ST2.3 | Magnetic reconnection and Kelvin-Helmholtz Instability in collisionless environments
EDI
Magnetic reconnection and Kelvin-Helmholtz Instability in collisionless environments
Convener: Seiji Zenitani | Co-conveners: Adriana Settino, Rongsheng Wang, Maria Elena Innocenti, Wenya Li, Kyoung-Joo (Joo) Hwang, Simone Benella
Orals
| Wed, 06 May, 10:45–12:30 (CEST)
 
Room -2.31
Posters on site
| Attendance Tue, 05 May, 14:00–15:45 (CEST) | Display Tue, 05 May, 14:00–18:00
 
Hall X4
Orals |
Wed, 10:45
Posters on site
Tue, 14:00
Magnetic reconnection and the Kelvin–Helmholtz instability (KHI) are two fundamental plasma processes that govern magnetic topology changes and plasma transport in a wide range of space and astrophysical environments. Magnetic reconnection is responsible for many explosive phenomena in space, while KHI occurs from the MHD scale to the electron kinetic scale. Magnetic reconnection can trigger the KHI and can be triggered in the process of KHI. Both of them play a dominant role at the boundaries between the solar wind and planetary magnetospheres, such as those of Earth, Mercury, Jupiter, and Saturn. Recent advances driven by high-resolution in situ measurements from spacecraft missions (e.g., MMS, Cluster, THEMIS, JUNO, MAVEN, Parker Solar Probe) and by state-of-the-art numerical simulations have led to significant progress in understanding these processes over the past several years.

Despite the advances, many key issues remain unresolved. In reconnection, the triggering mechanisms, quantitative aspects of the energy conversions, identification of the electron diffusion/dissipation region, and coupling across multiple scales remain unsolved. In KHI, plasma mixing across the boundary, an energy cascade from the large-scale vortices to kinetic scales, and the nonlinear evolution of the secondary modes are central issues. Furthermore, the interplay between reconnection and KHI introduces many new challenges. This joint session invites presentations on all of the aspects associated with magnetic reconnection and KHI from the spacecraft measurements, theoretical analysis, numerical simulations, and laboratory experiments.

Orals: Wed, 6 May, 10:45–12:30 | Room -2.31

Chairpersons: Seiji Zenitani, Rongsheng Wang, Adriana Settino
Highlighted talks
10:45–10:55
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EGU26-2361
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solicited
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On-site presentation
Fundamental understanding of magnetic reconnection via spiral CT scan 
Huishan Fu, Zhe Wang, and Jinbin Cao

It is still poorly understood at present how magnetic reconnection—a universal process in space, laboratory, and astrophysical plasmas—is triggered and developed, because there was no efficient technique to analyze such a super-dynamic and three-dimensional process. Even with the launch of NASA's MMS mission, studies of this process were still based on in-situ measurements along spacecraft trajectories and qualitative comparison with a schematic, which is stationary, two-dimensional, and oversimplified. As a result, using such conventional methodologies, the fundamental physics andinherent nature of magnetic reconnection cannot be uncovered. Here we invent a three-dimensional CT imaging technique, analogous to that in the hospital, and apply it to a magnetic reconnection in space. With the help of such an advanced technique, we at least made three exciting discoveries: (1) magnetic reconnection is triggered by whistler waves and developed by Hall effects; (2) magnetic reconnection accelerates electrons and converts energy via parallel electric fields; (3) magnetic reconnection converts magnetic energy to particle energy in the inflow region but inversely converts particle energy to magnetic energy near the X point, with the net conversion being (Binflow2-Boutflow2)/2m0during the whole process. These discoveries have upended the conventional concept and completely unraveled the fundamental nature of magnetic reconnection. 

How to cite: Fu, H., Wang, Z., and Cao, J.: Fundamental understanding of magnetic reconnection via spiral CT scan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2361, https://doi.org/10.5194/egusphere-egu26-2361, 2026.

10:55–11:05
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EGU26-11887
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solicited
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Virtual presentation
Giant Kelvin-Helmholtz waves at the Boundaries of the Mother's Day 2024 Coronal Mass Ejections Driving Geoeffectiveness and Motivating the sub-L1 Space Weather Measurements  
Katariina Nykyri

Starting in the evening of 10 May 2024 the Earth's magnetosphere was hit by the coronal mass ejections (CMEs) creating the largest geomagnetic storm since the Halloween Storm of 2003. The CME encounter was characterized by variations of plasma number density and magnetic field. Here, I present the ARTEMIS observations at the lunar orbit during this event and the MMS observations closer to the bow shock. The IMF Bz ranged from −60 to +40 nT both with hour to minutes periodicity with plasma jets propagating in +-z-direction within multi-scale wave structures. Similar signature has been recently reported at the magnetopause by MMS spacecraft (Li et al., 2023, https://doi.org/10.1029/2023GL105539; Nykyri, 2024, https://doi.org/10.1029/2024GL108605) of the Kelvin-Helmholtz (KH) wave observations during a strongly southward IMF. Here, I show that the CME boundaries were KH unstable leading to multi-scale density and magnetic field fluctuations including reconnection jets, with clear density compressions when spacecraft moved from southward ejecta field into oppositely orientated, draped sheath field region -a characterisitic signature of plasma compression driven by the KH waves. The wavelengths varied from 60 to 270 Re, suggesting that the magnetosphere was periodically exposed to successive intervals of strongly northward and southward IMF leading to enhanced mass and magnetic flux loading, enabling the strongest ring current growth in 20-years. The source region of the wave growth, driven by the sheared plasma flows at the CME boundaries by the KH-instability, was estimated to be about ~7 million km usptream of the Earth-Sun Lagrange 1 point, motivating the need for the new sub-L1 spacecraft constellations, allowing ~3-5 hr space weather predictions of the time-scale of the IMF Bz and By  variation estimates.

How to cite: Nykyri, K.: Giant Kelvin-Helmholtz waves at the Boundaries of the Mother's Day 2024 Coronal Mass Ejections Driving Geoeffectiveness and Motivating the sub-L1 Space Weather Measurements , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11887, https://doi.org/10.5194/egusphere-egu26-11887, 2026.

11:05–11:10
Discussion
Theory and modeling
11:10–11:20
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EGU26-2963
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ECS
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On-site presentation
Electron flat-top distributions in magnetic reconnection simulations 
Konrad Steinvall, Louis Richard, Ida Svenningsson, Tünde Fülöp, and István Pusztai

Collisionless magnetic reconnection hosts electron velocity distribution functions (VDFs; f) that are far from local thermodynamic equilibrium, as represented by the Maxwell-Boltzmann distribution function. One important example of such VDFs is the flat-top distribution, which is characterized by ∂f/∂v=0 in the VDF core. Simulations have shown that electron flat-top VDFs develop in the ion diffusion region of magnetic reconnection. There, large-scale parallel electric fields (E) trap the electrons that convect into the reconnection region with a small parallel velocity, leading to the formation of flat-top VDFs. During this process, the E strongly heats the trapped electrons parallel to the magnetic field, resulting in very large parallel temperature anisotropies. The formation of flat-tops is therefore believed to be an important contributor to electron heating and energization during reconnection. Spacecraft observations of electron flat-top distributions have recently provided indirect measurements of the total work done by E on the electrons. However, questions regarding the spatial distribution of flat-top VDFs and their role in electron energization during reconnection remain. Simulations are an essential complement to spacecraft observations, as they provide us with additional information about the spatial structure and temporal evolution of the reconnection event. 

Here, we will present results from 2D particle-in-cell simulations investigating electron flat-top distributions in symmetric collisionless reconnection. In particular, we will focus on where the flat-tops are generated, and on the energization mechanisms underlying their formation. We find that electron flat-top VDFs are most commonly found near the central reconnection region and in the outflow, correlating with the large-scale E present there. By decomposing the electric field into potential and solenoidal (inductive) parts, we find that the large-scale E is, to a large extent, due to an electric potential associated with charge separation in the diffusion region. The energy that the electrons gain from the potential part of the electric field as they enter the reconnection site, is lost as they exit it. Our results thus suggest that a large fraction of the heating associated with the formation of electron flat-tops should be considered temporary, as only the inductive part of the electric field can yield persistent energization.

How to cite: Steinvall, K., Richard, L., Svenningsson, I., Fülöp, T., and Pusztai, I.: Electron flat-top distributions in magnetic reconnection simulations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2963, https://doi.org/10.5194/egusphere-egu26-2963, 2026.

11:20–11:30
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EGU26-1819
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On-site presentation
Magnetic reconnection and high-speed jets downstream of a parallel shock: Full PIC simulation 
Zhongwei Yang, Quanming Lu, Heli Hietala, Hui Li, Wence Jiang, Can Huang, Xiaocheng Guo, Tianran Sun, San Lu, Xinliang Gao, Junyi Ren, and Chi Wang

Magnetic reconnection is one of the most fundamental processes governing energy conversion and particle acceleration in collisionless space plasmas. Observations have revealed the occurrence of magnetic reconnection (Phan et al., 2018) and transient structures such as high-speed jets (HSJs; Hietala et al., 2009) in planetary magnetosheaths. Statistical studies based on Cluster and MMS measurements have shown that most HSJs are preferentially located downstream of the quasi-parallel bow shock (Escoubet et al., 2020). In recent years, global hybrid simulations have been extensively employed to investigate the three-dimensional global distribution of HSJs and their associated ion kinetic properties (Palmroth et al., 2021; Yang et al., 2023; Guo et al., 2024; Fatemi et al., 2024). In this study, we perform full particle-in-cell (PIC) simulations spanning spatial scales of several Earth radii and, for the first time, demonstrate an “all-in-one” multiscale kinetic scenario linking foreshock ultra low frequency (ULF) wave, non-stationary shock front (Lembege & Savoini, 1992), downstream high-speed jet (Hietala et al., 2009) and bow wave (Liu et al., 2020) formation, and the subsequent triggering of magnetic reconnections. The simulations illustrate how high dynamic-pressure structures embedded in foreshock low-frequency waves can traverse a self-reforming shock, in good agreement with MMS observations reported by Raptis et al. (2022). Furthermore, the spatial relationships among HSJs, turbulent filamentary current sheets, and reconnection sites are identified. Using the guiding-center framework commonly applied to adiabatic electron acceleration in reconnection, we quantify the relative contributions of parallel electric fields, betatron acceleration, and Fermi processes to electron energization at different stages of HSJ-driven reconnection evolution. Finally, based on our simulation results, we present a preparatory investigation for the upcoming SMILE mission (launch scheduled for 2026), discussing the penetration depth of HSJs and their induced magnetopause deformation, and providing corresponding soft X-ray images.

How to cite: Yang, Z., Lu, Q., Hietala, H., Li, H., Jiang, W., Huang, C., Guo, X., Sun, T., Lu, S., Gao, X., Ren, J., and Wang, C.: Magnetic reconnection and high-speed jets downstream of a parallel shock: Full PIC simulation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1819, https://doi.org/10.5194/egusphere-egu26-1819, 2026.

11:30–11:40
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EGU26-15809
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ECS
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On-site presentation
A New X-line Model: Comparison to MHD Magnetic Separator 
Bayane Michotte de Welle, Hyunju Connor, David Sibeck, Alex Glocer, Stephen Fuselier, Karlheinz Trattner, Steven Petrinec, Austin Brenner, Fatemeh Bagheri, and Sangyun Lee

Predicting the location of magnetopause reconnection remains a major challenge. Existing models often fail to predict the location of the reconnection line seen in global MHD simulations, particularly under northward IMF. This work presents a new X-line model that identifies a dominant reconnection line by maximizing the reconnection rate on both local and global scales. First, it determines the orientations at the magnetopause that locally maximize the rate and then finds the global path with the highest integrated rate. Across four global MHD simulations with diverse dipole tilts and IMF orientations, the new model was consistently more accurate than both the maximum magnetic shear and the global rate maximization models in predicting the location of the magnetic separator in between the magnetospheric cusps. Crucially, it succeeds for a challenging northward IMF case where previous models have failed. This model suggests the X-line's location is determined by a fundamental principle of maximizing the conversion of magnetic to plasma energy.

How to cite: Michotte de Welle, B., Connor, H., Sibeck, D., Glocer, A., Fuselier, S., Trattner, K., Petrinec, S., Brenner, A., Bagheri, F., and Lee, S.: A New X-line Model: Comparison to MHD Magnetic Separator, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15809, https://doi.org/10.5194/egusphere-egu26-15809, 2026.

11:40–11:50
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EGU26-21774
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ECS
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On-site presentation
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Superposition of Doppler-shifting magnetopause Kelvin-Helmholtz modes through Dynamic Mode Decomposition of a global MHD simulation 
Harley Kelly, Martin Archer, Jonathan Eastwood, Mike Heyns, Joe Eggington, and Jeremy Chittenden

The Kelvin-Helmholtz instability mediates the viscous-like solar-terrestrial interaction by generating magnetopause surface waves that quickly become non-linear. Basic theory predicts the locally most-unstable linear wave dominates. However, Kelvin-Helmholtz is a broad, convective instability that also amplifies waves originating upstream. We address this conundrum by applying Dynamic Mode Decomposition to a Gorgon global magnetohydrodynamic simulation of the Kelvin-Helmholtz instability. While distinct modes quickly grow at points along the magnetopause, signalling local generation, their energy continues to slowly grow downtail. Thus, a superposition is present along the magnetopause, where the dominant mode is not always the locally fastest-growing. Each mode’s wavelength elongates downtail, correlating with the boundary layer flow speed due to the accelerating advective flow around the magnetosphere Doppler shifting the fixed-frequency waves. This may explain why longer wavelengths are observed in the tail than theory predicts and motivates further exploration of tangential inhomogeneities in basic Kelvin-Helmholtz theory.

How to cite: Kelly, H., Archer, M., Eastwood, J., Heyns, M., Eggington, J., and Chittenden, J.: Superposition of Doppler-shifting magnetopause Kelvin-Helmholtz modes through Dynamic Mode Decomposition of a global MHD simulation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21774, https://doi.org/10.5194/egusphere-egu26-21774, 2026.

Discussion Post a comment
11:50–12:00
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EGU26-13551
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ECS
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On-site presentation
Plasma Mixing in Collisionless Magnetized Plasmas Driven by the Kelvin–Helmholtz Instability 
Silvia Ferro, Fabio Bacchini, Giuseppe Arrò, Francesco Pucci, and Pierre Henri

Plasma mixing across velocity shear layers is a key process controlling mass and momentum transport at planetary magnetospheric boundaries. At the Earth’s magnetopause, the Kelvin–Helmholtz instability (KHI) is expected to facilitate such transport by generating large-scale vortices and turbulence. However, in collisionless and magnetized plasmas, the efficiency of KHI-driven mixing remains an open question, particularly in the presence of a magnetic field component aligned with the shear flow.

We investigate plasma mixing driven by the KHI using high-resolution, two-dimensional, fully kinetic particle-in-cell simulations of magnetized shear layers. We consider configurations with opposite orientations of vorticity relative to the flow-aligned magnetic field and analyze the nonlinear evolution of KHI vortices and the resulting turbulent boundary layer. Plasma mixing is quantified through particle tracing, allowing us to assess the degree of interpenetration between initially distinct plasma populations. Our results show that, despite the development of fully nonlinear KHI vortices that merge and evolve into complex dynamics, plasma mixing across the shear layer can remain strongly inhibited when even a modest magnetic field component is aligned with the flow. In this regime, magnetospheric and magnetosheath plasmas preserve partially distinct topologies within the turbulent layer, highlighting the stabilizing role of magnetic tension at kinetic scales. These findings demonstrate that KHI-driven turbulence does not necessarily imply efficient plasma mixing in collisionless magnetized environments and have important implications for solar wind–magnetosphere coupling and plasma transport at planetary boundaries.

How to cite: Ferro, S., Bacchini, F., Arrò, G., Pucci, F., and Henri, P.: Plasma Mixing in Collisionless Magnetized Plasmas Driven by the Kelvin–Helmholtz Instability, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13551, https://doi.org/10.5194/egusphere-egu26-13551, 2026.

Spacecraft observations
12:00–12:10
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EGU26-1830
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ECS
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On-site presentation
High-Speed Jets and Magnetic Reconnection in Earth’s magnetosheath: MMS observations 
Wanxin Ouyang, Shimou Wang, Binbin Tang, Zhongwei Yang, Wence Jiang, Rongsheng Wang, Quanming Lu, and Chi Wang

High-speed jets are transient structures in the magnetosheath characterized by high dynamic pressure. Their compressed magnetic field lines at the leading edge can generate bow waves, which are believed to accelerate particles and enhance energy dissipation in the magnetosheath. In this study, we analyzed a typical event within the magnetosheath, using observations from the Magnetospheric Multiscale (MMS) mission. Results reveal that during the MMS traversal of the magnetosheath, it captured for the first time a complete spatiotemporal co-occurrence of three structures: the high-speed jet, bow wave, and magnetic reconnection. Notably, the reconnection took place at the leading edge of the high-speed jet and bow wave, suggesting a potential mechanism for magnetosheath reconnection: high-speed jets propel bow waves, which then compress the pre-existing, curved magnetic field lines at the leading edge, thereby triggering reconnection. Concurrently, within the reconnection region, we observed distinct electron acceleration and heating signatures, including enhanced plasma flow, increased energy flux, and Joule dissipation. The electron characteristics exhibited significant differences between the two sides within the current sheet: on the left, the high-energy electron energy flux spectrum and pitch angle enhancements appeared only in the anti-parallel direction, whereas on the right, enhancements were observed in both parallel and anti-parallel directions. Furthermore, the electron velocity distribution function aligns with the distribution of local trapping mechanisms. These results suggest the potential existence of two different electron acceleration mechanisms during reconnection. This study reveals the coupling properties between high-speed jets and magnetosheath reconnection, providing new observational evidence for understanding how high-speed jets act as energy drivers influencing the energy transport and conversion in the magnetosheath.

How to cite: Ouyang, W., Wang, S., Tang, B., Yang, Z., Jiang, W., Wang, R., Lu, Q., and Wang, C.: High-Speed Jets and Magnetic Reconnection in Earth’s magnetosheath: MMS observations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1830, https://doi.org/10.5194/egusphere-egu26-1830, 2026.

12:10–12:20
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EGU26-8894
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ECS
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On-site presentation
Dynamic Component Reconnection and FTE-Driven Electron-Scale Processes under Large Dipole Tilt: A Multi-Spacecraft Study 
Enze Zhao, Malcolm Dunlop, Xiangcheng Dong, Karlheinz Trattner, Wending Fu, Huishan Fu, Keizo Fujimoto, Jinbin Cao, and Philippe Escoubet

We investigate the dynamics of magnetopause component reconnection under extreme winter solstice conditions with a dominant IMF-By component. Joint observations from ACE (at L1 point) and THEMIS-D (near the bow shock) reliably confirmed the solar wind conditions. At the low-latitude magnetopause, MMS observed multiple ion flow reversals within 7 minutes, indicating a dynamic component reconnection X-line topology. The component X-line’s central segment position deviates from Maximum Magnetic Shear model predictions. Consistent with previous research by Trattner et al., we demonstrate that under specific conditions, component X-lines are controlled by the interplay between the antiparallel reconnection region and magnetic shear maximization. On the kinetic scale, MMS detected two ion-scale flux transfer events (FTEs) with identical L-direction velocity but opposite helicity. The anomalous helical FTE1 was entirely located on the magnetosheath side, leading to a steep magnetic field gradient at the pressure-balanced interface between FTE1 and the magnetopause. In this region, MMS observed super-Alfvénic electron flows and varying electron agyrotropy but lacked classical EDR (electron diffusion region) signatures. We propose this represents an electron-only reconnection initiation mediated by diamagnetic current, triggered by the anomalous helical FTE contacting the magnetopause, rather than a traditional secondary reconnection site. Our study provides observational evidence for dynamic component reconnection and identifies a new mechanism for electron-only reconnection onset driven by diamagnetic currents.

How to cite: Zhao, E., Dunlop, M., Dong, X., Trattner, K., Fu, W., Fu, H., Fujimoto, K., Cao, J., and Escoubet, P.: Dynamic Component Reconnection and FTE-Driven Electron-Scale Processes under Large Dipole Tilt: A Multi-Spacecraft Study, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8894, https://doi.org/10.5194/egusphere-egu26-8894, 2026.

12:20–12:30
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EGU26-20404
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ECS
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On-site presentation
Exhaust structure of guide-field reconnection under shear flows as seen at the magnetopause flanks 
Tom Wakefield, Andrew N. Fazakerley, Colin Forsyth, Christopher J. Owen, and Karlheinz J. Trattner

Velocity shear at the magnetopause flanks often supports the development of Kelvin–Helmholtz waves, which can in turn facilitate favourable conditions for local magnetic reconnection. Under these flank conditions, reconnection typically occurs in a configuration characterised by a strong guide field. We extend previous two-dimensional models of shear-flow-modified reconnection to consider the resulting three-dimensional exhaust structure under strong guide-field conditions. This new framework suggests that the exhaust may be bounded by an asymmetric pair of standing shock-like structures, producing a three-dimensional geometry that extends into the out-of-plane (guide-field) direction. Using observations from the Magnetospheric Multiscale (MMS) mission during a dusk-flank magnetopause crossing under northward interplanetary magnetic field conditions, we confirm that reconnection exhausts exhibit these features.

How to cite: Wakefield, T., Fazakerley, A. N., Forsyth, C., Owen, C. J., and Trattner, K. J.: Exhaust structure of guide-field reconnection under shear flows as seen at the magnetopause flanks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20404, https://doi.org/10.5194/egusphere-egu26-20404, 2026.

Posters on site: Tue, 5 May, 14:00–15:45 | Hall X4

Display time: Tue, 5 May, 14:00–18:00
Chairpersons: Wenya Li, Simone Benella
X4.74
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EGU26-2391
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ECS
Energy Flux Densities in Electron-only Magnetic Reconnection in Space Plasma 
Zuzheng Chen, Tieyan Wang, Jiang Yu, Jing Wang, Yudong Ye, Huishan Fu, Jinbin Cao, Jun Cui, Hengyan Man, and Yuchen Jiang

Electron-only magnetic reconnection, a novel form of magnetic reconnection recently discovered in plasma turbulence, exhibits distinct features from the well-studied standard magnetic reconnection with ion coupling. Our study investigates its energy partition features by utilizing in situ measurements from the Magnetospheric Multiscale mission. Electron enthalpy flux exhibits a strong linear relationship with electron velocity. The spatial distributions of electron kinetic energy and enthalpy fluxes are influenced by the asymmetric effects and guide fields in a similar manner to how the spatial distributions of electron velocity are affected. The guide field enhances Poynting flux to a magnitude that rivals, or even surpasses, electron enthalpy flux, while also deflects it toward the outflow direction. These findings impact the understanding of energy partition in magnetic reconnection.

How to cite: Chen, Z., Wang, T., Yu, J., Wang, J., Ye, Y., Fu, H., Cao, J., Cui, J., Man, H., and Jiang, Y.: Energy Flux Densities in Electron-only Magnetic Reconnection in Space Plasma, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2391, https://doi.org/10.5194/egusphere-egu26-2391, 2026.

X4.75
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EGU26-2412
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ECS
Strong energy conversion by a magnetic hole behind a dipolarization front 
Zhenyu Xu, Huishan Fu, Wending Fu, Wenzhe Zhang, Zhe Wang, Zhizhong Guo, Chenxi Du, and Jinbin Cao

Dipolarization fronts (DFs) have been widely reported in the Earth’s magnetotail and are suggested to play an important role in energy conversion. Magnetic holes (MHs) are also usually observed near DFs, and recent spacecraft observations suggest that they can be excited by interchange instability (ICI). However, whether the MHs near DFs could contribute to energy conversion is still unknown. Here, by using the Magnetospheric Multiscale (MMS) mission observations, we find a sub-ion scale MH behind a DF. We present a two-dimensional illustration of the MH, revealing that such an MH was generated by the ICI. Inside this MH, a significant energy conversion up to ~ 2 nW/m3 (higher than typical observations near DFs) is caused by the local electron vortex current inside the MH and the background electric field on the DF. This study improves our understanding of energy injection during substorms and energy conversion near DFs.

How to cite: Xu, Z., Fu, H., Fu, W., Zhang, W., Wang, Z., Guo, Z., Du, C., and Cao, J.: Strong energy conversion by a magnetic hole behind a dipolarization front, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2412, https://doi.org/10.5194/egusphere-egu26-2412, 2026.

X4.76
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EGU26-3489
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ECS
Statistical evidence on the impact of foreshock effects on the Kelvin-Helmholtz waves at the Earth's magnetopause 
Niklas Grimmich, Shiva Kavosi, Martin Archer, Katariina Nykyri, Adrian Pöppelwerth, and Adriana Settino

An important mechanism for the transfer of energy across the boundary between the Earth's magnetic field and the solar wind involves the formation of waves and vortices at the magnetopause. These waves and vortices arise from the Kelvin–Helmholtz instability, which is caused by the difference in velocity between the magnetospheric plasma and the shocked solar wind plasma. From spacecraft observations and simulations, we know that Kelvin-Helmholtz waves can evolve and grow differently depending on their formation conditions and locations. Specifically, evidence from simulations indicates an impact from the foreshock on the development of the waves. However, this has not yet been fully confirmed by observations. 
    
Using the extensive in-situ data from the last solar cycle, we can compare the parameters of 3,335 KHI observations under different conditions. By applying different methods to determine the necessary plasma conditions at the boundary in wave parameter calculations, we are able to accumulate statistical evidence indicating whether the wave parameters of Kelvin–Helmholtz-induced waves are altered in the presence of a foreshock region upstream of the magnetopause. Our results suggest that, under certain solar wind conditions, the presence of the foreshock indeed alters the typical wave parameters of Kelvin-Helmholtz waves. This further reaffirms that the presence of the foreshock must be considered when understanding solar-terrestrial interactions.

How to cite: Grimmich, N., Kavosi, S., Archer, M., Nykyri, K., Pöppelwerth, A., and Settino, A.: Statistical evidence on the impact of foreshock effects on the Kelvin-Helmholtz waves at the Earth's magnetopause, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3489, https://doi.org/10.5194/egusphere-egu26-3489, 2026.

X4.77
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EGU26-3873
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ECS
Formation of the energetic electrons at the dipolarization front and the trailing flux pileup region during magnetic reconnection 
Jia Nan, Quanming Lu, Kai Huang, San Lu, Rongsheng Wang, and Shihang Hu

The dipolarization front (DF) and the flux pileup region (FPR) are crucial downstream structures in magnetic reconnection, where significant energetic electrons are frequently observed.  Using a two-dimensional particle-in-cell simulation model, we investigate the formation of energetic electrons in both the DF and the trailing FPR. Our results demonstrate that the energetic electrons at pitch angles near 90° at both regions undergo a two-stage acceleration process: an initial non-adiabatic acceleration by the reconnection electric field at the reconnection site followed by downstream adiabatic acceleration. We find that the 90° pitch-angle energetic electrons in the FPR reach substantially higher energies than those at the DF, as they encounter a stronger reconnection electric field at the reconnection site in the first stage. Furthermore, two populations of energetic electrons with distinct energy ranges at pitch angles near 0° and 180° are identified at the DF. The lower-energy population exhibits energies close to the magnitude of the parallel potential at the DF, which dominates the formation of this population byaccelerating the electrons towards the DF and providing the trapping mechanism. The higher-energy population is energized via Fermi mechanism through multiple reflections within the contracting magnetic island downstream. These findings provide new insights into the generation of energetic electrons during magnetic reconnection.

How to cite: Nan, J., Lu, Q., Huang, K., Lu, S., Wang, R., and Hu, S.: Formation of the energetic electrons at the dipolarization front and the trailing flux pileup region during magnetic reconnection, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3873, https://doi.org/10.5194/egusphere-egu26-3873, 2026.

X4.78
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EGU26-5307
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ECS
Energy conversion inside Kelvin-Helmholtz Vortices 
Adriana Settino, Zoltàn Vörös, Sohom Roy, Owen Roberts, Luca Sorriso-Valvo, Cyril Simon-Wedlund, and Rumi Nakamura

The Kelvin–Helmholtz instability (KHI) is a shear-driven phenomenon that generates a chain of vortices, located along the shear layer. As these vortices grow, they interact and fragment, eventually leading to turbulence and the dissipation of kinetic energy. The exact pathway through which KHI moves and converts energy across scales still remains elusive. Using Magnetospheric Multiscale (MMS) spacecraft data, we explore energy conversion pathways during KHI events at Earth's magnetopause.

We quantify, for the first time, KHI energy conversion channels via pressure–strain and J·E′ diagnostics. Enhanced energy conversion between flow and thermal energy is observed inside vortices, associated with both local non-thermal features and perpendicular temperature anisotropies. Conversely, at the boundaries, enhanced magnetic fluctuations are associated with peaks in the ion agyrotropy. Finally, we investigate how reconnecting current sheets, observed at the vortex boundaries, affect energy conversion terms.

How to cite: Settino, A., Vörös, Z., Roy, S., Roberts, O., Sorriso-Valvo, L., Simon-Wedlund, C., and Nakamura, R.: Energy conversion inside Kelvin-Helmholtz Vortices, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5307, https://doi.org/10.5194/egusphere-egu26-5307, 2026.

X4.79
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EGU26-8771
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ECS
Electron dynamics near the electron diffusion region with a moderate guild field 
Tongkuai Li, Wenya Li, Binbin Tang, Wenlong Guo, Huijie Liu, Chongle Zhang, and Chi Wang

We examine the detailed electron dynamics within an electron diffusion region (EDR) with a moderate guide field (normalized guide field ~0.5), as observed by the Magnetospheric Multiscale (MMS) spacecraft at the magnetopause. Due to the presence of a moderate guide field, high-energy electrons (>300 eV) can be scattered, while low-energy electrons (<150 eV) remain magnetized. This energy disparity results in the characteristic electron ‘ring’ distribution in velocity phase space. Additionally, we observe clear energization in the perpendicular direction during this event. We further investigate the electron dynamics of scattering and acceleration using test particle simulations. The results reproduce the energy disparity observed in electron scattering and suggest that the perpendicular energization of electrons is driven by the non-ideal electric field EM​. Due to the presence of a moderate guide field, this non-ideal electric field EM​ accelerates electrons through both V⊥⋅E⊥and V∥​⋅E∥​. The energy gained in the parallel direction via V∥​⋅E∥​ can then be transferred to the perpendicular direction through electron scattering. Based on spacecraft observations and test particle simulations, we show that the electron dynamics in reconnection with a moderate guide field are distinct from those in anti-parallel reconnection and strong guide field reconnection. These results enhance our understanding of how electron dynamics transition from anti-parallel to strong guide field reconnection.

How to cite: Li, T., Li, W., Tang, B., Guo, W., Liu, H., Zhang, C., and Wang, C.: Electron dynamics near the electron diffusion region with a moderate guild field, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8771, https://doi.org/10.5194/egusphere-egu26-8771, 2026.

X4.80
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EGU26-9619
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ECS
Data-driven Fully Kinetic Simulations of Magnetic Reconnection in Earth’s Magnetotail  
Nadja Reisinger and Fabio Bacchini

Magnetic reconnection is an energy conversion process that accelerates particles to high energies. This explosive process occurs in near-Earth space as well as in many other astrophysical environments. While direct measurements of plasma parameters, including particle energy distributions, are often not possible, Earth’s magnetosphere is one of the few natural laboratories where such observations can be made. However, satellite observations are limited in spatial and temporal coverage, whereas simulations can offer a more comprehensive view of the reconnection process. Thus, we want to bridge both techniques and use kinetic simulations initialized with observational constraints.   

We present fully kinetic simulations of magnetic reconnection in Earth’s magnetotail, using parameters derived from a well-studied event observed by the Magnetospheric Multiscale (MMS) mission. The simulations are performed using the energy-conserving particle-in-cell (PIC) code ECsim/RelSIM, which includes both ion and electron dynamics to investigate particle energization during reconnection. We investigate the impact of initial plasma conditions and numerical parameters on the resulting energy distributions, and compare the simulation outputs with in-situ observations to assess the simulations’ ability to reproduce key features of the event. This work presents a comparison of particle energy distributions between fully kinetic simulations and spacecraft observations for a magnetotail reconnection event. 

How to cite: Reisinger, N. and Bacchini, F.: Data-driven Fully Kinetic Simulations of Magnetic Reconnection in Earth’s Magnetotail , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9619, https://doi.org/10.5194/egusphere-egu26-9619, 2026.

X4.81
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EGU26-12292
Recurrent Tailward-Propagating Auroral Arcs during northward IMF 
Hyangpyo Kim, Rumi Nakamura, Kazuo Shiokawa, Adriana Settino, Kyoung-Joo Hwang, Hiroshi Hasegawa, Keisuke Hosokawa, and Jaeheung Park

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.

How to cite: Kim, H., Nakamura, R., Shiokawa, K., Settino, A., Hwang, K.-J., Hasegawa, H., Hosokawa, K., and Park, J.: Recurrent Tailward-Propagating Auroral Arcs during northward IMF, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12292, https://doi.org/10.5194/egusphere-egu26-12292, 2026.

X4.82
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EGU26-14287
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ECS
On the ambiguity between electron diffusion region and magnetospheric separatrix: a revisited MMS event 
Thomas Faure, Olivier Le Contel, Mohammed Baraka, Soboh Alqeeq, Alessandro Retinò, Thomas Chust, Yuri Khotyaintsev, Victoria Wilder, Narges Ahmadi, Daniel J. Gershman, Hanying Wei, Jim Burch, Roy Torbert, Robert Ergun, and Per-Arne Lindqvist

At the magnetopause, the boundary between the magnetosphere and the shocked solar-wind-dominated region, a fundamental process takes place: magnetic reconnection, which allows part of the solar wind plasma to enter the magnetosphere. In order to investigate this process at the scale of electron dynamics, within a region known as the electron diffusion region (EDR), the Magnetospheric Multiscale (MMS) mission was launched in 2015.

Our work focuses on the analysis of a magnetic reconnection event at the magnetopause observed by MMS. This event was initially reported as a crossing of the EDR (Webster et al., 2018).

We carried out a detailed investigation of this event to determine the spacecraft trajectories within the reconnection region. The signatures of the electric and magnetic fields, particle velocities and energies, energy dissipation, current analysis, as well as the presence of highly structured whistler and lower-hybrid waves, suggest that the EDR may have been confused with another adjacent region: the magnetospheric separatrix. This region corresponds to the boundary between electrons moving toward the reconnection site and those moving away from it. Both the EDR and the magnetospheric separatrix are electron-scale regions that exhibit a number of similar observational signatures.

Our results raise an important question: could some previously reported EDR crossings actually correspond to magnetospheric separatrices? What are the differences in terms of energy conversion and partitioning, wave activity, plasma acceleration and heating between the near-EDR magnetospheric separatrix and the EDR?

How to cite: Faure, T., Le Contel, O., Baraka, M., Alqeeq, S., Retinò, A., Chust, T., Khotyaintsev, Y., Wilder, V., Ahmadi, N., Gershman, D. J., Wei, H., Burch, J., Torbert, R., Ergun, R., and Lindqvist, P.-A.: On the ambiguity between electron diffusion region and magnetospheric separatrix: a revisited MMS event, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14287, https://doi.org/10.5194/egusphere-egu26-14287, 2026.

X4.83
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EGU26-15653
Plasma waves in the high-speed electron flows 
Huijie Liu, Wenya Li, Binbin Tang, Cecilia Norgren, Kaijun Liu, Daniel Graham, Yuri V. Khotyaintsev, and Chi Wang

High-speed electron flows (HSEFs) are widely regarded as a significant source of various plasma waves and instabilities, which can subsequently interact with electrons and significantly impact electron dynamics. Using high-resolution data from the Magnetospheric Multiscale (MMS) mission, Liu et al. (2025) collected HSEFs in the Earth's magnetotail from 2017 to 2021, proving an excellent basis for a statistical investigation of associated plasma waves. Here, we perform a statistical investigation of the plasma waves in and out of the plasma sheet, respectively. In the plasma sheet, the observed fluctuations, including upper-hybrid waves, broadband electrostatic waves (BEWs), and low-frequency electrostatic waves, are mainly associated with perpendicular-moving electrons. Out of the plasma sheet, MMS observed Langmuir waves, BEWs, and low-frequency electrostatic fluctuations, primarily related to field-aligned electrons. The association of the observed plasma waves with magnetic reconnection is also discussed.

How to cite: Liu, H., Li, W., Tang, B., Norgren, C., Liu, K., Graham, D., Khotyaintsev, Y. V., and Wang, C.: Plasma waves in the high-speed electron flows, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15653, https://doi.org/10.5194/egusphere-egu26-15653, 2026.

X4.84
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EGU26-17498
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ECS
Role of Cold Ions in Parallel Current and Hall Field Enhancement along Separatrices in Magnetopause Reconnection 
Mohammed Baraka, Olivier Le Contel, Alessandro Retino, Jérémy Dargent, Arnaud Beck, Sergio Toledo-Redondo, Maria-Elena Innocenti, Giulia Cozzani, Souhail Dahani, Thomas Faure, Soboh Alqeeq, Inmaculada Albert, and Cecilia Norgren

Magnetic reconnection at the magnetopause involves complex multiscale dynamics, in which most particles do not traverse the electron or ion diffusion regions directly but are instead energized along the separatrices and within the reconnection outflow. Multiple ion populations, i.e., hot and cold ions strongly influence current systems and Hall physics. Cold ions, which have small gyroradii, remain magnetized longer than hot ions and follow the E x B drift along the separatrices together with electrons, whereas hotter ions decouple at larger spatial scales. This difference modifies the Hall physics.

In a previous study using a 2.5D fully kinetic particle-in-cell simulation setup with and without cold ions, it was shown that the delayed demagnetization of cold ions near the separatrices reduces the perpendicular ion current. Using the same simulations, we further find that the presence of cold ions enhances both the parallel electron current and the Hall magnetic field. These results provide a framework for future studies of energy dissipation during magnetopause magnetic reconnection in the presence of cold ions.  

How to cite: Baraka, M., Le Contel, O., Retino, A., Dargent, J., Beck, A., Toledo-Redondo, S., Innocenti, M.-E., Cozzani, G., Dahani, S., Faure, T., Alqeeq, S., Albert, I., and Norgren, C.: Role of Cold Ions in Parallel Current and Hall Field Enhancement along Separatrices in Magnetopause Reconnection, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17498, https://doi.org/10.5194/egusphere-egu26-17498, 2026.

X4.85
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EGU26-15023
Limited cold ion heating in the magnetopause boundary layers 
Sarah Vines, Stephen Fuselier, Karlheinz Trattner, Sergio Toledo-Redondo, Robert Allen, Kyunghwan Dokgo, Kristie Llera, Jason Beedle, Kyoung-Joo (Joo) Hwang, Kevin Genestreti, Eunjin Choi, Steven Petrinec, Christopher Russell, Hanying Wei, Robert Ergun, Craig Pollock, Daniel Gershman, Roy Torbert, and James Burch

For cold and heavy magnetospheric ion populations that reach the dayside magnetopause, how those populations evolve across magnetopause separatrices into the reconnection exhaust, and how the populations may affect or be affected by reconnection, are still not well understood. Observations from the Magnetospheric Multiscale (MMS) mission from January 2019 are analyzed for a series of magnetopause crossings during a time period with a “string-of-pearls” configuration of the MMS constellation. With inter-spacecraft separations of ~100-300 km, this configuration allows for simultaneous measurements of the cold ion populations in different regions of the magnetopause boundary and current layers. For several magnetopause crossings on 2019-01-25, while magnetospheric heavy ions (He+ and O+) are not observable, a significant amount of cold (temperatures of ~1’s-10 eV) magnetospheric H+ is present in the outer magnetosphere. This cold H+ population is accelerated by the E×B drift near the magnetopause, but remains as a cold beam (temperatures of 10’s eV) well into the boundary layers and reconnection exhaust. While wave modes are present that could potentially contribute to ion heating, temperature changes are small and occur primarily at the edge of the boundary layer, and so more likely related to the initial acceleration by the normal electric field than wave-particle interactions. The lack of heating for the magnetopause crossings on 2019-01-25 differs from that observed in previous work where MMS was farther away from the X-line, pointing to the highly spatially structured nature of reconnection sites along the separatrices and the importance of the relative density of the cold ion population reaching the magnetopause.

How to cite: Vines, S., Fuselier, S., Trattner, K., Toledo-Redondo, S., Allen, R., Dokgo, K., Llera, K., Beedle, J., Hwang, K.-J. (., Genestreti, K., Choi, E., Petrinec, S., Russell, C., Wei, H., Ergun, R., Pollock, C., Gershman, D., Torbert, R., and Burch, J.: Limited cold ion heating in the magnetopause boundary layers, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15023, https://doi.org/10.5194/egusphere-egu26-15023, 2026.