Current Sheet Research Articles

Synchrotron Pair Production Equilibrium in Relativistic Magnetic Reconnection

Magnetic reconnection is ubiquitous in astrophysical systems, and in many such systems the plasma suffers from significant cooling due to synchrotron radiation. We study relativistic magnetic reconnection in the presence of strong synchrotron cooling, where the ambient magnetization, σ, is high and the magnetic compactness, ℓ B , of the system is of order unity. In this regime, e± pair production from synchrotron photons is inevitable, and this process can regulate the magnetization σ surrounding the current sheet. We investigate this self-regulation analytically and find a self-consistent steady state for a given magnetic compactness of the system and initial magnetization. This result helps estimate the self-consistent upstream magnetization in systems where plasma density is poorly constrained, and can be useful for a variety of astrophysical systems. As illustrative examples, we apply it to study the properties of reconnecting current sheets near the supermassive black hole of M87, as well as the equatorial current sheet outside the light cylinder of the Crab pulsar.

Slow wind belt in the quiet solar corona

The slow solar wind belt in the quiet corona, observed with the Metis coronagraph on board Solar Orbiter on May 15, 2020, during the activity minimum of the cycle 24, in a field of view extending from 3.8 R⊙ to 7.0 R⊙, is formed by a slow and dense wind stream running along the coronal current sheet, accelerating in the radial direction and reaching at 6.8 R⊙ a speed within 150 and 190 km s−1, depending on the assumptions on the velocity distribution of the neutral hydrogen atoms in the coronal plasma. The slow stream is separated by thin regions of high velocity shear from faster streams, almost symmetric relative to the current sheet, with peak velocity within 175 and 230 km s−1 at the same coronal level. The density–velocity structure of the slow wind zone is discussed in terms of the expansion factor of the open magnetic field lines that is known to be related to the speed of the quasi-steady solar wind, and in relation to the presence of a web of quasi-separatrix layers, S-web, the potential sites of reconnection that release coronal plasma into the wind. The parameters characterizing the coronal magnetic field lines are derived from 3D MHD model calculations. The S-web is found to coincide with the latitudinal region where the slow wind is observed in the outer corona and is surrounded by thin layers of open field lines expanding in a non-monotonic way.

A Model for Gradual-phase Heating Driven by MHD Turbulence in Solar Flares

Coronal flare emission is commonly observed to decay on timescales longer than those predicted by impulsively driven, one-dimensional flare loop models. This discrepancy is most apparent during the gradual phase, where emission from these models decays over minutes, in contrast to the hour or more often observed. Magnetic reconnection is invoked as the energy source of a flare, but should deposit energy into a given loop within a matter of seconds. Models which supplement this impulsive energization with a long, persistent ad hoc heating have successfully reproduced long-duration emission, but without providing a clear physical justification. Here we propose a model for extended flare heating by the slow dissipation of turbulent Alfvén waves initiated during the retraction of newly reconnected flux tubes through a current sheet. Using one-dimensional simulations, we track the production and evolution of MHD wave turbulence trapped by reflection from high-density gradients in the transition region. Turbulent energy dissipates through nonlinear interaction between counter-propagating waves, modeled here using a phenomenological one-point closure model. Atmospheric Imaging Assembly EUV light curves synthesized from the simulation were able to reproduce emission decay on the order of tens of minutes. We find this simple model offers a possible mechanism for generating the extended heating demanded by observed coronal flare emissions self-consistently from reconnection-powered flare energy release.

MAVEN Observations of Tailward Reconnection Front in the Martian Magnetotail

AbstractIntense electromagnetic energy conversion and local magnetic flux transport occur at reconnection fronts (RFs) formed by magnetic reconnection. Tailward RFs (TRFs), characterized by sharply enhanced negative magnetic field normal components accompanied by tailward plasma flows, are the tailward propagating dipolarization fronts in the Earth's magnetotail. So far RFs and TRFs are observed merely at magnetized planets. Using data from Mars Atmosphere and Volatile EvolutioN, we report, for the first time, the observations of TRF embedded in a current sheet (possibly diffusion region of magnetic reconnection) in the Martian magnetotail. At the TRF, O+ and O2+ are accelerated in tailward direction; behind the TRF, the perpendicular electron fluxes enhance at the energy of 251–905 eV, which should be caused by betatron acceleration. The energized plasmas indicate that the electromagnetic energy converts to plasma kinetic energy around the TRF. Our observations suggest that RFs could occur in the planetary magnetotail without the global intrinsic dipole magnetic field.

Formation of an Intermediate Filament Driven by Small-scale Magnetic Reconnection

We present the formation process of a filament in NOAA active region 12765 from 2020 June 5 to 8, using observations from the New Vacuum Solar Telescope, the Solar Dynamics Observatory, and the Global Oscillation Network Group. We found that intermittent small-scale magnetic reconnection occurs at the northern part of the filament, and the small-scale magnetic reconnection shows the characteristics of the oscillatory reconnections. During the magnetic reconnection process, a large amount of material is continuously injected into the filament channel. Furthermore, there are bidirectional inflow and outflow, current sheets, and bright cusp-shaped structures. The velocities of the material injections range from 17 to 183 km s−1 with an average velocity of about 57 km s−1. A total of 53 material injections were found from 03:10 UT on 2020 June 5 to 00:10 UT on June 8. The total mass carried by the injection events is about 7.39 × 1014 g, and the total kinetic energy released through magnetic reconnection is approximately 3.09 × 1021 J. The projection area of the filament increased from less than 1 × 102 Mm2 to around 7 × 102 Mm2. We conclude that the filament is formed by direct material injection into the filament channel due to the small-scale magnetic reconnections.

Simulation of current sheet in the Jupiter’s near magnetotail

We considered numerical modeling of the current sheet in the Jupiter’s near magnetotail to estimate the possible range of parameters of this layer depending on the presence of hot sulfur and oxygen ions. The characteristics of the obtained stationary configurations of the current sheet formed by counterflows of either protons alone, or oxygen ions alone, or sulfur ions alone, are in good agreement with the Juno’s data.

Magnetic field amplification and structure formation by the Rayleigh-Taylor instability

We report our results from a set of high-resolution, two-fluid, non-linear simulations of the magnetized Rayleigh Taylor instability (RTI) at the interface between a solar prominence and the corona. These data follow results reported earlier on linear and early non-linear RTI dynamics in this environment. This paper is focused on the generation and amplification of magnetic structures by RTI. The simulations use a two-fluid model that includes collisions between neutrals and charges, including ionization and recombination, energy and momentum transfer, and frictional heating. The 2.5D magnetized RTI simulations demonstrate that in a fully developed state of RTI, a large fraction of the gravitational energy of a prominence thread can be converted into quasi-turbulent energy of the magnetic field. The RTI magnetic energy generation is further accompanied by magnetic and plasma density structure formation, including dynamic formation, break-up, and merging of current sheets and plasmoid sub-structures. The flow decoupling between neutrals and charges, as well as ionization and recombination reactions, are shown to have significant impact on the structure formation in a magnetized RTI.

Multicomponent Current Sheet of the Magnetopause with an Arbitrary Energy Distribution of Particles

An exact solution to the Maxwell–Vlasov equations has been found for a large class of multicomponent current sheets in collisionless plasma, which describe the spatial structure of the current in the magnetopause and consistent inhomogeneous anisotropic momentum distributions of particles with different effective temperatures. Devised sheets allow a nonmonotonic variation of the magnetic field and can have asymmetric, multihump, and sign-alternating profiles of the current density. Profiles of the current of different particle populations can have different scales, contain countercurrents, and be spatially shifted with respect to each other. The model under consideration is applicable to qualitatively describe a magnetopause separating a magnetosphere of a planet from a solar wind or separating regions of the solar wind with different parameters of the plasma and magnetic field.

COCONUT, a Novel Fast-converging MHD Model for Solar Corona Simulations. II. Assessing the Impact of the Input Magnetic Map on Space-weather Forecasting at Minimum of Activity

This paper is dedicated to the new implicit unstructured coronal code COCONUT, which aims at providing fast and accurate inputs for space-weather forecasting as an alternative to empirical models. We use all 20 available magnetic maps of the solar photosphere covering the date of 2019 July 2, which corresponds to a solar eclipse on Earth. We use the same standard preprocessing on all maps, then perform coronal MHD simulations with the same numerical and physical parameters. We conclude by quantifying the performance of each map using three indicators from remote-sensing observations: white-light total solar eclipse images for the streamers’ edges, EUV synoptic maps for coronal holes, and white-light coronagraph images for the heliospheric current sheet. We discuss the performance of space-weather forecasting and show that the choice of the input magnetic map has a strong impact. We find performances between 24% and 85% for the streamers’ edges, 24%–88% for the coronal hole boundaries, and a mean deviation between 4° and 12° for the heliospheric current sheet position. We find that the HMI runs perform better on all indicators, with GONG-ADAPT being the second-best choice. HMI runs perform better for the streamers’ edges, and GONG-ADAPT for polar coronal holes, HMI synchronic for equatorial coronal holes, and the streamer belt. We especially illustrate the importance of the filling of the poles. This demonstrates that the solar poles have to be taken into account even for ecliptic plane previsions.

MESSENGER Observations of Reconnection in Mercury's Magnetotail Under Strong IMF Forcing

AbstractThe MESSENGER spacecraft typically crossed Mercury's magnetotail current sheet relatively close to the planet, that is, less than 2.5 RM (planet radius; 2,440 km). Magnetometer measurements are used to detect active reconnection events by identifying the quadrupole Hall magnetic field signatures that form about X‐lines. Statistical analyses of the 51 active reconnection events detected in this manner indicate that they occur most frequently on the duskside and typically at a mean altitude greater than 1.5 RM. In contrast, the dawnside events occur at altitudes of ∼1 RM. In addition, a higher recurrence rate of flux ropes formed in the Hall region was observed on the dawnside. Applying the Kan‐Lee solar wind‐magnetosphere coupling function confirmed that these near‐tail reconnection events at Mercury are observed under strong forcing by the interplanetary magnetic field. We further propose that the reconnection‐driven nightside magnetosphere‐planet interaction is characterized by a pronounced dawn‐dusk asymmetry and may significantly influence the near‐Mercury space environment.

The electron diffusion region dominated by electromagnetic turbulence in the reconnection current layer

Most of the plasma fluid equations have employed the electrical resistivity to generate the magnetic dissipation required for magnetic reconnection to occur in collisionless plasma. However, there has been no clear evidence that such a model is indeed appropriate in the reconnection diffusion region in terms of the kinetic physics. The present study demonstrates that, using a large-scale 3D kinetic simulation and analytical analysis, the spatial distribution of the non-ideal electric field is consistent with the dissipation due to the viscosity rather than the resistivity, when electromagnetic (EM) turbulence is dominant in the electron diffusion region (EDR). The effective viscosity is caused by the EM turbulence that is driven by the flow shear instabilities leading to the electron momentum transport across the EDR. The result suggests a fundamental modification of the fluid equations using the resistivity in the Ohm's law. In contrast, for the 2D current sheet without significant turbulence activity, the non-ideal field profile does not obey the simple form based on the viscosity, so that further investigation is needed for a better description.

Plasmoids, Flows, and Jets during Magnetic Reconnection in a Failed Solar Eruption

We report a detailed analysis of a failed eruption and flare in active region 12018 on 2014 April 3 using multiwavelength observations from the Solar Dynamics Observatory/Atmospheric Imaging Assembly, IRIS, STEREO, and Hinode/Solar Optical Telescope. At least four jets were observed to emanate from the cusp of this small active region (large bright point) with a null-point topology during the 2 hr prior to the slow rise of a filament. During the filament slow rise multiple plasma blobs were seen, most likely formed in a null-point current sheet near the cusp. The subsequent filament eruption, which was outside the IRIS field of view, was accompanied by a flare but remained confined. During the explosive flare reconnection phase, additional blobs appeared repetitively and moved bidirectionally within the flaring region below the erupting filament. The filament kinked, rotated, and underwent leg–leg reconnection as it rose, yet it failed to produce a coronal mass ejection. Tiny jet-like features in the fan loops were detected during the filament slow rise/preflare phase. We interpret them as signatures of reconnection between the ambient magnetic field and the plasmoids leaving the null-point sheet and streaming along the fan loops. We contrast our interpretation of these tiny jets, which occur within the large-scale context of a failed filament eruption, with the local nanoflare-heating scenario proposed by Antolin et al.

Data Mining Inspired Localized Resistivity in Global MHD Simulations of the Magnetosphere

AbstractRecent advances in reconstructing Earth's magnetic field and associated currents by utilizing data mining of in situ magnetometer observations in the magnetosphere based on geomagnetic indices and solar wind parameters have proven remarkably accurate at reproducing observed ion diffusion regions. We investigate the effect of placing regions of localized resistivity in global simulations of the magnetosphere at specific locations inspired by the data mining results for the substorm occurring on 6 July 2017. When explicit resistivity is included, the simulation forms an x‐line at the same time and location as the Magnetospheric Multiscale Mission observation of an ion diffusion region at 15:35 UT on that day. Without this explicit resistivity, reconnection forms later in the substorm and far too close to Earth (≳−15RE), a common problem with global simulations of Earth's magnetosphere. A consequence of reconnection taking place farther down the tail due to localized resistivity is that the reconnection outflows transport magnetic flux Earthward and thus prevent the current sheet from thinning enough for reconnection to take place nearer Earth. As these flows rebound tailward from the inner magnetosphere, they can temporarily and locally (in the dawn‐dusk direction) stretch the magnetic field allowing for small scale x‐lines to form in the near Earth region. Due to the narrow cross‐tail extent of these x‐lines (≲5RE) and their short lifespan (≲5 min), they would be difficult to observe with in situ measurements. Future work will explore time‐dependent resistivity using 5 min cadence data mining reconstructions.

In Situ Observations of Whistler-mode Waves in Magnetic Reconnection at Mars

Whistler-mode waves are one of the most important plasma waves potentially affecting the triggering and development processes of magnetic reconnection. They are widely present in the Earth’s reconnection ion diffusion region but have not yet been reported in Mars’. Based on in situ measurements from MAVEN, the Mars Atmosphere and Volatile Evolution mission, we report for the first time whistler-mode waves in the ion diffusion region at the center of the current sheet in the Martian magnetotail. Simultaneously, pancake electron distributions with high temperature anisotropy are observed. Linear instability analyses imply that these unstable electrons can trigger the observed whistler-mode waves. Such findings not only fill the gap in observations of whistler-mode waves in the magnetic reconnection at Mars but also enrich our understanding of their generation mechanism in the reconnection region at unmagnetized planets.

Magnetic Energy Dissipation and γ-Ray Emission in Energetic Pulsars

Some of the most energetic pulsars exhibit rotation-modulated γ-ray emission in the 0.1–100 GeV band. The luminosity of this emission is typically 0.1%–10% of the pulsar spin-down power (γ-ray efficiency), implying that a significant fraction of the available electromagnetic energy is dissipated in the magnetosphere and reradiated as high-energy photons. To investigate this phenomenon we model a pulsar magnetosphere using 3D particle-in-cell simulations with strong synchrotron cooling. We particularly focus on the dynamics of the equatorial current sheet where magnetic reconnection and energy dissipation take place. Our simulations demonstrate that a fraction of the spin-down power dissipated in the magnetospheric current sheet is controlled by the rate of magnetic reconnection at microphysical plasma scales and only depends on the pulsar inclination angle. We demonstrate that the maximum energy and the distribution function of accelerated pairs is controlled by the available magnetic energy per particle near the current sheet, the magnetization parameter. The shape and the extent of the plasma distribution is imprinted in the observed synchrotron emission, in particular, in the peak and the cutoff of the observed spectrum. We study how the strength of synchrotron cooling affects the observed variety of spectral shapes. Our conclusions naturally explain why pulsars with higher spin-down power have wider spectral shapes and, as a result, lower γ-ray efficiency.

Nonequilibrium Ionization Modeling of Petschek-type Shocks in Reconnecting Current Sheets in Solar Eruptions

Nonequilibrium ionization (NEI) is essentially required for astrophysical plasma diagnostics once the plasma status departs from the assumption of ionization equilibrium. In this work, we perform fast NEI calculations combined with magnetohydrodynamic (MHD) simulations and analyze the ionization properties of a Petschek-type magnetic reconnection current sheet during solar eruptions. Our simulation reveals Petschek-type slow-mode shocks in the classical Spitzer thermal conduction models and conduction flux-limitation situations. The results show that under-ionized features can be commonly found in shocked reconnection outflows and thermal halo regions outside the shocks. The departure from equilibrium ionization strongly depends on plasma density. In addition, this departure is sensitive to the observable target temperature: the high-temperature iron ions are strongly affected by the effects of NEI. The under-ionization also affects the synthetic SDO/AIA intensities, which indicates that the reconstructed hot reconnection current sheet structure may be significantly underestimated either for temperature or apparent width. We also perform an MHD-NEI analysis on the reconnection current sheet in the classical solar flare geometry. Finally, we show the potential reversal between the under-ionized and over-ionized states at the lower tip of reconnection current sheets where the downward outflow collides with closed magnetic loops, which can strongly affect multiple SDO/AIA band ratios along the reconnection current sheet.

Numerical Study on Excitation of Turbulence and Oscillation in Above-the-loop-top Region of a Solar Flare

Extreme-ultraviolet imaging spectroscopic observations often show an increase in line width around the loop-top or above-the-loop-top (ALT) region of solar flares, suggestive of turbulence. In addition, recent spectroscopic observations found the oscillation in the Doppler velocity around the ALT region. We performed 3D magnetohydrodynamic (MHD) simulations to investigate the dynamics in the ALT region, with a particular focus on the generation of turbulence and the excitation of the oscillatory motion. We found a rapid growth of MHD instabilities around the upper parts of the ALT region (arms of the magnetic tuning fork). The instabilities grow more rapidly than the magnetic Rayleigh–Taylor-type instabilities at the density interface beneath the reconnecting current sheet. Eventually, the ALT region is filled with turbulent flows. The arms of the magnetic tuning fork have bad-curvature and transonic flows. Therefore, we consider that the rapidly growing instabilities are combinations of pressure-driven and centrifugally driven Rayleigh–Taylor-type instabilities. Despite the presence of turbulent flows, the ALT region shows a coherent oscillation driven by the backflow of the reconnection jet. We examine the numerical results by reanalyzing the solar flare presented in Reeves et al. We find that the highest nonthermal velocity is always at the uppermost visible edge of the ALT region, where oscillations are present. This result is consistent with our models. We also argue that the turbulent magnetic field has a significant impact on the confinement of nonthermal electrons in the ALT region.

Optical modeling of single and multilayer two-dimensional materials and heterostructures

Bidimensional materials are ideally viewed as having no thickness, as their name suggests. Their optical response have been previously modelled by a purely bidimensional surface current or by a very thin film with some contradictory results. The advent of multilayer stacks of bidimensional materials and combinations of different materials in vertical van der Waals heterostructures highlights, however, that these materials have a finite thickness. In this article, we propose a new model that reconciles both approaches and we show how volume properties of stacked bidimensional layers can be calculated from the bidimensional response of each individual layer, and conversely. In our approach, each bidimensional layers is surrounded by vacuum and described as a kind of transfer matrix with intrinsic parameters that do not depend on the external medium. This provides a link between continuous thin films and discrete layers. We show how to model heterostructures of bidimensional materials and identify the parameters of the current sheet that represents the bidimensional material in the zero-thickness limit, namely the in-plane surface susceptibility and the out-of-plane displacement susceptibility. We show that our unified model is perfectly compatible with existing ellipsometric data with the same reliability as the existing interface model but with different values of the surface susceptibility or bulk dielectric function. We discuss in detail the origin of the discrepancies and show that our approach allows to determine intrinsic properties of the bidimensional materials with the advantage that multilayer and monolayer systems are described in a same framework.

Tracing Hot Spot Motion in Sagittarius A* Using the Next-Generation Event Horizon Telescope (ngEHT)

We propose the tracing of the motion of a shearing hot spot near the Sgr A* source through a dynamical image reconstruction algorithm, StarWarps. Such a hot spot may form as the exhaust of magnetic reconnection in a current sheet near the black hole horizon. A hot spot that is ejected from the current sheet into an orbit in the accretion disk may shear and diffuse due to instabilities at its boundary during its orbit, resulting in a distinct signature. We subdivide the motion into two different phases: the first phase refers to the appearance of the hot spot modeled as a bright blob, followed by a subsequent shearing phase. We employ different observational array configurations, including EHT (2017, 2022) and the next-generation Event Horizon Telescope (ngEHTp1, ngEHT) arrays, with several new sites added, and make dynamical image reconstructions for each of them. Subsequently, we infer the hot spot angular image location in the first phase, followed by the axes ratio and the ellipse area in the second phase. We focus on the direct observability of the orbiting hot spot in the sub-mm wavelength. Our analysis demonstrates that for this particular simulation, the newly added dishes are better able to trace the first phase as well as part of the second phase before the flux is reduced substantially, compared to the EHT arrays. The algorithm used in this work can be easily extended to other types of dynamics, as well as different shearing timescales. More simulations are required to prove whether the current set of newly proposed sites are sufficient to resolve any motions near variable sources, such as Sgr A*.

Simulation of Solar Wind Turbulence near Corotating Interaction Regions: Superposed Epoch Analysis of Simulations and Observations

The effect of the turbulence that is associated with solar wind corotating interaction regions (CIRs) on transport of galactic cosmic rays remains an outstanding problem in space science. Observations show that the intensities of the plasma and magnetic fluctuations are enhanced within a CIR. The velocity shear layer between the slow and fast wind embedded in a CIR is thought to be responsible for this enhancement in turbulent energy. We perform physics-based magnetohydrodynamic simulations of the plasma background and turbulent fluctuations in the solar wind dominated by CIRs for radial distances between 0.3 and 5 au. A simple but effective approach is used to incorporate the inner boundary conditions for the solar wind and magnetic field for the periods 2007–2008 and 2017–2018. Legendre coefficients at the source surface obtained from the Wilcox Solar Observatory library are utilized for dynamic reconstructions of the current sheet and the fast and slow streams at the inner boundary. The dynamic inner boundary enables our simulations to generate CIRs that are reasonably comparable with observations near Earth. While the magnetic field structure is reasonably well reproduced, the enhancements in the turbulent energy at the stream interfaces are smaller than observed. A superposed epoch analysis is performed over several CIRs from the simulation and compared to the superposed epoch analysis of the observed CIRs. The results for the turbulent energy and correlation length are used to estimate the diffusion tensor of galactic cosmic rays. The derived diffusion coefficients could be used for more realistic modeling of cosmic rays in a dynamically evolving inner heliosphere.