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1. Mercury's Altered Magnetosphere During a Sub‐Alfvénic ICME Event: MESSENGER Observations and Inferred Asymmetric Alfvén Wing Formation From Global MHD Simulations

   https://doi.org/10.1029/2025JA034248  

   Bowers, Charles F; Jackman, Caitríona M; Jia, Xianzhe; Hadid, Lina Z; Sun, Weijie; Hayes, Laura A; Dewey, Ryan M; Burkholder, Brandon; Hollman, Daragh M; Cervantes, Sebastian; et al (November 2025, Journal of Geophysical Research: Space Physics)

   Abstract We present observations of a rare configuration of Mercury's magnetosphere in response to sub‐Alfvénic upstream conditions, driven by an interplanetary coronal mass ejection (ICME) that impacted the planet on 1 May 2013. Using data from the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft, supported by a global three‐dimensional magnetohydrodynamic (MHD) simulation of the event, we demonstrate that Mercury's magnetospheric response during this interval was distinct from the typical super‐Alfvénic state. During the sub‐Alfvénic upstream conditions, MESSENGER measured a distorted magnetotail with a depleted southern magnetotail lobe. An MHD simulation closely reproduces these observations, providing a plausible global context for the reconfiguration of Mercury's magnetosphere under sub‐Alfvénic conditions. The simulation predicts that a pair of Alfvén wings formed during this event, redirecting magnetic flux and plasma within the magnetosphere. The interplanetary magnetic field orientation during this event was primarily sunward/dawnward, generating asymmetric Alfvén wings with respect to the flow direction, in contrast to previously observed north–south wing configurations at the planet. Using Solar Orbiter observations in the inner heliosphere, we estimate that the solar wind is sub‐Alfvénic approximately 2.5 times per Earth year near solar maximum, with intervals lasting between 10 s and 12 hr. Studies of these rare, sub‐Alfvénic solar wind‐magnetospheric interactions provide valuable insights into exoplanet–stellar wind interactions under similarly sub‐Alfvénic conditions where in situ observations are not available. 

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2. Interplanetary Magnetic Field B _y Controlled Alfvén Wings at Earth During an Encounter of a Coronal Mass Ejection

   https://doi.org/10.1029/2024GL113416  

   Chen, Yuxi; Dong, Chuanfei; Chen, Li‐Jen; Sarantos, Menelaos; Burkholder, Brandon L (March 2025, Geophysical Research Letters)

   Abstract On 24 April 2023, an ICME reached Earth's orbit. The solar wind density dropped to 0.3 amu/cc while the IMF strength was about 25 nT. As a result, the solar wind flow transitions to a sub‐Alfvénic state with an Alfvén Mach number of 0.4. We carry out global magnetohydrodynamic simulations to investigate the responses of Earth's magnetosphere to the ICME ejecta. The results show the formation of Alfvén wings as the solar wind becomes sub‐Alfvénic. Furthermore, the sub‐Alfvénic period was characterized by the dominance of the IMF component, causing the Alfvén wings to extend toward the dawn and dusk flanks. We investigate the global magnetospheric convection of this sub‐ Alfvénic case and find that the overall convection is mediated by the Alfvén wings, while the magnetic field convection in inner magnetosphere is similar to the super‐Alfvénic case. 

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3. Suppression of Magnetopause Reconnection in the Presence of Cold Magnetospheric Plasma

   https://doi.org/10.1029/2025EA004408  

   Khanal, Krishna; Zou, Ying; Zank, Gary P (September 2025, Earth and Space Science)

   Abstract The temporal variability of magnetopause reconnection is an important aspect of solar wind magnetosphere coupling. Even under stable solar wind driving, reconnection can be triggered, modulated, or suppressed because of magnetic field and plasma conditions near the magnetopause boundary. We analyze a unique event in which a THEMIS satellite crosses the subsolar magnetopause three times within a 5 min interval in the presence of a cold‐ion population on the magnetospheric side of the boundary. During the first crossing, the satellite detects reconnection outflow and a D‐ shaped ion velocity distribution earthward from the boundary, indicating an active reconnection. The signatures disappear during the second crossing when the magnetospheric cold‐ion density increases significantly and reappear during the third crossing when the magnetospheric density drops to a level comparable to that of the first crossing. The solar wind and magnetosheath conditions do not change much during the interval. The magnetospheric population is evidently associated with a plasmaspheric plume with considerable variation in density. According to the theory of mass loading, the presence of such a plume population results in the local Alfvén speed at the second crossing being 40% smaller compared to the first and third crossings. However, the theory itself does not suggest suppression. We discuss possible suppression mechanisms considering the additional effects of the prevailing solar wind and local magnetopause conditions. 

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4. 3-D global hybrid simulations of magnetospheric response to foreshock processes

   https://doi.org/10.1186/s40623-021-01469-2  

   Shi, Feng; Lin, Yu; Wang, Xueyi; Wang, Boyi; Nishimura, Yukitoshi (December 2021, Earth, Planets and Space)

   Abstract It has been suggested that ion foreshock waves originating in the solar wind upstream of the quasi-parallel ( Q -||) shock can impact the planetary magnetosphere leading to standing shear Alfvén waves, i.e., the field line resonances (FLRs). In this paper, we carry out simulations of interaction between the solar wind and terrestrial magnetosphere under radial interplanetary magnetic field conditions by using a 3-D global hybrid model, and show the properties of self-consistently generated field line resonances through direct mode conversion in magnetospheric response to the foreshock disturbances for the first time. The simulation results show that the foreshock disturbances from the Q -|| shock can excite magnetospheric ultralow-frequency waves, among which the toroidal Alfvén waves are examined. It is found that the foreshock wave spectrum covers a wide frequency range and matches the band of FLR harmonics after excluding the Doppler shift effects. The fundamental harmonic of field line resonances dominates and has the strongest wave power, and the higher the harmonic order, the weaker the corresponding wave power. The nodes and anti-nodes of the odd and even harmonics in the equatorial plane are also presented. In addition, as the local Alfvén speed increases earthward, the corresponding frequency of each harmonic increases. The field-aligned current in the cusp region indicative of the possibly observable aurora is found to be a result of magnetopause perturbation which is caused by the foreshock disturbances, and a global view substantiating this scenario is given. Finally, it is found that when the solar wind Mach number decreases, the strength of both field line resonance and field-aligned current decreases accordingly. 

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5. Transport and Acceleration of O ⁺ Ions in Upstream Solar Wind Due To Impact of an IMF Discontinuity: 3D Global Hybrid Simulation

   https://doi.org/10.1029/2023GL103883  

   Wang, Chih‐Ping; Wang, Xueyi; Lin, Yu (August 2023, Geophysical Research Letters)

   Key Points First global 3D hybrid simulations to investigate the fate of O + ions after escaping the dayside magnetosphere New transport/acceleration path for escaped O + ions in upstream solar wind region after impact of an IMF tangential discontinuity New transport/acceleration path brings some of escaped O + ions back to the inner magnetosphere, contributing to O + ring current pressure 

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