Jih‐Hong Shue

The plasmapause boundary can efficiently modulate magnetosphere-ionosphere coupling. During typical geomagnetic storms, the eroded plasmapause displays periodic oscillations that manifest as giant undulations of the aurora in the ionosphere. However, it remains unresolved whether such plasmapause oscillations occur during extreme storms. Here, we confirmed the occurrence of periodic plasmapause oscillations during the most intense superstorm over the past 20 years. Based on the auroral giant undulations, the extremely eroded plasmapause, down to 2 Earth radii (R _E ), oscillated periodically at a scale of 1 R _E in response to the May 2024 superstorm. Furthermore, we analyzed different and similar signatures of plasmapause oscillations during this superstorm and compared them with the signatures during typical storms. Our results suggest that plasmapause oscillations are an intrinsic and systematic pattern of external explosive energy input into the inner magnetosphere, thus providing physical constraints for global coupling models when addressing the extreme space weather conditions.

The magnetopause is a key boundary formed by the interaction between Earth's magnetic field and the solar wind. While empirical models simplify the magnetopause as a smooth surface, observations reveal its complex fine‐scale structure. Using data from the Cluster satellites, we identified 78 magnetopause crossing events and analyzed their geometric features through principal and Gaussian curvatures. The results show that the magnetopause commonly exhibits ripple‐like structures, with approximately 72% of the events characterized by saddle‐shaped geometries, with no significant correlation to solar wind parameters. In the flank region and under quasi‐perpendicular shock conditions, the occurrence of ellipsoidal structures increases significantly, likely due to the impacts of Kelvin‐Helmholtz instability and isolated high‐speed jet collisions. Additionally, we estimated the effective phase velocity of the ripple‐like boundary structures relative to the local plasma and found an overall preference for sunward propagation along the magnetopause, with magnitudes generally smaller than the bulk plasma speed.

We investigated the spatial distributions and pitch‐angle distributions of 30–300 eV H ⁺ and O ⁺ ion fluxes using data obtained by the low‐energy particle experiments‐ion mass analyzer (LEPi) onboard the Arase satellite. A statistical analysis over 3.7 years reveals that the H ⁺ ion flux is enhanced at L > 4 from the premidnight sector through dawn to noon, with the peak location shifting inward during geomagnetic disturbances. The pitch‐angle distribution of H ⁺ ions varies with both L and geomagnetic activity, which can be interpreted as the influence of transport processes and the ring current effect. In contrast, the O ⁺ ion flux exhibits a pronounced enhancement in a confined region at L = 3–5 and 19–9 magnetic local time, with a strong dawn‐dusk asymmetry. The O ⁺ flux increases with geomagnetic activity at L = 3–5, while remaining nearly unchanged at L = 6. Its pitch‐angle distribution is consistently bidirectional and field‐aligned, with little dependence on geomagnetic conditions. These distinct spatial and pitch‐angle characteristics indicate that the observed H ⁺ ions correspond to the inner part of the warm plasma cloak, whereas the observed O ⁺ ions represent the high‐energy tail of the oxygen torus. The results further suggest that low‐energy O ⁺ ions are supplied directly from the nightside ionosphere along geomagnetic field lines, rather than being transported inward from the outer magnetosphere. Our findings demonstrate that the warm plasma cloak and the oxygen torus constitute likely independent plasma populations in the inner magnetosphere.

Energy redistribution during plasma compression or expansion remains poorly understood, particularly within induced magnetospheric systems. The polytropic index serves as an important diagnostic of this process by characterizing the thermodynamic response of temperature and pressure to density variations. This study presents the first statistical determination of the ion polytropic index in the Martian magnetosheath, leveraging 9 yr of Mars Atmosphere and Volatile EvolutioN (MAVEN) observations. Derived indices largely fall within the subisothermal range, yielding a median value of 0.42. Such polytropic behavior signifies systematic ion cooling during local compression—a thermodynamic response that departs significantly from both the adiabatic condition and the near-isothermal trends typically observed in Earth’s magnetosheath. Beyond this characteristic, a well-defined spatial trend is also identified, with the index increasing radially from the bow shock toward the induced magnetosphere boundary. The polytropic index displays a clear anticorrelation with the amplitude of low-frequency magnetic field fluctuations, indicating a close association between wave activity and ion thermodynamics in the Martian magnetosheath. In summary, this study provides a pioneering quantitative characterization of ion polytropic behavior at Mars, offering a critical benchmark for comparative analyses between induced and intrinsic magnetospheric environments.

When interplanetary magnetic field discontinuities interact with planetary bow shocks, hot flow anomalies (HFAs) form in the solar wind and can extend into the magnetosheath. Here we reconstruct the three‐dimensional geometry of an HFA bounded by two jet regions in the terrestrial magnetosheath. Using a previously established conceptual model of HFA evolution together with in situ measurements in the magnetosheath and pristine solar wind, we derive the structure's geometrical characteristics and show that its normal aligns with the discontinuity normal. It spans most of the dayside magnetosheath. Ground magnetometer data corroborate the reconstruction, revealing both the scale of the disturbance and its dusk‐to‐dawn propagation. Notably, one bounding jet reaches 11 in width, significantly larger than the sizes of typical magnetosheath jets reported in the literature.

Anomalous reconnection layer (ARL) usually appears near the magnetopause when the solar wind is in low Alfvén Mach number. The structure of an ARL is similar to the magnetic reconnection outflow region, i.e., a decrease in the total magnetic field and an increase in the high-energy ion flux. The ARL is seldom observed in the Earth’s magnetospheric environment because the solar wind at Earth is mostly in high Alfvén Mach number regime. According to previous observations, the solar wind at Mercury is usually in low Alfvén Mach number. Therefore, we assume that such an ARL can be observed frequently near Mercury’s magnetopause. To test this assumption, we examined the magnetic fields and ion fluxes obtained at the Mercury’s magnetosheath by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. With 120 events of ARLs we identified from MESSENGER’s data, we validate the assumption that ARLs frequently appear on Mercury. These ARL events were extracted from the list of MESSENGER bowshock and magnetopause crossing times compiled by Winslow et al. [2013]. The number of the ARL events found on Mercury is much larger than those found on Earth. The thickness of each ARL was estimated from the data, finding that the ARLs occupy, on average, one-fifth the thickness of the magnetosheath for Mercury. This work helps deepen our understanding of the comparative magnetospheric environment of Mercury and Earth. ReferencesWinslow, R. M., B. J. Anderson, C. L. Johnson, J. A. Slavin, H. Korth, M. E. Purucker, D. N. Baker, and S. C. Solomon (2013), Mercury's magnetopause and bow shock from MESSENGER Magnetometer observations, J. Geophys. Res. Space Physics, 118, 2213–2227, doi:10.1002/jgra.50237.

Geomagnetic activity of the magnetosphere is contributed by the direct-driven and loading-unloading processes. Since the loading-unloading process for the quiet IMF conditions is minimum, we are allowed to focus on the direct-driven process. Quasi-northward interplanetary magnetic field (IMF) has been well known to be a quiet IMF condition for direct-driven geomagnetic activity of the magnetosphere in contrast to that for a disturbed southward IMF condition. Quasi-radial and near-zero IMF are the other quiet IMF conditions, which were usually less studied in the field of magnetospheric physics. A comparison among the three quiet IMF conditions for which one is for the quietest direct-driven geomagnetic activity has not been performed in the past. Here we use two solar cycles of OMNI solar wind data and the three high-latitude geomagnetic indices (AE, PCN, PCS) to perform the comparison. We find that the quasi-northward IMF condition is the winner. The geomagnetic activity for quasi-radial IMF was never the quietest because of an extra dayside reconnection in addition to a lobe reconnection, contributing more energy into the magnetosphere than the other two quiet IMF conditions do.

Recent studies suggest that magnetosheath jets can form at the boundaries of a hot flow anomaly (HFA) during shock-discontinuity interaction by solar wind's compression and less efficient deceleration from a curved bow shock. Here, based on Magnetospheric Multiscale (MMS) data and an 3D global hybrid simulation, we report two large-scale jets at the boundaries of an HFA that together with the HFA reached more than 20 Earth radii in width, thus representing a large-scale restructuring of the dayside magnetosheath. Since shock-discontinuity interaction is a universal process that can occur at all planets, we expect that magnetosheath restructuring under such mechanisms is also universal across the solar system.

In this research, the validity of Bernoulli's theorem for the solar wind crossing the Earth's bow shock has been investigated based on space explorations. Using solar wind plasma and interplanetary magnetic field data from the OMNI database, along with magnetosheath plasma and magnetic field data measured by the magnetospheric multiscale satellite in 2016 and 2018, 95 events were selected to study the Bernoulli invariant from the upstream to the downstream of Earth's bow shock (from the unshocked solar wind region to the shocked magnetosheath region). Statistical investigations show that the differences in the characteristic energy of plasmas between the solar wind region and the magnetosheath region are less than 5%, regardless of the upstream solar wind speed and the satellite's location in the magnetosheath. These results quantitatively support the generalization of Bernoulli's equation to the solar wind flowing through the bow shock, including the contribution of magnetic field energy.