Current Sheet Research Articles (Page 1)

Abstract In this study, we investigated the role of interplanetary magnetic field (IMF) orientation in controlling the location and structure of the current sheet (CS) in the Martian magnetotail. Based on carefully selected cases as well as statistical studies using the magnetic field and plasma data of Mars Atmosphere and Volatile EvolutioN from 2014 October to 2020 February, our work shows that the IMF orientation can systematically influence the magnetotail CS structure of Mars. Our results reveal a systematic dawn–dusk ( Y -axis) asymmetry in CS positioning under different IMF orientations. Specifically, a sunward-directed IMF (cone angle < 60°) shifts the CS toward the dusk (+ Y ) hemisphere, while a tailward-directed IMF (cone angle > 120°) shifts it toward the dawn (− Y ) hemisphere. Under cross-flow IMF conditions (70° < cone angle < 110°), no significant CS displacement is observed. This pattern persists even after accounting for possible confounding influences such as Martian crustal magnetic fields and solar EUV intensity. Our findings suggest that the cone angle of the IMF can systematically control the CS structure and magnetic lobes in the magnetotail of Mars. This IMF-controlled CS asymmetry has implications for understanding planetary ion escape and magnetotail dynamics at unmagnetized bodies.

Neutron emission mechanisms in a plasma focus: insights from current sheet dynamics

Abstract Understanding the mechanisms behind fast magnetic reconnection and subsequent plasma heating is essential to explain the temperatures of a few million kelvin observed in the solar corona. This study investigates the influence of initial current sheet width and stratification on the reconnection process and coronal temperature within a large-scale magnetohydrodynamics framework. Additionally, it further addresses the contribution of small- to large-scale dynamics through the Hall term. Using numerical simulations, we demonstrate that even modest reductions in the initial current sheet width significantly speed up the reconnection process, yielding faster rates of temperature rise and achieving high peak values without requiring external fluctuations. Moreover, introducing a dependence on scale height in the initial coronal magnetic field and density profiles enhances reconnection rates and temperature increases. We also observe earlier evolution of fast reconnection with narrower current sheets and shorter scale heights, resulting in a more frequent formation of magnetic islands and X-points that evolve dynamically over time. Under these initial settings, the Hall term’s curl is amplified by three orders of magnitude, reaching 1% of the characteristic scale required to influence large-scale dynamics and offering insight into the multiscale interactions essential for coronal heating processes. This study underscores the significance of minor adjustments in initial conditions for enhancing magnetic reconnection, coronal heating, and amplifying the Hall term’s influence on large-scale dynamics.

Abstract In this paper, we report multiwavelength and multipoint observations of the prominence eruption originating from active region 11163, which generated an M3.5 class flare and a coronal mass ejection (CME) on 2011 February 24. The prominence lifts off and propagates nonradially in the southeast direction. Using the revised cone model, we carry out three-dimensional reconstructions of the ice-cream-like prominence. It is found that the latitudinal inclination angle decreases from ∼60° to ∼37°, indicating that the prominence tends to propagate more radially. The longitudinal inclination angle almost keeps constant (−6°). The highly inclined prominence eruption and the related CME drive an extreme-ultraviolet (EUV) wave, which propagates southward at speeds of ∼381.60 and ∼398.59 km s −1 observed in 193 and 304 Å, respectively. The M3.5 class flare presents quasiperiodic pulsations (QPPs) in soft X-ray, hard X-ray, EUV, and radio wavelengths with periods of 80−120 s. Contemporary with the flare QPPs, a thin current sheet and multiple plasmoids are observed following the eruptive prominence. Combining with the appearance of drifting pulsation structure, the QPPs are most probably generated by quasiperiodic magnetic reconnection and particle accelerations as a result of plasmoids in the current sheet.

Abstract Understanding the spatial variability and correlation lengths throughout Stream Interaction Regions (SIRs) has importance both for fundamentally understanding the structuring of the solar wind and SIRs and for space weather operations. SIR observations from Advanced Composition Explorer and Wind are utilized to characterize the spatial variability of these structures and to compute correlation lengths of the solar wind speed and magnetic field magnitude for different regions of the SIR. In general, the probability of agreement between time‐shifted magnetic field magnitudes and solar wind speeds decreases with increasing spacecraft separation, with this degradation being more pronounced in the magnetic field magnitude. While the slow compressed region of the SIR has the steepest decline in agreement for the magnetic field magnitude, it is the shallowest decline for the percent agreement of solar wind speed. Despite these trends, the correlation length is longest in the slow compressed region for both magnetic field magnitude and solar wind speed, with the magnetic field magnitude generally having a longer correlation length than speed. These findings are consistent with current sheet layering within the compressed regions of the SIR and impacts of large‐amplitude Alfvénic fluctuations within the compressed and pristine fast solar wind. These results also further demonstrate the limitations of treating structures in the solar wind as laminar slabs, rather than being highly structured and variable, which may have impacts on uncertainties in space weather forecasting.

Abstract The quiescent or dynamic nature of fine-scale raylike features in the Sun corona, observed in visible light, is still an open question. Here, we show that most of the daily and hourly periodic variations in visible light brightness of the high corona (up to 15 R ⊙ ) are aligned to the tip of streamers and are consistent with the periodicity of plasma release from simulations of tearing-induced magnetic reconnection at the heliospheric current sheet. The areas in which we detect periodicities can be used as tracers of nonquiescent fine coronal rays. This also allows their distinction from coronal rays more likely to be real quiescent features or associated with smaller and/or faster unresolved brightness variations. In the low- and middle-corona (down to 1.4 R ⊙ ) similar brightness variations are observed along loop-like and cusp-like features marking boundaries of streamers, which then connect to radial features in the high corona. This suggests the presence of additional mechanisms in the low- and middle-corona periodically releasing density structures in the solar wind. The periodicity distributions show a solar cycle modulation with shorter periods (smaller structures) during solar maximum. Periodicities are observed within streamers during solar minimum but are visible at all latitudes, even extending radially from the poles, during solar maximum.

Magnets in the accelerator interaction region (IR) present significant challenges because of high field requirements and limited available space. Conical-shaped magnets offer advantages in these environments by allowing closer placement to the interaction point while maintaining clearance from synchrotron radiation. Interestingly, numerical studies have shown that conical canted-cosine-theta (CCT) designs produce a constant field distribution along the axial direction in the IR quadrupoles for the Electron-Ion Collider (EIC) at Brookhaven National Laboratory. However, the field harmonics generated by conical CCT windings are not yet fully understood. This paper presents an analytical approach to describe the magnetic field produced by a conical surface current and proposes a method for designing conical CCT magnets for accelerator applications. First, we begin with a surface current sheet having a general cosine-theta distribution in spherical coordinates and solve the vector potential using the Green’s function. The magnetic fields generated by the conical current sheet are expressed using associated Legendre polynomials. These results are then related to circular field harmonics and integral field harmonics for designing a coil that produces a pure multipole field. Next, a single layer of the conical CCT winding path is produced based on the cosine-theta current distribution. Finally, the magnetic field quality of dipole and quadrupole conical CCT coils with multiple layers is verified using the Biot-Savart law.

Abstract This study presents comprehensive observations of intense high‐speed jets (HSJs) and their global impacts on the inner magnetosphere and ionosphere, using multi‐satellite and ground‐based observations. Cluster‐4, located near the bow shock, observed signatures associated with foreshock transients generated by a solar wind directional discontinuity. Downstream of the bow shock, THEMIS‐A, positioned post‐noon (∼12.8 MLT) in the magnetosheath, detected a sunward plasma flow prior to crossing into the magnetosphere. Nearly simultaneously, THEMIS‐E, inside the magnetosphere at ∼13.4 MLT, suddenly crossed into the magnetosheath and observed intense earthward HSJs. The strong compression of the magnetopause current sheet forced GOES‐13 to temporarily enter the magnetosheath, while SuperDARN radars registered enhanced poleward ionospheric convection and magnetometers detected signatures of westward currents. These sequential observations provide a rare, integrated demonstration of how an upstream foreshock disturbance transfers energy and momentum throughout the coupled magnetosphere‐ionosphere system.

Abstract Solar flares are major space weather events that result from the explosive conversion of stored magnetic energy into bulk motion, plasma heating, and particle acceleration. While the standard flare model has proven highly successful in explaining key morphological features of flare observations, many aspects of the energy release are not yet understood. In particular, the turbulent three-dimensional structure of the flare current sheet is thought to play an important role in fast reconnection, particle acceleration, and bursty dynamics. Although direct diagnosis of the magnetic field dynamics in the corona remains highly challenging, rich information may be gleaned from flare ribbons, which represent the chromospheric imprints of reconnection in the corona. Intriguingly, recent solar imaging observations have revealed a diversity of fine structure in flare ribbons that hints at corresponding complexity in the reconnection region. We present high-resolution three-dimensional magnetohydrodynamics simulations of an eruptive flare and describe our efforts to interpret fine-scale ribbon features in terms of the current sheet dynamics. In our model, the current sheet is characterized by many coherent magnetic structures known as plasmoids. We derive a model analog for ribbons by generating a time series of field-line length maps ( L -maps) and identifying abrupt shortenings as flare reconnection events. We thereby demonstrate that plasmoids imprint transient “spirals” along the analog of the ribbon front, with a morphology consistent with observed fine structure. We discuss the implications of these results for interpreting SolO, Interface Region Imaging Spectrograph, and Daniel K. Inouye Solar Telescope observations of explosive flare energy release.

Abstract High energy electron bunches from the laser-driven relativistic magnetic reconnection has been intensively studied. However, diagnostic methods for identifying such acceleration mechanism remain inadequate. This study utilizes 2.5-dimensional Particle-In-Cell simulations to explore a diagnostic approach based on electron polarization dynamics governed by the Thomas-Bargmann-Michel-Telegdi equation. The trajectories of electrons accelerated by the MR are confined in the current sheet, where the magnetic field is effectively annihilated. The resulting electron beam exhibits extremely low depolarization, distinguishing it from other accelerated populations and serving as a definitive signature of magnetic reconnection. This diagnostic approach enables more detailed investigations of magnetic reconnection-driven acceleration mechanisms in future studies.

Abstract Magnetic reconnection explosively converts magnetic energy into plasma heating and particle acceleration. At Earth's magnetopause, reconnection governs solar wind‐magnetosphere coupling and drives global convection. Understanding these processes requires resolving reconnection's spatiotemporal evolution, which is difficult for in situ observations but achievable in laboratory experiments. However, building a geometry like the real magnetopause in laboratory remains a key challenge. Here, we present the first laboratory experiment at the Space Plasma Environment Research Facility trying to replicates Earth's magnetopause configuration. Using a dipole magnet (mimicking magnetosphere) and coaxial flux cores (simulating solar wind), we establish a magnetopause‐like current sheet via dynamic plasma compression. Measured Hall magnetic fields confirm Hall reconnection, transitioning from an asymmetric multiple x ‐lines state to a symmetric single x ‐line state. This dynamic evolution will impact global energy conversion and transport at magnetopause.

Abstract We study electron energization in turbulence-generated current sheets in the shock transition region by means of fully kinetic collisionless plasma simulations and theory. Using parameters in the Earth’s bow shock, we perform a two-dimensional particle-in-cell simulation of a quasi-parallel shock. In shock turbulence, many current sheets are produced, including those exhibiting magnetic reconnection and those that are not reconnecting. The electron temperature is enhanced in nonreconnecting current sheets as well as in reconnecting current sheets and magnetic islands. Performing electron trajectory tracing analysis, we find that energetic electrons are produced in nonreconnecting thinning current sheets. The motional electric field during the thinning process of a current sheet energizes both magnetized and unmagnetized electrons. We analytically show that the energization rate for unmagnetized electrons is slightly less than that of adiabatic energization for magnetized electrons, but unmagnetized electrons can be effectively trapped in magnetic field structures formed in thinning current sheets and continue to be energized. These nonreconnecting current sheets produce energetic electrons whose energies are comparable to the energetic electrons produced in magnetic islands, and they can reach the injection energy for diffusive shock acceleration, which is an acceleration mechanism for producing cosmic rays. The number of electrons that are energized in nonreconnecting current sheets is about a quarter of that in reconnection regions. The energization mechanism can be applicable to various space and astrophysical environments, including planetary bow shocks and supernova remnant shocks.

Abstract Magnetic reconnection is a fundamental plasma process that alters the magnetic field topology and releases magnetic energy. Most numerical simulations and spacecraft observations assume a two‐dimensional diffusion region, with the electron diffusion region (EDR) embedded in the same plane as the ion diffusion region (IDR) and a uniform guide field throughout. Using observations from Magnetospheric Multiscale mission, we report a non‐coplanar, knotted EDR in Earth's magnetotail current sheet. The reconnection plane of the knotted EDR deviates by approximately 38° from that of the IDR, with the guide field exhibiting both a 38° directional shift and a twofold increase in amplitude. Moreover, the Hall magnetic field is bipolar in the EDR but quadrupolar in the IDR, indicating different Hall current structures at electron and ion scales. These observations highlight the importance of three‐dimensional effects and illustrate the complexity of multiscale coupling between the EDR and IDR during reconnection studies.

Solar flares accelerate electrons, creating non-thermal energy distributions. However, the acceleration sites and dominant acceleration mechanisms remain largely unknown. We study the characteristics of electron acceleration and subsequent non-thermal energy distribution in a 2D coronal plasmoid-mediated reconnecting current sheet. We used test particles and the guiding centre approximation to transport electrons in a static coronal 2D fan-spine topology magnetohydrodynamic (MHD) snapshot. The snapshot was from a Bifrost simulation that featured plasmoid-mediated reconnection at a current sheet. To sample initial particle conditions that lead to non-thermal energies, we used importance sampling. In this way, the characteristics of the non-thermal electrons were statistically representative of the MHD plasma. The energy distribution of the electrons forms a non-thermal power law that varies with our tolerance of the guiding centre approximation's validity, from no obvious power law to a power law with an exponent of -4 (the power law also depends on the statistical weighing of the electrons). The non-thermal electrons gain energy through a gradual betatron acceleration close to magnetic null points associated with plasmoids. In this static, asymmetric, coronal, 2D fan-spine topology MHD configuration, non-thermal electron acceleration occurs only in the vicinity of null points associated with magnetic gradients and electric fields induced by plasmoid formation and ejection. However, the guiding centre approximation alone is not sufficient to properly estimate the shape of the non-thermal power law since, according to our results, electron acceleration is correlated with the adiabaticity of the particles' motion. The results also show that the particle power law formation is biased by the test particle sampling procedure.

Abstract The corona is a crucial region that connects the solar surface to the solar wind and serves as the primary site of solar activity. The 2024 total solar eclipse (TSE) provides a unique opportunity to investigate the large-scale coronal structure. Combined with TSE observations, we study the impact of the magnetic structure of the farside active region, located in the eastern hemisphere of the Sun that has not yet rotated into the Earth field of view, on a global magnetohydrodynamic simulation. To address the limitation of single-view measurements for the routine synoptic map, we correct the magnetic field in the farside region by incorporating full-disk magnetograms measured several days after the TSE, allowing us to capture the temporal evolution of the photospheric magnetic field in near real time. Simulation results demonstrate that the local magnetic field in the farside active region can significantly influence the global coronal structure, by altering the position of the heliospheric current sheet, and further affect the global distribution of plasma parameters, even in polar regions. A comparison of the simulation results with white-light TSE + Large Angle and Spectrometric Coronagraph C2 observations and in situ measurements by the Parker Solar Probe reveals that the composite synoptic map improves the accuracy of the coronal modeling. This work provides robust support for advancing our understanding of coronal evolution, as well as deepening the link between the photosphere and large-scale coronal structure. Furthermore, it establishes a theoretical foundation for the future development of multiview stereoscopic measurements of the photospheric magnetic field.

Abstract To understand the ion injection for diffusive shock acceleration in space and astrophysical shock waves, such as in planetary bow shocks and supernova remnant shocks, ion heating and acceleration in high-Alfvén-Mach-number (M A) quasi-parallel shock waves are studied by means of full particle-in-cell (PIC) simulations and theory. We perform 2D and 3D PIC simulations in the regime of M A ≥ 10, where shock-driven turbulence contains a number of current sheets and magnetic reconnection sites. The nonresonant ion–ion beam mode grows in the shock transition region, which creates current sheets, and some of them drive magnetic reconnection. The ion temperature in magnetic islands produced by ion-coupled reconnection becomes significantly higher than those in the surrounding regions. This temperature enhancement is due to two physical reasons: the reduction of the number density of the incident ions in the nonresonant wave, and the energization and heating in the magnetic islands. Ions that enter magnetic islands are energized by the Hall electric field pointing toward the island center. Ions are also energized by the motional electric field produced in the outer regions of the islands and the current sheets. The energy increase rate of ions by these energization mechanisms is much larger than that of the conventional shock drift acceleration. These energetic ions are unmagnetized, and they escape from the shock transition region toward the upstream region; therefore, ions in shocks with reconnecting current sheets can be injected into diffusive shock acceleration more efficiently than those in laminar shock waves.

Abstract We investigate the acceleration and transport of electrons in the highly fine-structured current sheet that develops during magnetic flux rope (MFR) eruptions. Our work combines ultraresolved magnetohydrodynamic (MHD) simulations of MFR eruption, with test-particle studies performed using the guiding center approximation. Our grid-adaptive, fully 3D, high-resolution MHD simulations model MFR eruptions that form complex current-sheet topologies, serving as background electromagnetic fields for particle acceleration. Within the current sheet, tearing-mode instabilities give rise to mini flux ropes. Electrons become temporarily trapped within these elongated structures, undergoing acceleration and transport processes that significantly differ from those observed in 2D or 2.5D simulations. Our findings reveal that these fine-scale structures act as efficient particle accelerators, surpassing the acceleration efficiency of single X-line reconnection events, and are capable of energizing electrons to energies exceeding 100 keV. High-energy electrons accelerated in different mini flux ropes follow distinct trajectories, due to spatially varying magnetic field connectivity, ultimately precipitating onto opposite sides of flare ribbons. Remarkably, double electron sources at the flare ribbons originate from different small-flux-rope acceleration regions, rather than from the same reconnecting field line, as previously suggested. Distinct small flux ropes possess opposite magnetic helicity, to accelerate electrons to source regions with different magnetic polarities, establishing a novel conjugate double-source configuration. Furthermore, electrons escaping from the lower regions exhibit a broken-power-law energy spectrum. This spectral break arises from electrons accelerated in disparate mini flux ropes, each exhibiting magnetic reconnection rates and acceleration efficiencies, which reflect the varying local reconnection conditions.

Abstract Magnetic reconnection is a fundamental and omnipresent energy conversion process in plasma physics. Novel observations of fields and particles from Parker Solar Probe (PSP) have shown the absence of reconnection in a large number of current sheets in the near-Sun solar wind. Using near-Sun observations from PSP encounters 4–11 (2020 January–2022 March), we investigate whether reconnection onset might be suppressed by velocity shear. We compare estimates of the tearing mode growth rate in the presence of shear flow for time periods identified as containing reconnecting current sheets versus nonreconnecting times, finding systematically larger growth rates for reconnection periods. Upon examination of the parameters associated with reconnection onset, we find that 85% of the reconnection events are embedded in slow, non-Alfvénic wind streams. We compare with fast, slow non-Alfvénic, and slow Alfvénic streams, finding that the growth rate is suppressed in highly Alfvénic fast and slow wind, and reconnection is not seen in these wind types, as would be expected from our theoretical expressions. These wind streams have strong Alfvénic flow shear, consistent with the idea of reconnection suppression by such flows. This could help explain the frequent absence of reconnection events in the highly Alfvénic, near-Sun solar wind observed by PSP. Finally, we find a steepening of both the trace and magnitude magnetic field spectra within reconnection periods in comparison to ambient wind. We tie this to the dynamics of relatively balanced turbulence within these reconnection periods and the potential generation of compressible fluctuations.

Abstract We present an unprecedented simulation of how two large-scale heliospheric transients—a coronal mass ejection (CME) and a corotating stream interaction region—collide, producing a dramatic increase in the complexity of the CME due to formation of mesoscale flux ropes. These structures are captured for the first time by a numerical simulation using high-resolution numerical grids. The circumstances that lead to the formation of these complex structures occur during solar maximum. At the time of the solar maximum taken for this study, 2014 September, the heliospheric current sheet is vertically inclined running over the poles, allowing the CME to impact a preceding slow-fast stream interaction region. The simulation is performed with the Alfvén Wave Solar Atmosphere Model (or AWSoM), with which we initiate a fast CME from active region (AR) 12158 by applying a Gibson–Low magnetic flux rope. Magnetic reconnection within the leading extremity of the CME results in the formation of mesoscale flux ropes, which contain sufficiently strong magnetic fields (∼30 nT) to affect planetary magnetospheres. Finally, we use a tetrahedral configuration of four virtual probes, corresponding to the Space Weather Investigation Frontier mission concept, to show that the mission can uniquely resolve the spatial characteristics and temporal evolution of reconnecting current sheets within the CME, as well as the resulting mesoscale structures.

Context. Coronal jets are ubiquitous, collimated million-degree ejections that contribute to the energy and mass supply of the upper solar atmosphere and the solar wind. Solar Orbiter observations provide an unprecedented opportunity to study fine-scale jets from a unique vantage point close to the Sun. Aims. We aim to uncover thin jets originating from coronal bright points (CBPs) and investigate observable features of plasmoid-mediated reconnection. Methods. We analyzed eleven datasets from the High Resolution Imager 174 Å of the Extreme Ultraviolet Imager (HRIEUV) on board Solar Orbiter, focusing on narrow jets from CBPs and signatures of magnetic reconnection within current sheets and outflow regions. To aid in the interpretation, we compared the observations with radiative-magnetohydrodynamic simulations of a CBP conducted with the Bifrost code. Results. We identified thin coronal jets originating from CBPs with widths ranging from 253 km to 706 km. These are scales that could not be resolved with previous EUV imaging instruments. Remarkably, these jets are 30−85% brighter than their surroundings and can extend up to 22 Mm, while maintaining their narrow form. For one of the datasets, we directly identified plasmoid-mediated reconnection through the development within the current sheet of a small-scale plasmoid that reaches a length of 332 km and propagates at 40 km s−1. For another dataset, we inferred indirect traces of plasmoid-mediated reconnection through the intermittent boomerang-like pattern that appears in the outflow region. The simulation self-consistently produces a current sheet and small-scale plasmoids similar to those observed, whose synthetic HRIEUV emission reproduces both direct imprints within the current sheet and intermittent patterns in the outflow region associated with their ejection. Conclusions. Our findings highlight Solar Orbiter’s unique capability to capture narrow jets and sub-megameter-scale plasmoid-mediated reconnection signatures in the corona. These results motivate future statistical studies aimed at assessing the role of such fine-scale phenomena in coronal dynamics and solar wind formation.