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.