Plasmapause surface wave oscillates the magnetosphere and diffuse aurora

Space weather research heavily focuses on geospace energy circulation. The inner magnetosphere's plasmapause, a steep boundary, divides the corotating, cold, dense plasmasphere from the hot ring current/plasma sheet. Theoretical models predict plasmapause surface waves (PSWs) due to sharp inhomogeneities, acting as a source of geomagnetic pulsations. However, until now, direct evidence of these waves and their impact on magnetospheric dynamics has been lacking. This study addresses this gap by directly observing a PSW and its effects during a geomagnetic storm, leveraging multi-satellite and ground-based data. The solar wind and its embedded interplanetary magnetic field (IMF) are key drivers of magnetospheric energy dissipation. A southward IMF allows solar wind mass and energy to enter the magnetosphere, ultimately releasing into the ionosphere and upper atmosphere, creating auroras. Ultra-low frequency (ULF) waves carry much of this electromagnetic energy, playing crucial roles in geomagnetic perturbations and energizing radiation belt particles. These ULF waves can originate externally (solar wind perturbations or magnetopause surface waves) or internally (plasma instabilities). Many auroral phenomena, such as substorm onset, auroral arcs, and intensity fluctuations, are linked to magnetospheric ULF waves. While ULF waves from magnetopause surface waves propagate inward, the possibility of a reverse process remains unexplored. The plasmasphere, the inner magnetosphere, contains cold, dense plasma corotating with Earth. Its boundary, the plasmapause, separates plasmas of differing temperatures and densities. Energetic particles outside the plasmapause precipitate into the ionosphere, generating auroras. Typically, the diffuse auroral boundary is smooth under quiet conditions, mirroring the plasmapause's flat shape. However, during geomagnetic storms, particularly in the main phase, enhanced solar wind convection fields erode the plasmapause, creating a sharp radial boundary. MHD theory suggests that impulses acting on this sharp boundary generate discrete eigenmodes—PSWs. Although predicted, direct PSW observation and their role in magnetospheric dynamics remain unknown. Sawtooth-shaped auroral undulations (sawtooth aurora, SA) at the diffuse aurora's equatorward edge, observed during disturbed periods in the afternoon-evening sector, lack a conclusive explanation, despite proposed plasma instability mechanisms. The proximity of SAs and the plasmapause suggests a potential link between PSWs and SAs, which requires further investigation. This study offers the first direct observational evidence of a PSW, using conjugated satellite and ground observations, demonstrating its role as a systematic ULF wave driver in the magnetosphere.

Previous research has extensively studied the role of the solar wind and IMF in driving magnetospheric energy dissipation and the generation of ULF waves. Studies have shown the importance of ULF waves in various magnetospheric phenomena, including substorms, auroral arcs, and particle energization. Theoretical work has predicted the existence of plasmapause surface waves (PSWs) based on magnetohydrodynamic (MHD) theory, but direct observational evidence has been lacking. The origin of sawtooth-shaped auroral undulations (SAs) has been explored through several plasma instability mechanisms, but no definitive conclusion has been reached. The existing literature on ULF wave propagation primarily focuses on inward propagation from external sources, with limited understanding of outward propagation from internal sources. Previous studies have also investigated the structure and dynamics of the plasmapause and its response to geomagnetic storms, highlighting its importance in magnetosphere-ionosphere coupling. This study builds on this existing literature by providing the first direct observational evidence of a PSW and its connection to SAs, offering a new perspective on magnetospheric dynamics.

This study used a combination of multi-satellite and ground-based measurements to investigate the plasmapause surface wave (PSW) and its associated phenomena. The data included observations from the Van Allen Probes (VAP) A and B spacecraft, the Exploration of energization and Radiation in Geospace (ERG) spacecraft, the Defense Meteorological Satellite Program (DMSP) F17 and F18 satellites, the Fengyun-3D (FY-3D) satellite, and the ground-based International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer array. The VAP and ERG spacecraft provided in-situ measurements of plasma density, electric and magnetic fields, and energetic particle fluxes. The DMSP and FY-3D satellites provided images of the aurora, allowing for the observation of sawtooth aurora (SA). The IMAGE magnetometer array provided measurements of ground-based magnetic field perturbations. Specific instruments used included the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) on VAP, the Plasma Wave Experiment (PWE) on ERG, the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) on DMSP, the wide-field auroral imager (WAI) on FY-3D, and magnetometers from the IMAGE array. Data analysis involved spectral analysis of magnetic and electric field perturbations to identify the characteristics of the PSW, including frequency, wavelength, and propagation direction. Wave characteristics and phase relationships were evaluated in a field-aligned coordinate system. The phase speed and azimuthal wavelength of the SA were determined through cross-correlation analysis of emission intensity profiles from the DMSP satellites. The propagation characteristics of ULF waves generated by the PSW were examined using ground-based magnetometer data. The plasmaspheric electron density was calculated using the upper hybrid resonance frequency from the electric field data. The empirical plasmaspheric model was used to model the shape of the plasmapause for visualization. The Tsyganenko 96 magnetic field model was employed for field line mapping between the equatorial plane and the ionosphere. Statistical analysis of SA occurrences during geomagnetic storms from 2014 to 2018 was conducted using DMSP and FY-3D data. The study also calculated the azimuthal wave number from ground-based measurements and estimated the eigenfrequency of the PSW using plasma parameters from the spacecraft observations. The radial width of the plasmapause was estimated from satellite data to determine whether the conditions for surface wave excitation were met.

The key findings of this study include the following: 1. Direct observational evidence of a plasmapause surface wave (PSW) was obtained using data from multiple satellites and ground-based magnetometers during a geomagnetic storm. The VAP and ERG spacecraft observed alternating distributions of cold and hot plasmas, indicating oscillations at the plasmapause boundary. These oscillations manifested as a sawtooth-shaped plasmapause structure. 2. The PSW exhibited clear coherent phase relationships between magnetic and electric fields, consistent with a surface wave. The wave propagated sunward/westward with an azimuthal wavelength of approximately 10° and a phase speed of 0.01°s⁻¹. The azimuthal mode number (m) was estimated to be around 36. 3. The PSW was shown to generate conjugated sunward/westward-propagating sawtooth aurora (SA) in both hemispheres. The SA showed consistent azimuthal wavelength and propagation speed with the PSW, confirming the causal relationship. 4. The PSW was found to generate outward-propagating ultra-low-frequency (ULF) waves outside the plasmapause, which drove field line resonance (FLR). The ULF waves exhibited clear poleward (radially outward) propagation, starting near the SA location, which contrasts with the typical inward propagation of ULF waves generated externally. The ULF waves also showed sunward/westward propagation, consistent with the PSW and SA. 5. Statistical analysis revealed that the PSW-driven sawtooth auroras occurred in more than 90% of geomagnetic storms between 2014 and 2018, indicating that this phenomenon is a systematic and significant process in magnetosphere-ionosphere energy dissipation. 6. The characteristics of the SA, including AACGM latitude of crest, azimuthal wavelength, and crest-to-trough amplitude, varied with the intensity of the geomagnetic storms, as evidenced by FY-3D satellite observations. This suggests that PSW characteristics are also modulated by geomagnetic activity. The radial width of the plasmapause was found to be much smaller than the transverse wave scale of the PSW, satisfying the condition for surface wave excitation.

This study's findings provide direct observational evidence for the existence and importance of plasmapause surface waves (PSWs) in magnetospheric dynamics. The observed sunward/westward propagation of the PSW, its generation of outward-propagating ULF waves, and its driving of field line resonances are novel observations that expand our understanding of magnetosphere-ionosphere coupling. The high occurrence rate of PSW-driven sawtooth auroras strongly suggests that PSWs are a systematic and crucial consequence of geomagnetic storms, playing a major role in energy dissipation. The observed correlation between PSW characteristics and geomagnetic storm intensity implies a dynamic interplay between solar wind forcing and magnetospheric response. This study's findings have implications for improving space weather models and forecasting, as understanding the generation, propagation, and consequences of PSWs is critical for predicting the impacts of geomagnetic storms. While the study focuses on a single event, the high statistical occurrence rate of the associated sawtooth aurora strengthens the generalizability of the findings.

This study provides the first direct observational evidence of a plasmapause surface wave (PSW) and its significant role in driving magnetospheric dynamics and auroral activity. The observed sunward/westward propagating PSW triggered sawtooth auroras and outward-propagating ULF waves, highlighting a crucial mechanism for energy dissipation during geomagnetic storms. The high occurrence rate of this phenomenon underscores its importance in space weather processes. Future research should focus on investigating the excitation mechanisms of PSWs, including exploring the roles of various plasma instabilities and solar wind drivers. Further studies are also needed to improve our understanding of the wave-particle interactions involved and to quantify the energy transfer between different magnetospheric regions. The relationship between PSW properties and geomagnetic storm intensity warrants further investigation, which requires both advanced theoretical modelling and statistical studies based on extensive data sets.

While this study presents strong evidence for the existence and impact of plasmapause surface waves, some limitations should be acknowledged. The analysis is based on a single geomagnetic storm event, although the high statistical occurrence rate of sawtooth aurora strengthens the generalizability of the results. The spatial and temporal coverage of the auroral images used to study sawtooth auroras were limited, potentially influencing the accuracy of the wave propagation parameters. The assumptions made in calculations, such as those regarding ion composition in the plasmasphere and the use of empirical models, could introduce some uncertainties. Further research with improved spatial and temporal resolution and more detailed plasma measurements is needed to fully characterize the properties of the PSW and its effects.