Hiroshi Hasegawa

Abstract 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.

Abstract We investigate Magnetospheric Multiscale (MMS) observation of the rising‐tone whistler waves during the magnetopause asymmetric magnetic reconnection on 24 December 2016. The rising‐tone whistler wave propagates toward the electron diffusion region of magnetic reconnection along the magnetic field on the magnetosheath side of the separatrix. The fundamental frequency of the whistler wave is slightly above 0.5 (electron cyclotron frequency) and rises to 1.0 . This study shows that the separatrix can provide favorable conditions for (a) the generation of whistler waves, (b) the frequency chirping of whistler waves, and (c) the local confinement of whistler waves within the narrow separatrix. These rising‐tone whistler waves, generated under such conditions, may contribute to the rapid energization of electrons during the magnetic reconnection.

Abstract 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.

Abstract This short article highlights unsolved problems of magnetic reconnection in collisionless plasma. Advanced in-situ plasma measurements and simulations have enabled scientists to gain a novel understanding of magnetic reconnection. Nevertheless, outstanding questions remain concerning the complex dynamics and structures in the diffusion region, cross-scale and regional couplings, the onset of magnetic reconnection, and the details of particle energization. We discuss future directions for magnetic reconnection research, including new observations, new simulations, and interdisciplinary approaches.

Abstract 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.

Abstract We present observations on 24 April 2023 by the Magnetospheric Multiscale spacecraft at the dayside, mid‐latitude magnetopause, when an interplanetary magnetic cloud (MC) with sub‐Alfvénic flows and northward and dawnward interplanetary magnetic field components impacted Earth's magnetosphere. The aim is to reveal the processes of solar wind‐magnetosphere interaction under sub‐Alfvénic solar wind with northward magnetic field. Our analysis of electron and ion data suggests that magnetopause reconnection occurred near both polar cusps, forming boundary layers on closed magnetic field lines on both the solar wind (i.e., MC) and magnetospheric sides of the magnetopause. Grad‐Shafranov, electron‐magnetohydrodynamics, and polynomial reconstructions of magnetopause current layers show that local (equator‐of‐the‐cusp) reconnection occurred in a sub‐ion‐scale magnetopause current sheet with a low magnetic shear angle (30°). Interestingly, the local reconnection was observed between the two (MC‐side and magnetosphere‐side) layers of closed field lines. It indicates that reconnected field lines from double cusp reconnection were interacting to induce another reconnection at the mid‐latitude magnetopause. Our results suggest that magnetopause reconnection was more efficient or frequent under sub‐Alfvénic solar wind with much lower beta plasma conditions than typical conditions. We discuss the role of such efficient reconnection in the formation of low‐latitude boundary layers.

Abstract Alongside magnetic reconnection, turbulence is another fundamental nonlinear plasma phenomenon that plays a key role in energy transport and conversion in space and astrophysical plasmas. From a numerical, theoretical, and observational point of view there is a long history of exploring the interplay between these two phenomena in space plasma environments; however, recent high-resolution, multi-spacecraft observations have ushered in a new era of understanding this complex topic. The interplay between reconnection and turbulence is both complex and multifaceted, and can be viewed through a number of different interrelated lenses - including turbulence acting to generate current sheets that undergo magnetic reconnection (turbulence-driven reconnection), magnetic reconnection driving turbulent dynamics in an environment (reconnection-driven turbulence) or acting as an intermediate step in the excitation of turbulence, and the random diffusive/dispersive nature of the magnetic field lines embedded in turbulent fluctuations enabling so-called stochastic reconnection. In this paper, we review the current state of knowledge on these different facets of the interplay between turbulence and reconnection in the context of collisionless plasmas, such as those found in many near-Earth astrophysical environments, from a theoretical, numerical, and observational perspective. Particular focus is given to several key regions in Earth’s magnetosphere – namely, Earth’s magnetosheath, magnetotail, and Kelvin-Helmholtz vortices on the magnetopause flanks – where NASA’s Magnetospheric Multiscale mission has been providing new insights into the topic.

Abstract We examine characteristics of the boundaries of 11 Kelvin‐Helmholtz vortex crossings observed by MMS on 23 September 2017 under southward IMF conditions. At both the leading and trailing edges, boundary regions of mixed plasma are observed together with lower‐hybrid wave activity. We found that thicker boundary regions feature a higher number of sub‐ion scale current sheets, of which only one shows clear reconnection signatures. Moreover, the lower‐hybrid waves along the vortex spine region are identified as an effective mechanism for plasma transport with an estimated diffusion coefficient of s. Comparisons with 3D simulations performed under the same conditions as the MMS event suggest that the extension of the boundary regions as well as the number of current sheets are related to different evolutionary stages of the vortices. Such observations can be explained by changes in the upstream magnetic field conditions.

Abstract There is ample evidence for magnetic reconnection in the solar system, but it is a nontrivial task to visualize, to determine the proper approaches and frames to study, and in turn to elucidate the physical processes at work in reconnection regions from in-situ measurements of plasma particles and electromagnetic fields. Here an overview is given of a variety of single- and multi-spacecraft data analysis techniques that are key to revealing the context of in-situ observations of magnetic reconnection in space and for detecting and analyzing the diffusion regions where ions and/or electrons are demagnetized. We focus on recent advances in the era of the Magnetospheric Multiscale mission, which has made electron-scale, multi-point measurements of magnetic reconnection in and around Earth’s magnetosphere.

Abstract Flux transfer events (FTEs) are a type of magnetospheric phenomena that exhibit distinctive observational signatures from the in situ spacecraft measurements. They are generally believed to possess a magnetic field configuration of a magnetic flux rope and formed through magnetic reconnection at the dayside magnetopause, sometimes accompanied with enhanced plasma convection in the ionosphere. We examine two FTE intervals under the condition of southward interplanetary magnetic field (IMF) with a dawn‐dusk component. We apply the Grad‐Shafranov (GS) reconstruction method to the in situ measurements by the Magnetospheric Multiscale (MMS) spacecraft to derive the magnetic flux contents associated with the FTE flux ropes. In particular, given a cylindrical magnetic flux rope configuration derived from the GS reconstruction, the magnetic flux content can be characterized by both the toroidal (axial) and poloidal fluxes. We then estimate the amount of magnetic flux (i.e., the reconnection flux) encompassed by the area “opened” in the ionosphere, based on the ground‐based Super Dual Auroral Radar Network (SuperDARN) observations. We find that for event 1, the FTE flux rope is oriented in the approximate dawn‐dusk direction, and the amount of its total poloidal magnetic flux falls within the range of the corresponding reconnection flux. For event 2, the FTE flux rope is oriented in the north‐south direction. Both the FTE flux and the reconnection flux have greater uncertainty. We provide a detailed description about a formation scenario of sequential magnetic reconnection between adjacent field lines based on the FTE flux rope configurations from our results.

Abstract An LMN coordinate system for magnetic reconnection events is sometimes determined by defining N as the direction of the gradient across the current sheet and L as the direction of maximum variance of the magnetic field. The third direction, M, is often assumed to be the direction of zero gradient, and thus the orientation of the X line. But when there is a guide field, the X line direction may have a significant component in the L direction defined in this way. For a 2D description, a coordinate system describing such an event would preferably be defined using a different coordinate direction M′ oriented along the X line. Here we use a 3D particle‐in‐cell simulation to show that the X line is oriented approximately along the direction bisecting the asymptotic magnetic field directions on the two sides of the current sheet. We describe two possible ways to determine the orientation of the X line from spacecraft data, one using the minimum gradient direction from Minimum Directional Derivative analysis at distances of the order of the current sheet thickness from the X line, and another using the bisection direction based on the asymptotic magnetic fields outside the current sheet. We discuss conditions for validity of these estimates, and we illustrate these conditions using several Magnetospheric Multiscale (MMS) events. We also show that intersection of a flux rope due to secondary reconnection with the primary X line can destroy invariance along the X line and negate the validity of a two‐dimensional description.

Abstract Various physical processes in association with magnetic reconnection occur over multiple scales from the microscopic to macroscopic scale lengths. This paper reviews multi-scale and cross-scale aspects of magnetic reconnection revealed in the near-Earth space beyond the general global-scale features and magnetospheric circulation organized by the Dungey Cycle. Significant and novel advancements recently reported, in particular, since the launch of the Magnetospheric Multi-scale mission (MMS), are highlighted being categorized into different locations with different magnetic topologies. These potentially paradigm-shifting findings include shock and foreshock transient driven reconnection, magnetosheath turbulent reconnection, flow shear driven reconnection, multiple X-line structures generated in the dayside/flankside/nightside magnetospheric current sheets, development and evolution of reconnection-driven structures such as flux transfer events, flux ropes, and dipolarization fronts, and their interactions with ambient plasmas. The paper emphasizes key aspects of kinetic processes leading to multi-scale structures and bringing large-scale impacts of magnetic reconnection as discovered in the geospace environment. These key features can be relevant and applicable to understanding other heliospheric and astrophysical systems.

Abstract We present Magnetospheric Multiscale observations of an electron‐scale reconnecting current sheet in the mixing region along the trailing edge of a Kelvin‐Helmholtz vortex during southward interplanetary magnetic field conditions. Within this region, we observe intense electrostatic wave activity, consistent with lower‐hybrid waves. These waves lead to the transport of high‐density magnetosheath plasma across the boundary layer into the magnetosphere and generate a mixing region with highly compressed magnetic field lines, leading to the formation of a thin current sheet associated with electron‐scale reconnection signatures. Consistencies between these reconnection signatures and a realistic, local, fully‐kinetic simulation modeling this current sheet indicate a temporal evolution of the observed electron‐scale reconnection current sheet. The multi‐scale and inter‐process character of this event can help us understand plasma mixing connected to the Kelvin‐Helmholtz instability and the temporal evolution of electron‐scale reconnection.

About sixty years ago it was proposed that the solar wind entry and changes in magnetospheric magnetic topology via dayside magnetic reconnection initiate the magnetospheric convection over the poles. On the other hand, the quasi-viscous interaction via Kelvin-Helmholtz waves/vortices was proposed to lead to the solar wind entry and magnetospheric convection. Since then, the two processes have been thought to regulate the solar wind and earth’s magnetosphere coupling. However, their relative efficiency and importance leave a lot of room for enhanced and quantitative understanding. Kelvin-Helmholtz instability operating on the entire surface of the magnetopause also provide a place for not only solar wind transport but also energetic particle transport or escape, thus, being an efficient channel for two-way transport. Recent observations and simulations indicate that the flanks of the earth’s magnetosphere can act as a pathway to/from the central magnetotail current sheet. Possible causality between the flank-side dynamics and magnetotail current sheet stability has never been explored. In this paper we discuss our perspective on these unsolved areas of Heliophysics research with brief suggestions of observational and numerical approaches.

Magnetic reconnection is a key fundamental process in collisionless plasmas, which converts magnetic energy to plasma kinetic energy. Past observation and simulation studies suggested that this process causes an efficient energy conversion through the formation and coalescence of multiple magnetic islands. In this study, based on a large-scale two-dimensional fully kinetic simulations of coalescing multiple islands with a moderate guide magnetic field, we first examined the spatial dimensions of the internal structures of the coalescing islands. The results show that the dimensions of the structures in the directions normal to and along the initial current sheet depend on the initial thickness of the current sheet and the number of coalescing islands. We then found that the horizontal dimension of the structures controls the evolution time scale of the island coalescence process. We further found that when the vertical dimension of the structures, which corresponds to the length of the reconnection X-line in the reconnection outflow direction at the merging point between the two coalescing islands, is sufficiently longer than the ion inertial length, reconnection in the merging current sheet can well mature and both ions and electrons can be effectively heated around the merging X-line. The obtained scaling predicts that such a strong heating by well-matured reconnection in the island coalescence process would be seen in various plasma environments, such as the Earth's magnetotail and solar flares.

Abstract Dayside transients, such as hot flow anomalies, foreshock bubbles, magnetosheath jets, flux transfer events, and surface waves, are frequently observed upstream from the bow shock, in the magnetosheath, and at the magnetopause. They play a significant role in the solar wind-magnetosphere-ionosphere coupling. Foreshock transient phenomena, associated with variations in the solar wind dynamic pressure, deform the magnetopause, and in turn generates field-aligned currents (FACs) connected to the auroral ionosphere. Solar wind dynamic pressure variations and transient phenomena at the dayside magnetopause drive magnetospheric ultra low frequency (ULF) waves, which can play an important role in the dynamics of Earth’s radiation belts. These transient phenomena and their geoeffects have been investigated using coordinated in-situ spacecraft observations, spacecraft-borne imagers, ground-based observations, and numerical simulations. Cluster, THEMIS, Geotail, and MMS multi-mission observations allow us to track the motion and time evolution of transient phenomena at different spatial and temporal scales in detail, whereas ground-based experiments can observe the ionospheric projections of transient magnetopause phenomena such as waves on the magnetopause driven by hot flow anomalies or flux transfer events produced by bursty reconnection across their full longitudinal and latitudinal extent. Magnetohydrodynamics (MHD), hybrid, and particle-in-cell (PIC) simulations are powerful tools to simulate the dayside transient phenomena. This paper provides a comprehensive review of the present understanding of dayside transient phenomena at Earth and other planets, their geoeffects, and outstanding questions.