Hiroshi Hasegawa

This study demonstrates the effectiveness of physics-informed neural networks (PINNs) for reconstructing Grad–Shafranov (GS) equilibrium structures from synthetic, noise-free trajectory-like data representative of satellite measurements under both GS and magnetohydrostatic constraints. The proposed framework focuses on inferring continuous two-dimensional equilibrium structures from limited and sparsely sampled measurements along trajectories analogous to satellite sampling using static equilibrium solutions without explicitly provided boundary conditions. The sinusoidal representation network-enhanced architecture played a key role in capturing fine-scale structures. By incorporating uncertainty weighting, the model ensured balanced optimization, leading to higher accuracy and more effective enforcement of physical constraints. Additionally, the study highlights the importance of diverse training data, including multi-component measurements and data from multiple trajectories. The integration of such data significantly improved model performance by reducing errors and uncertainty while stabilizing training dynamics. These findings suggest that, even in the absence of explicit boundary conditions, which are commonly incorporated in PINN research, multi-component and multi-trajectory data can provide additional directional information that guides the solution process, reduces reconstruction ambiguity, and improves the reliability of the reconstructed solution.

We present a novel data analysis technique based on physics‐informed neural networks (PINNs) to reconstruct two‐dimensional (2D) magnetohydrodynamic (MHD) and Hall MHD equilibria in a space plasma from in situ spacecraft measurements. Our method incorporates the steady‐state MHD or Hall MHD equations—a set of partial differential equations (PDEs) as physical constraints—into a deep learning framework. In contrast to traditional reconstruction techniques relying on explicit spatial integration of the PDEs from spacecraft trajectories, the PINN approach allows us to obtain a physically consistent equilibrium by minimizing a composite loss function that includes both prediction errors and PDE residuals. We validate the method through benchmark tests using exact solutions of axially symmetric MHD and Hall MHD equilibria, and further demonstrate its utility through application to an extensively investigated magnetotail reconnection event observed by the Magnetospheric Multiscale mission. The resulting reconstruction reproduces key structural features, including the X‐type current sheet geometry and quadrupolar Hall magnetic field, consistent with prior results and numerical simulations. We also present empirical estimates of the reconnection rate and spatially varying resistivity based on the reconstructed fields. Our results demonstrate that the PINN‐based method provides a flexible and powerful new tool for revealing plasma structures around observing spacecraft, particularly in regions where model assumptions may be only approximately valid.

Auroral arcs in the high latitude are frequently observed during periods of northward interplanetary magnetic field (IMF). However, their generation mechanisms are not yet fully understood, due to the limited availability of simultaneous in-situ and optical auroral observations. In this talk, we present observations of recurrent tailward-propagating auroral arcs in the dawnside during a period of northward IMF. THEMIS satellites located near the dawnside magnetopause in the equatorial plane observed strong fluctuations in magnetic field and plasma parameters with periods of 5–7 minutes, suggesting the presence of surface waves or ULF waves. Subsequently, shorter-period fluctuations of 2-3 minutes were observed for an hour, which may indicate the development of Kelvin–Helmholtz instability (KHI). During these intervals, a ground-based all-sky imager detected recurrent auroral arcs detached from the poleward boundary of the auroral oval, with a period of 5–10 minutes, propagating predominantly tailward. DMSP observations revealed multiple auroral arcs on the dawnside, accompanied by electron precipitation associated with paired upward and downward field-aligned currents. Based on these coordinated space- and ground-based observations, we discuss several possible generation mechanisms for the observed auroral arcs, including surface waves, ULF waves, and KHI.

Magnetic reconnection is a fundamental physical process operating throughout the Universe, yet its onset in planetary magnetotails remains elusive. Current-sheet flapping is a common dynamic phenomenon in planetary magnetotails, with its roles in driving reconnection largely underexplored. This paper reports the first observation of electron-dominant and ion-coupled reconnection accompanied by current sheet flapping. The Magnetospheric Multiscale mission (MMS) spacecraft was located in the premidnight sector current sheet, which was undergoing kink-mode flapping that propagated duskward, away from the midnight sector, with a frequency of 0.06–0.1 Hz and a wavelength of around 0.5 R _E . The flapping led MMS to cross the current sheet repeatedly, from north to south and vice versa. During the initial crossing of the current sheet, distinct electron outflows were observed, while ion responses were weak. This indicates either electron-only reconnection, suggesting an early stage of reconnection (temporal effect) or MMS passing through the exhaust region close to an X-line (spatial effect). Signatures of the electron diffusion region’s limited length, yielding an unusually high aspect ratio, suggest that the former scenario is more likely. Regular ion-coupled reconnection that involves ion outflows became more prominent until MMS eventually crossed the X-line from tailward to earthward. This event indicates a potential causal relationship between electron-only reconnection, current sheet flapping, and subsequent ion reconnection. We discuss feasible mechanisms for current sheet flapping and the feedback between flapping and reconnection, providing new insights into the onset of magnetotail reconnection and cross-scale processes of importance to other heliospheric and astrophysical systems.

We present a novel data analysis technique based on physics-informed neural networks (PINNs) to reconstruct two-dimensional (2D), magnetohydrodynamic (MHD) and Hall MHD equilibria in a space plasma from in situ spacecraft measurements. Our method incorporates the steady-state MHD or Hall MHD equations—a set of partial differential equations (PDEs) as physical constraints—into a deep learning framework. In contrast to traditional reconstruction techniques relying on explicit spatial integration of the PDEs from spacecraft trajectories, the PINN approach allows us to derive a physically consistent equilibrium by minimizing a composite loss function that includes both prediction errors and PDE residuals. We validate the method through benchmark tests using exact solutions of axially symmetric MHD and Hall MHD equilibria, and further demonstrate its utility through application to an extensively investigated magnetotail reconnection event observed by the Magnetospheric Multiscale mission. The resulting reconstruction reproduces key structural features, including the X-type current sheet geometry and quadrupolar Hall magnetic field, consistent with prior results and numerical simulations. We also present empirical estimates of the reconnection rate and spatially varying resistivity based on the reconstructed fields. Our results demonstrate that the PINN-based method provides a flexible and powerful new tool for revealing plasma structures around observing spacecraft, particularly in regions where model assumptions may be only approximately valid.

Magnetic reconnection is a fundamental plasma process responsible for the sometimes explosive release of magnetic energy in space and laboratory plasmas. Inside the diffusion regions of magnetic reconnection, the plasma becomes demagnetized and decouples from the magnetic field, enabling the change in magnetic topology necessary to power the energy release over larger scales. Since it was launched in 2015, the Magnetospheric MultiScale (MMS) mission has significantly advanced the understanding of the particle dynamics key to magnetic reconnection by providing high-resolution, in-situ measurements able to resolve ion and electron kinetic scales, i.e. a fraction of a gyroradius, that have confirmed theoretical predictions, revealed new phenomena, and refined existing models. These breakthroughs are critical for understanding not only space plasmas but also laboratory and astrophysical plasmas where magnetic reconnection occurs. In this work, we review the ion and electron dynamics occurring within the diffusion regions, in the inflow, along the separatrices, and downstream of the diffusion regions, in different reconnection configurations: symmetric, asymmetric, antiparallel, and guide field reconnection.

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.

The magnetic cloud (MC) of the Coronal Mass Ejection on 24 April 2023, contains sub‐Alfvénic solar wind, transforming Earth's magnetosphere from conventional bow‐shock magnetotail configuration to Alfvén wings. Utilizing measurements from the Magnetosphere Multiscale (MMS) mission, we present for the first time electron distribution signatures as the spacecraft traverses through various magnetic topologies during this transformation. Specifically, we characterize electrons inside the sub‐Alfvénic MC, on the dawn‐dusk wing field lines and on the closed field lines. The signatures include strahl electrons in MC regions and energetic keV electrons streaming along the dawn and dusk wing field lines. We demonstrate the distribution signatures of dual wing reconnection, defined as reconnection between dawn‐dusk Alfvén wing field lines and the interplanetary magnetic field (IMF). These signatures include four electron populations comprised of partially depleted MC electrons and bi‐directional energetic electrons with variations in energy and pitch‐angle. The distributions reveal evidence of bursty magnetic reconnection under northward IMF.