Zhifang Guo

Magnetopause flux ropes (MFRs) act as a critical mediator of solar wind–magnetosphere coupling, yet their complete global lifecycle, including generation, poleward transport, and topological collapse, remains poorly understood. We investigate this evolution with a three‐dimensional global hybrid simulation under a purely southward interplanetary magnetic field (IMF). We find that small‐scale MFRs form at low latitudes via multiple X‐line reconnections and drift poleward along the magnetopause boundary layer, driven mainly by reconnection outflows. Trailing MFRs compress leading ones, and discrete small‐scale MFRs coalesce into larger coherent structures via re‐reconnection. During poleward propagation, merged MFRs undergo coupled axial stretching induced by field‐aligned ion velocity difference and latitudinal flow difference, such that their axial extent initially expands to a maximum and then gradually contracts. As MFRs propagate toward mid‐ and high‐latitudes, magnetosheath plasma flows replace low‐latitude reconnection outflows as the necessary external driver and significantly accelerate poleward advection. Spatially inhomogeneous magnetosheath plasma flows further stretch MFRs in the dawn–dusk azimuthal direction, eroding their helical stability and inducing topological collapse. MFR helical structures therefore dissipate before reaching the magnetospheric cusp, without cusp‐mediated re‐reconnection. Our results reveal a global evolutionary pathway for dayside MFRs, in which coalescence and collapse are driven by spatially variable magnetosheath flows under purely southward IMF conditions.

Using the gyrokinetic electron and fully kinetic ion particle simulation model, we conduct an in-depth investigation into the dynamics of magnetic reconnection in the presence of a strong guide field ( B G / B y 0 ), which is a prevalent condition in solar wind and laboratory plasmas. Here, B y 0 denotes the antiparallel component of the magnetic field, and B G represents the guide field. Our simulations explore both 2D and 3D force-free current sheets, incorporating a strong guide field ( B G / B y 0 = 5 ), a low plasma beta ( β ~ 0.02 ), and a near-realistic mass ratio ( m i / m e = 900 ). In the 2D simulations, as the mass ratio m i / m e increases, a remarkable acceleration in the formation of the primary current profile is observed. This finding highlights the substantial impact of the mass ratio on the evolution of the primary reconnection current sheet. In 3D simulations, following primary reconnection, secondary instabilities develop in the separatrix region, driven by the finite m i / m e effect. A comparative analysis of 3D and 2D results reveals that these secondary instabilities are a direct manifestation of nonlinear, 3D effects. Furthermore, the structure and growth rate of these instabilities align closely with the predictions from eigenmode theory, identifying them as ideal MHD kink modes. Notably, kinetic Alfvén waves are emitted outside the reconnection region, sharing the same wavenumber k y as the secondary instabilities, which play a crucial role in facilitating parallel electron acceleration and heating via Landau damping. These findings enhance our understanding of the intricate dynamics associated with magnetic reconnection in the presence of a strong guide field.

Magnetic reconnection under a strong guide field B G / B y 0 ≫ 1 , as in solar wind and laboratory plasmas, is investigated using the gyrokinetic electron and fully kinetic ion (GeFi) particle simulation model, where B y 0 is the antiparallel component of the magnetic field and B G is the guide field. Cases with two-dimensional force-free current sheets are presented in conditions with a large guide field and low plasma beta, similar to the reconnection events associated with solar flares. To gain insights into how the ion-to-electron mass ratio ( m i / m e ) and normalized guide field ( B G / B y 0 ) affect the rate and structures of reconnection, we present a series of simulations by varying m i / m e from 100 to 1836 and B G / B y 0 from 5 to 15. First, linear eigenmode analysis is performed for the collisionless tearing mode. We find that the eigenmode structure and growth rate of the tearing mode obtained from the GeFi simulation are in excellent agreement with the eigenmode theory for strong B G . Subsequently, we employ GeFi simulations to explore the reconnection rate and current sheet structure across a wide range of m i / m e and B G / B y 0 . Intriguingly, it is found that the reconnection rate is insensitive to m i / m e . The growth rate of the tearing mode, however, has a strong dependence on m i / m e . The reconnection rate exhibits a clear increase with decreasing B G / B y 0 and also strongly depends on B G / B y 0 . Furthermore, the length of the electron diffusion region increases with m i / m e . These findings advance understanding of the reconnection dynamics in solar flares and other astrophysical phenomena.

Magnetic reconnection is one of the most universal processes in space plasma that is responsible for charged particle acceleration and the mixing and heating of plasma populations. In this paper we consider a triggering process of reconnection that is driven by interaction of two discontinuities: solar wind rotational discontinuity and tangential discontinuity at Earth’s magnetospheric boundary, the magnetopause. Combining multispacecraft measurements and global hybrid simulations, we show that solar wind discontinuities may drive the magnetopause reconnection and cause the mixing of the solar wind and magnetosphere plasmas around the magnetopause, well downstream of the solar wind flow. Since large-amplitude discontinuities are frequently observed in the solar wind and predicted for various stellar winds, our results of reconnection driven by the discontinuity–discontinuity interaction may have a broad application beyond the magnetosphere.

Particle injection by magnetotail reconnection plays an important role in the magnetospheric physics, since these particles may be accelerated to high energy and constituent energetic particles in the inner magnetosphere. In this paper, we trace such ion particles from a near‐Earth reconnection region to the inner magnetosphere self‐consistently in a 3‐D global hybrid simulation using ANGIE3D, and demonstrate the important roles of the earthward fast flows in the ion acceleration process in the tail plasma sheet. Although the particles can gain several times of their initial energy from the reconnection X‐line region, a dramatic increase of the ion energy (from a few keV up to a few tens of keV) occurs in a very short period of time due to their encounter with the earthward fast flows. A large portion (≳70%) of the earthward moving particles around the focused reconnection site encounter fast flows and are significantly accelerated. Our results indicate that fast flow electric fields play major roles, as opposed to other common mechanisms such as the adiabatic Betatron and Fermi processes, in ion acceleration from the near‐tail region to the inner magnetosphere. In addition, dipolarization fronts associated with magnetic reconnection also affect the particle acceleration. On the global scale, it is found that particles can encounter the reconnection region first and then return to the fast flows multiple times, and thus the acceleration is through multiple local acceleration processes, leading to particle energy ∼50 keV by reconnection/fast flows in the tail. The global simulation shows that particle acceleration involves multiple regions during their injection into the ring current.

The nightside magnetic field at the middle and low latitude observatory is usually disturbed, decreasing in its north component (noted as Bx) during the periods of enhanced space weather activity. As indicated by the SYM‐H index, the decreasing field mainly originated from the magnetospheric westward ring current and the cross‐tail currents. In the meridian direction, the maximum of the disturbed field has been observed not always located at the geomagnetic dipole equator. One possible explanation is that the latitudinal position of the magnetotail current systems is affected by the orientation of solar wind to the Earth's geomagnetic dipole axis (noted as tilt angle). For a non‐zero tilt angle, the current deviates from the equatorial position (e.g., Malin & Mete Işikara, 1976; 10.1111/j.1365-246x.1976.tb07096.x). In this study, the mid‐low latitudinal distribution of the nightside disturbed field is systematically investigated every ∼1.5 hr, based on the magnetic surveys of the Swarm satellite constellation at polar orbits of ∼450 km altitude. The meridian distribution of Bx regularly appears like an upward‐opening parabolic shape with its vertex representing the magnetic minimum. In summer, the minimum is found generally located in the southern hemisphere, while in winter the pattern is reversed. The overall distribution demonstrates north‐south asymmetry. When the disturbed level is low, the deviation of the magnetic minimum from the dipole equator usually gets larger. The north‐south asymmetric distribution during storm times is further analyzed. In the initial phase, the magnetic minimum was shifted prominently (about 10°–30°) from the equator, then moved back (with the offset less than 10°) in the main phase and recovery phase in most cases. Moreover, the latitudinal deviation undergoes a diurnal variation, which is conspicuous in spring and autumn. It seems the asymmetric distribution is anti‐correlated to the contribution of the ring current, indicated by the SYM‐H index. The warping structure of the cross‐tail current sheet probably plays a predominant role in generating the north‐south asymmetry of the magnetic field disturbances for non‐zero dipole tilt angles. Monitoring the external magnetic configuration via the low‐Earth‐orbit satellites will provide fundamental evidence to identify the relative position of the ring current and the cross‐tail current, as well as the dynamic processes in the near magnetotail.

Using a three‐dimensional global hybrid simulation, we investigate the formation and evolution of ion‐scale magnetic reconnection inside an interplanetary rotational discontinuity (RD) owing to its interaction with the quasi‐perpendicular (Q‐) bow shock and the magnetosphere. The interplanetary magnetic field (IMF) is initially predominantly northward, while it changes to purely southward across the RD. A significantly thinned RD current layer with a width ∼ ion skin depth and normal magnetic field is formed by a shock compression process as the RD interacts with the Q‐ shock. Magnetic reconnection thereupon takes place inside the thin RD current layer, where Hall magnetic and electric fields, reconnection electric field, and high‐speed ion outflow jets are identified. Simultaneously, flux ropes form with an extension of a few ion inertial lengths. As the RD is transmitted into the magnetosheath, multiple reconnection sites lead to the formation of longer flux ropes. Moreover, magnetosheath reconnection is also found at new reconnection sites inside the RD. No reconnection is found in the Q‐ shock alone outside the RD. The flux ropes propagate poleward and tailward in the magnetosheath. In addition, magnetopause reconnection takes place under the southward IMF on the sunward side of the RD after the RD passes through the magnetopause.

Ion‐scale magnetic reconnection has been observed downstream of the terrestrial quasi‐parallel (Q‐∥) shock. Whether it is driven by interplanetary discontinuities or turbulent Q‐∥ shock, however, is unclear. Using three‐dimensional global hybrid simulation, we investigate the generation of magnetic reconnection downstream of the Q‐∥ shock, while an interplanetary rotational discontinuity (RD) is launched to the bow shock. Cases with various solar wind Alfvén Mach numbers, M_A = 3.0 to 8, and propagation directions of the RD are presented. The propagation direction n is assumed to be in the GSE xz plane and pointing earthward, with n = (−sin(θ₁₂/2),0,−cos(θ₁₂/2)), where θ₁₂ is the angle between the upstream (B₁) and downstream (B₂) magnetic fields across the transmitted RD. It is found that magnetic reconnection occurs inside the RD downstream of the Q‐∥ shock, forming flux ropes extending along the dawn‐dusk direction about tens of ion inertial lengths. Large‐amplitude low‐frequency waves originated from the Q‐∥ shock lead to the bending and squeezing of the field lines around the RD, which play an important role in triggering reconnection inside the RD. As the RD impacts the dayside magnetopause, magnetopause reconnection takes place between the field lines behind the RD and geomagnetic field lines. Nevertheless, no reconnection is found downstream of the Q‐∥ shock itself or outside the RD in the magnetosheath. The existent and structure of reconnection in the magnetosheath are found to strongly depend on the parameters M_A and n. Our simulation shows that ion‐scale magnetic reconnection is driven by an external driver in the form of the compression of an RD around the bow shock and in the magnetosheath, rather than caused by the turbulent Q‐∥ shock alone.

Magnetosheath reconnection due to the interaction of an interplanetary directional discontinuity with the bow shock and Earth's magnetosphere under an initially northward interplanetary magnetic field (IMF) has been investigated in previous simulations (e.g., Guo et al., 2018, https://doi.org/10.1029/2018ja025679). Under an initially southward IMF, the magnetosheath reconnection could interact with reconnection at the magnetopause. In this study, using three‐dimensional (3D) global‐scale hybrid simulations, we present cases with incoming tangential discontinuities (TDs) in an initially southward IMF, which possess various magnetic field rotation angles (ΔΦ) and half‐width (w), to study the effects of pre‐existing magnetopause reconnection on the formation of magnetosheath flux ropes, as well as the subsequent interaction between the magnetopause reconnection and magnetosheath reconnection, with downstreams of both Quasi‐perpendicular (Q‐) and Quasi‐parallel (Q‐||) shock examined. The initial IMF is assumed to be oblique, with a finite B_x and B_z, similar to that in Guo et al. (2018), https://doi.org/10.1029/2018ja025679 but with a southward B_z < 0. Compared with the cases with an initially northward IMF, magnetopause reconnection weakens the compression processes of the TD and leads to less frequent reconnection in the magnetosheath. The existence and the structure of magnetosheath reconnection are found to strongly depend on the parameters w and of the TD. When interacting with the magnetopause reconnection, the magnetosheath flux ropes can re‐reconnect with the geomagnetic dipole field lines, forming new structures of magnetopause flux ropes. The resulting evolution of flux rope configuration is illustrated.

We use the magnetohydrodynamic (MHD) with embedded particle‐in‐cell model (MHD‐EPIC) to study the Geospace Environment Modeling (GEM) dayside kinetic processes challenge event at 01:50–03:00 UT on 18 November 2015, when the magnetosphere was driven by a steady southward interplanetary magnetic field (IMF). In the MHD‐EPIC simulation, the dayside magnetopause is covered by a PIC code so that the dayside reconnection is properly handled. We compare the magnetic fields and the plasma profiles of the magnetopause crossing with the MMS3 spacecraft observations. Most variables match the observations well in the magnetosphere, in the magnetosheath, and also during the current sheet crossing. The MHD‐EPIC simulation produces flux ropes, and we demonstrate that some magnetic field and plasma features observed by the MMS3 spacecraft can be reproduced by a flux rope crossing event. We use an algorithm to automatically identify the reconnection sites from the simulation results. It turns out that there are usually multiple X‐lines at the magnetopause. By tracing the locations of the X‐lines, we find that the typical moving speed of the X‐line endpoints is about 70 km/s, which is higher than but still comparable with the ground‐based observations.