 s, accounting for 91.49% of the entire polar pass ΣJE = 2.71×10^14 eV/(cm^2·sr). Average electron energy flux in hurricane region ~2.25×10^12 eV/(cm^2·s·sr), about 28.8× higher than adjacent auroral oval (~7.81×10^10) and 71.7× higher than diffuse aurora (~3.14×10^10) during the same pass. Whole-pass average during the hurricane event was ~15.1× higher than a typical quiet pass, ~8.3× higher than a typical southward IMF non-storm pass, and ~3.2× higher than a superstorm pass. Hurricane-region fluxes are comparable to superstorm levels and exceed typical substorm values. - Formation mechanism: Observations and PPMLR-MHD simulations attribute the hurricane to steady high-latitude lobe reconnection under northward IMF with dominant By, producing a strong upward-FAC funnel and circular convection lobe-cell embedded in the afternoon convection cell. Field-aligned electric fields (Knight-type current–voltage relation) accelerate electrons, generating the auroral hurricane. Simulation reproduced the FAC spot with arms, associated flow, and duskside current sheets consistent with observed arcs. - Implications: Despite quiet geomagnetic indices, the hurricane enabled large, rapid energy and momentum deposition into the polar ionosphere, suggesting standard indices may miss such high-latitude activity. Potential space weather impacts include enhanced satellite drag, HF communication disturbances, and navigation/radar errors.

The findings demonstrate that efficient solar wind–magnetosphere–ionosphere coupling, with intense energy and momentum deposition, can occur during otherwise quiet geomagnetic conditions via steady high-latitude lobe reconnection. The space hurricane’s eye, strong circular shear flows, and concentrated upward FAC funnel explain the observed auroral cyclone and intense electron precipitation. The flow pattern (clockwise ion drift with anticlockwise auroral arm trend) highlights differences from tropospheric hurricanes and reflects electrodynamic constraints of magnetosphere–ionosphere coupling and current continuity. Simulations validate the proposed mechanism, showing reconnection tailward of the cusp under northward IMF with strong By, transport of open field lines dawnward then tailward in the lobe, and closure of upward FAC through cusp-aligned downward FAC. The hurricane functions as a particularly efficient conduit for energy transfer when solar wind speed and density are low, challenging the assumption that quiet indices imply weak high-latitude coupling. Its location poleward of geomagnetic observatories suggests current indices underrepresent such events. The phenomenon may be common at other magnetized planets and moons, with broader implications for plasma interactions with interplanetary or interstellar flows.

This work reports and explains a large, long-lived space hurricane over the northern polar ionosphere during extremely quiet geomagnetic conditions. Multi-instrument observations (DMSP, AMPERE, THEMIS-B) revealed a cyclone-shaped aurora, an eye with near-zero flow, strong circular shears, and a localized upward FAC spot producing intense inverted-V electron precipitation (~10 keV). A data-driven PPMLR-MHD simulation reproduced a strong upward-FAC funnel linked to steady high-latitude lobe reconnection under northward IMF with dominant By, confirming the formation mechanism and current-closure pathways. The study shows that significant energy and momentum deposition can occur poleward of the auroral oval even when global indices are quiet, implying a need to refine space weather monitoring to capture high-latitude, polar cap processes. Future work should survey the occurrence rate and morphology of space hurricanes across solar wind conditions, develop indices sensitive to polar cap electrodynamics, investigate thermospheric responses (e.g., neutral winds and density/drag), and assess the prevalence of analogous phenomena at other magnetized bodies.

- Event-based study centered on a single well-observed interval (20 Aug 2014); broader occurrence rates and variability remain to be established. - Simulation employs an inner boundary at 3 RE, a dipole field approximation, and height-integrated ionospheric conductance, which may limit representation of inner-magnetospheric and plasmaspheric effects. - Instrument coverage, spatial resolution, and mapping uncertainties (e.g., DMSP in-situ altitude vs. auroral mapping altitude) can introduce offsets between flows and auroral features and may limit detailed structure resolution.