Plasmapause surface wave oscillates the magnetosphere and diffuse aurora

The study investigates how sharp gradients at the Earth's plasmapause can support discrete surface eigenmodes and influence magnetosphere–ionosphere coupling. Ultra-low frequency (ULF) waves (≈0.1 mHz to 10 Hz) are key carriers of electromagnetic energy in the system and can be driven externally (solar wind, magnetopause surface waves) or internally (magnetospheric instabilities). Theory predicted a plasmapause surface wave (PSW) could be excited at the sharp plasmapause boundary, but direct in situ evidence and its dynamical consequences were lacking. Large-scale sawtooth-shaped undulations at the equatorward edge of diffuse aurora (sawtooth aurora, SA) occur during disturbed times and may be linked to plasmapause processes. The research question is whether PSWs exist and, if so, whether they modulate the plasmapause and drive SAs and ULF activity, thus contributing significantly to storm-time energy dissipation.

Background work establishes that ULF waves structure magnetospheric dynamics and auroral phenomena and are driven by solar wind perturbations and magnetopause surface waves, or internal plasma instabilities. The plasmasphere–plasmapause system exhibits sharp density and temperature gradients; MHD theory (Chen & Hasegawa, 1974) predicts that impulses at such boundaries can excite surface eigenmodes. Observational studies have reported auroral undulations (giant undulations) and links between ULF waves and auroral features, but lacked conjugate evidence tying SAs to a magnetospheric boundary wave at the plasmapause. Prior works also document field line resonances and inward-propagating ULF waves from the magnetopause; whether the reverse (outward from plasmapause) occurs was unknown.

The study uses coordinated multi-point observations during the 16 July 2017 geomagnetic storm and a multi-year survey (2014–2018):

• Spacecraft: Van Allen Probes (VAP A/B: EMFISIS magnetometer and high-frequency receiver; EFW electric fields; HOPE particle spectrometer), ERG/Arase (MGF magnetic field; PWE high-frequency analyzer; LEP-e/i particle analyzers), DMSP F17/F18 (SSUSI auroral imaging; RPA ion densities), FY-3D WAI (global auroral imaging), THEMIS-E (EFI/ESA for density), MMS-1 (FEEPS, HCPA), GOES-15 (EPS).
• Ground-based: IMAGE magnetometer array latitudinal and longitudinal chains; SuperMAG chain for additional geomagnetic pulsations.
• Data processing: Electron density derived from upper hybrid resonance frequency and cyclotron frequency; coordinate transformations (SM, GSM, AACGM); bandpass filtering around 1.4–1.5 mHz; spectral analysis; phase relations among B and E components in field-aligned coordinates; cross-correlation of auroral intensity profiles from sequential DMSP passes to derive SA phase speed and azimuthal wavelength; dual-satellite timing between VAP-A/B to estimate PSW azimuthal wavelength, m-number, and westward phase speed; ground cross-phase between IMAGE stations to estimate m and propagation speed, and to identify FLR via amplitude maxima and 180° phase reversal; estimation of PSW eigenfrequency using plasmapause field line parameters (B≈400 nT, ne≈1500 cm−3, typical ion composition) and field line length from T96 model; assessment of plasmapause sharpness (~0.1–0.2 RE width) vs PSW transverse scale (~0.8 RE at ~4.5 RE) to validate surface-wave conditions.
• Event survey: DMSP and FY-3D auroral imagery during storms with Dst < −40 nT (2014–2018) to quantify SA occurrence; subset with dusk-sector ground magnetometers to assess ULF propagation and FLR during SAs.

• Direct identification of a plasmapause surface wave (PSW) during the 16 July 2017 storm: strong wave activity localized at the plasmapause, ceasing inside plasmasphere; mixed poloidal (Br, Eθ) and toroidal (Bθ, Er) components with characteristic phase relationships (e.g., Br nearly antiphase with Eθ at −164±9°, Bθ ~90° out of phase with Eθ and Vr ~90° lagging Br).
• Wave frequency: peaks at ~1.4–1.5 mHz (VAP and ERG).
• Azimuthal properties from VAP-A/B: wavelength ≈10°±0.3°, westward (sunward in MLT) phase speed ≈0.010±0.001° s−1, azimuthal mode number m ≈36±1.
• Equatorial magnetic perturbations show 180° phase differences across the magnetic equator and the plasmapause boundary, consistent with a fundamental PSW eigenmode.
• Conjugate sawtooth aurora (SA) at the equatorward edge of diffuse aurora observed by DMSP in both hemispheres map to the sawtooth-shaped plasmapause; SA phase speed ≈0.01±0.001° s−1 westward and azimuthal wavelength ~6.4–10.2°, decreasing with increasing MLT, matching PSW parameters.
• PSW drives outward (poleward) propagating ULF waves outside the plasmapause, producing field line resonances (FLR) observed by IMAGE: at ~1.4±0.5 mHz, N-component amplitude maximizes near ~66° MLAT (MAS), with ~180° phase change across the peak; ground east-component shows westward propagation speed ≈−0.012° s−1 and m ≈36.8±0.6, consistent with space-based PSW/SA.
• Estimated fundamental poloidal eigenfrequency from plasmapause parameters is ~1.35–2.11 mHz, consistent with observations (~1.5 mHz).
• Excitation: storm-time particle injections (11:00–11:30 UT) provided periodic pressure enhancements with power spectral density peaking at ~1.5 mHz, supplying energy to excite and sustain the PSW.
• Statistical survey (2014–2018): SAs occur in >90% of geomagnetic storms (94/103). In 24 events with dusk-sector ground data, ULF pulsations coincided with SAs, propagated radially outward (mapped to equator), and peaked outside the plasmapause.
• FY-3D WAI imaging shows SA structural parameters vary with storm strength (Dst), e.g., azimuthal wavelengths ~3.4–12.8° and crest-to-trough amplitudes ~1.8–8.8° across cases.

Findings confirm the existence of a discrete plasmapause surface eigenmode that couples fast compressional waves with shear Alfvén waves at a sharp Alfvén speed gradient. The PSW modulates plasmapause geometry into a sawtooth pattern, enabling hot plasma intrusions into low-density regions; wave–particle interactions (e.g., ECH and chorus) scatter particles, producing sawtooth diffuse aurora that maps to the PSW crest–trough pattern and shares its kinematics. Unlike magnetopause-driven waves that propagate inward, PSW-driven ULF waves propagate outward, causing FLR outside the plasmapause, a previously undocumented process. The excitation is consistent with storm-time injections whose pressure variations include discrete (~1.5 mHz) and broadband components, providing sustained energy input. The results distinguish PSW/SA signatures from other auroral forms (e.g., omega bands, giant pulsations) by sector, propagation, and resonance characteristics. The ubiquity of SAs during storms implies PSWs are a common pathway for energy redistribution in the inner magnetosphere and ionosphere, with implications for other rapidly rotating planetary magnetospheres where corotation breakdown and sharp boundaries are prevalent.

This work provides the first direct in situ and conjugate observational evidence of plasmapause surface waves, links them causally to sawtooth aurora as their optical manifestation, and demonstrates that PSWs drive outward-propagating ULF waves and FLR outside the plasmapause. The PSW parameters (frequency ~1.5 mHz, m ~36, wavelength ~10°, westward phase speed ~0.01° s−1) are consistent across satellite and ground measurements. A multi-year survey shows SAs occur in >90% of storms, indicating PSWs are a systematic, storm-time mechanism for magnetosphere–ionosphere energy coupling. Future research should develop theory for PSW azimuthal mode selection and evolution, quantify dependencies on plasmapause sharpness and external/internal pressure drivers, improve ground signature characterization, and conduct statistical studies with expanded instrument coverage to refine propagation and resonance diagnostics.

• In situ detection of PSWs is challenging due to limited satellite crossings of the plasmapause at appropriate times and locations; the study relies on a few conjunctions and model-based mapping (e.g., T96) to connect aurora to the equatorial plasmapause.
• Ground signature interpretation of surface waves (including PSW) is not yet fully established; propagation direction and speed estimates are constrained by the available station geometry and may be affected by plasmapause shape.
• Phase differences (e.g., Br vs Er) are not perfectly stable; some parameters have uncertainties.
• Ion composition in the plasmasphere was not directly measured during the storm; eigenfrequency estimates use typical composition assumptions.
• The SA occurrence survey has incomplete spatial/temporal coverage; 9 storms could not be classified due to imaging limitations.