Understanding Of Magnetic Reconnection Research Articles

The Nonorthogonal X-line in a Small Guide-field Reconnection Event in the Magnetotail

Magnetic reconnection is a fundamental plasma process that has been studied with analytical theory, numerical simulations, in situ observations, and laboratory experiments for decades. The models that have been established to describe magnetic reconnection often assume a reconnection plane normal to the current sheet in which an antiparallel magnetic field annihilates. The annihilation points, also known as the X-points, form an x-line, which is believed to be perpendicular to the reconnection plane. Recently, a new study using Magnetospheric Multiscale mission observations has challenged our understanding of magnetic reconnection by providing evidence that the x-line is not necessarily orthogonal to the reconnection plane. In this study we report a second nonorthogonal x-line event with similar features as that in the previous case study, supporting that the sheared x-line phenomenon is not an aberrant event. We employ a detailed directional derivative analysis to identify the x-line direction and show that the in-plane reconnection characteristics are well maintained even with a nonorthogonal x-line. In addition, we find the x-line tends to follow the magnetic field on one side of the current sheet, which suggests an asymmetry across the current sheet. We discuss the possibility that the nonorthogonal x-line arises from an interplay between the two aspects of reconnection: the macroscopic magnetic field topology and microscopic particle kinetics.

Multifluid Simulations of Upper-chromospheric Magnetic Reconnection with Helium–Hydrogen Mixture

Our understanding of magnetic reconnection (MR) under chromospheric conditions remains limited. Recent observations have demonstrated the important role of ion–neutral interactions in the dynamics of the chromosphere. Furthermore, the comparison between the spectral profiles and synthetic observations of reconnection events suggests that current MHD approaches appear to be inconsistent with observations. First, collisions and multithermal aspects of the plasma play a role in these regions. Second, hydrogen and helium ionization effects are relevant to the energy balance of the chromosphere. This work investigates the multifluid multispecies (MFMS) effects on MR in conditions representative of the upper chromosphere using the multifluid Ebysus code. We compare an MFMS approach based on a helium–hydrogen mixture with a two-fluid MHD model based on hydrogen only. The simulations of MR are performed in a Lundquist number regime high enough to develop plasmoids and instabilities. We study the evolution of the MR and compare the two approaches including the structure of the current sheet and plasmoids, the decoupling of the particles, the evolution of the heating mechanisms, and the composition. The presence of helium species leads to more efficient heating mechanisms than the two-fluid case. This scenario, which is out of reach of the two-fluid or single-fluid models, can reach transition region temperatures starting from upper-chromospheric thermodynamic conditions, representative of a quiet Sun scenario. The different dynamics between helium and hydrogen species could lead to chemical fractionation and, under certain conditions, enrichment of helium in the strongest outflows. This could be of significance for recent observations of helium enrichment in the solar wind in switchbacks and coronal mass ejections.

A magnetic reconnection model for hot explosions in the cool atmosphere of the Sun

Context. Ultraviolet (UV) bursts and Ellerman bombs (EBs) are transient brightenings observed in the low solar atmospheres of emerging flux regions. Magnetic reconnection is believed to be the main mechanism leading to formation of the two activities, which are usually formed far apart from each other. However, observations also led to the discovery of co-spatial and co-temporal EBs and UV bursts, and their formation mechanisms are still not clear. The multi-thermal components in these events, which span a large temperature range, challenge our understanding of magnetic reconnection and heating mechanisms in the partially ionized lower solar atmosphere. Aims. We studied magnetic reconnection between the emerging magnetic flux and back ground magnetic fields in the partially ionized and highly stratificated low solar atmosphere. We aim to explain the multi-thermal characteristics of UV bursts, and to find out whether EBs and UV bursts can be generated in the same reconnection process and how they are related with each other. We also aim to unearth the important small-scale physics in these events. Methods. We used the single-fluid magnetohydrodynamic (MHD) code NIRVANA to perform simulations. The background magnetic fields and emerging fields at the solar surface are reasonably strong. The initial plasma parameters are based on the C7 atmosphere model. We simulated cases with different resolutions, and included the effects of ambipolar diffusion, radiative cooling, and heat conduction. We analyzed the current density, plasma density, temperature, and velocity distributions in the main current sheet region, and synthesized the Si IV emission spectrum. Results. After the current sheet with dense photosphere plasma emerges and reaches 0.5 Mm above the solar surface, plasmoid instability appears. The plasmoids collide and coalesce with each other, which causes the plasmas with different densities and temperatures to be mixed up in the turbulent reconnection region. Therefore, the hot plasmas corresponding to the UV emissions and colder plasmas corresponding to the emissions from other wavelengths can move together and occur at about the same height. In the meantime, the hot turbulent structures concentrate above 0.4 Mm, whereas the cool plasmas extend to much lower heights to the bottom of the current sheet. These phenomena are consistent with published observations in which UV bursts have a tendency to be located at greater heights close to corresponding EBs and all the EBs have partial overlap with corresponding UV bursts in space. The synthesized Si IV line profiles are similar to that observed in UV bursts; the enhanced wing of the line profiles can extend to about 100 km s−1. The differences are significant among the numerical results with different resolutions, indicating that the realistic magnetic diffusivity is crucial to revealing the fine structures and realistic plasmas heating in these reconnection events. Our results also show that the reconnection heating contributed by ambipolar diffusion in the low chromosphere around the temperature minimum region is not efficient.

Spectropolarimetric Insight into Plasma Sheet Dynamics of a Solar Flare

Abstract We examine spectropolarimetric data from the Coronal Multi-channel Polarimeter (CoMP) instrument, acquired during the evolution of the 2017 September 10 X8.2 solar flare on the western solar limb. CoMP captured linearly polarized light from two emission lines of Fe xiii at 1074.7 and 1079.8 nm, from 1.03 to 1.5 solar radii. We focus here on the hot plasma sheet lying above the bright flare loops and beneath the ejected coronal mass ejection. The polarization has a striking and coherent spatial structure, with unexpectedly small polarization aligned with the plasma sheet. By elimination, we find that small-scale magnetic field structure is needed to cause such significant depolarization, and suggest that plasmoid formation during reconnection (associated with the tearing-mode instability) creates magnetic structure on scales below instrument resolution of 6 Mm. We conclude that polarization measurements with new coronagraphs, such as the upcoming Daniel K. Inouye Solar Telescope, will further enhance our understanding of magnetic reconnection and development of turbulence in the solar corona.

Diffusion regions and 3D energy mode development in spontaneous reconnection

The understanding of magnetic reconnection in three-dimensions (3D) is far shallower than its counterpart in two-dimensions due to its potential complication, not to mention the evolving of the spontaneously growing turbulence. We investigate the reason for reconnection acceleration on the characters and development of diffusion regions and sheared 3D energy modes (energy modes that are not parallel to the antiparallel magnetic fields) during the turbulence building stage. We found that multiple reconnection layers emerge due to the growth of 3D sheared tearing instability. Diffusion regions in adjacent reconnection layers form an inflow-outflow coupling that enhances the local reconnection. Further coupling of the existing energy modes breeds new energy modes near the current sheet edge. As reconnection layers span and interact with each other across the whole current sheet, global magnetic energy consumption accelerates. The significant contribution of 3D energy modes and their interaction to the reconnection rate enhancement seems to be independent of magnetic diffusivity. On the other hand, the global guide field changes the layout of the 3D reconnection layer and thus determines whether the system is fast-reconnection-preferable.

Magnetic Nulls in the Reconnection Driven by Turbulence

Abstract For the first time, we use spacecraft measurements to investigate the magnetic nulls in a reconnection event driven by turbulence in the magnetosheath. We particularly focus on the relation between magnetic-null topologies and currents, which can be decomposed into a component perpendicular to spine ( ) and a component parallel to spine ( ). Our new observations include: (1) the total current at spiral nulls is much larger than that at radial nulls; (2) is large at spiral nulls but small at radial nulls; (3) at radial nulls, is dominant, while at spiral nulls, is dominant; (4) the fan-spine angle θ at both radial and spiral nulls decreases with , with a clear upper boundary; (5) with a database of 715 nulls, we statistically resolve the relation between θ and as with the correlation coefficient of cc = 0.71 (radial null) and with cc = 0.76 (spiral null), where is in the unit of . Our physical interpretations of these observations are: (1) the current parallel to spine significantly determines the null topology, with large producing spiral nulls and small producing radial nulls; (2) the current perpendicular to spine serves to tilt the fan plane to the spine, for both spiral and radial nulls. All of these observations and conclusions significantly improve our understanding of magnetic reconnection.

Plasmoid Instability in Forming Current Sheets

Abstract The plasmoid instability has revolutionized our understanding of magnetic reconnection in astrophysical environments. By preventing the formation of highly elongated reconnection layers, it is crucial in enabling the rapid energy conversion rates that are characteristic of many astrophysical phenomena. Most previous studies have focused on Sweet–Parker current sheets, which are unattainable in typical astrophysical systems. Here we derive a general set of scaling laws for the plasmoid instability in resistive and visco-resistive current sheets that evolve over time. Our method relies on a principle of least time that enables us to determine the properties of the reconnecting current sheet (aspect ratio and elapsed time) and the plasmoid instability (growth rate, wavenumber, inner layer width) at the end of the linear phase. After this phase the reconnecting current sheet is disrupted and fast reconnection can occur. The scaling laws of the plasmoid instability are not simple power laws, and they depend on the Lundquist number (S), the magnetic Prandtl number (P m ), the noise of the system ( ), the characteristic rate of current sheet evolution ( ), and the thinning process. We also demonstrate that previous scalings are inapplicable to the vast majority of astrophysical systems. We explore the implications of the new scaling relations in astrophysical systems such as the solar corona and the interstellar medium. In both of these systems, we show that our scaling laws yield values for the growth rate, wavenumber, and aspect ratio that are much smaller than the Sweet–Parker–based scalings.

Testing nowcasts of the ionospheric convection from the expanding and contracting polar cap model

AbstractThe expanding/contracting polar cap (ECPC) model, or the time‐dependent Dungey cycle, provides a theoretical framework for understanding solar wind‐magnetosphere‐ionosphere coupling. The ECPC describes the relationship between magnetopause reconnection and substorm growth phase, magnetotail reconnection and substorm expansion phase, associated changes in auroral morphology, and ionospheric convective motions. Despite the many successes of the model, there has yet to be a rigorous test of the predictions or nowcasts made regarding ionospheric convection, which remains a final hurdle for the validation of the ECPC. In this study we undertake a comparison of ionospheric convection, as measured in situ by ion drift meters on board DMSP (Defense Meteorological Satellite Program) satellites and from the ground by SuperDARN (Super Dual Auroral Radar Network), with motions nowcasted by a theoretical model. The model is coupled to measurements of changes in the size of the polar cap made using global auroral imagery from the IMAGE FUV (Imager for Magnetopause to Aurora Global Exploration Far Ultraviolet) instrument, as well as the dayside reconnection rate, estimated using the OMNI data set. The results show that we can largely nowcast the magnitudes of ionospheric convection flows using the context of our understanding of magnetic reconnection at the magnetopause and in the magnetotail.

Plasmoid instability in high-Lundquist-number magnetic reconnection

Our understanding of magnetic reconnection in resistive magnetohydrodynamics has gone through a fundamental change in recent years. The conventional wisdom is that magnetic reconnection mediated by resistivity is slow in laminar high Lundquist (S) plasmas, constrained by the scaling of the reconnection rate predicted by Sweet-Parker theory. However, recent studies have shown that when S exceeds a critical value ∼104, the Sweet-Parker current sheet is unstable to a super-Alfvénic plasmoid instability, with a linear growth rate that scales as S1/4. In the fully developed statistical steady state of two-dimensional resistive magnetohydrodynamic simulations, the normalized average reconnection rate is approximately 0.01, nearly independent of S, and the distribution function f(ψ) of plasmoid magnetic flux ψ follows a power law f(ψ)∼ψ−1. When Hall effects are included, the plasmoid instability may trigger onset of Hall reconnection even when the conventional criterion for onset is not satisfied. The rich variety of possible reconnection dynamics is organized in the framework of a phase diagram.

Kinetic simulations of the structures of magnetic island in multiple X line guide field reconnection

Magnetic reconnection is one of the most important processes in astrophysical, space, and laboratory plasmas, and magnetic island is an important feature in reconnection. Therefore, identifying the structures of magnetic island is crucial to improving our understanding of magnetic reconnection. Using two-dimensional (2-D) particle-in-cell (PIC) simulations, we demonstrate that the out-of-plane magnetic field has a dip in the center of magnetic island, which is formed during multiple X line guide field reconnection. Such structures are considered to be produced by the current system in the magnetic island. At the edge of the magnetic island, there exists a current anti-parallel to the in-plane magnetic field, while the current is parallel to the in-plane magnetic field inside the magnetic island. Such a dual-ring current system, which is attributed to the electron dynamics in the magnetic island, leads to the dip of the out-of-plane magnetic field in the center of the island. The relevance between our simulations and crater flux transfer events (C-FTEs) is also discussed.

THE ROLE OF TURBULENT MAGNETIC RECONNECTION IN THE FORMATION OF ROTATIONALLY SUPPORTED PROTOSTELLAR DISKS

The formation of protostellar disks out of molecular cloud cores is still not fully understood. Under ideal MHD conditions, the removal of angular momentum from the disk progenitor by the typically embedded magnetic field may prevent the formation of a rotationally supported disk during the main protostellar accretion phase of low mass stars. This has been known as the magnetic braking problem and the most investigated mechanism to alleviate this problem and help removing the excess of magnetic flux during the star formation process, the so called ambipolar diffusion (AD), has been shown to be not sufficient to weaken the magnetic braking at least at this stage of the disk formation. In this work, motivated by recent progress in the understanding of magnetic reconnection in turbulent environments, we appeal to the diffusion of magnetic field mediated by magnetic reconnection as an alternative mechanism for removing magnetic flux. We investigate numerically this mechanism during the later phases of the protostellar disk formation and show its high efficiency. By means 3D MHD simulations, we show that this mechanism is able to transport magnetic flux to the outskirts of the disk progenitor at time scales compatible with the collapse, allowing the formation of a rotationally supported disk around the protostar of dimensions ~100 AU. Since MHD turbulence is expected to be present in protostellar disks, this is a natural mechanism for removing magnetic flux excess and allowing the formation of these disks. This mechanism dismiss the necessity of postulating a hypothetical increase of the Ohmic resistivity as discussed in the literature. Together with our earlier work which showed that magnetic flux removal from molecular cloud cores is very efficient, this work calls for reconsidering the relative role of AD for the processes of star and planet formation.

The Terrestrial Web

E arth is controlled by an integrated system of dynamic forces acting on all phases (gas, liquid, and solid) of the matter that makes up our terrestrial web. These forces and their phases are linked in an intricate, and in many ways unknown, pattern. Here we review some of the latest models and observations of five subsystems of Earth: the expansive plasma- dominated magnetosphere, the gaseous atmosphere, the fluid ocean, the solid convective mantle, and the fluid-to-solid core. The magnetosphere is dominated by Earth's magnetic field, which protects the planet from the solar wind while providing the connective conduit between the solar wind and the ionosphere, creating aurorae and geomagnetic storms. Lyon (p. [1987][1]) reviews the recent observations of a substorm that provides an improved understanding of magnetic reconnection between the solar wind's magnetic field and Earth's magnetic field. Coupled general circulation models (CGCMs) are one of the most promising approaches for understanding the complex interactions between the atmosphere and oceans. Grassl (p. [1991][2]) reviews the current state of CGCMs, including the ways in which they are tested and evaluated, how they have improved our understanding of climate variability, how well we think we can predict climate, the extent and causes of modern climate variability, and how CGCMs can be improved. El Nino and La Nina, the warm and cold phases of sea surface temperature in the equatorial Pacific Ocean, are the largest single sources of global interannual climate variability. How El Nino and La Nina have varied in the past; are varying now; and will vary in the future, perhaps in response to global warming, are questions with more than just academic importance. Fedorov and Philander (p. [1997][3]) review what we have learned recently about why they occur and whether or not they may be changing as a consequence of changing climate. They suggest that understanding El Nino and La Nina must include a better knowledge of the background state of climate within which they exist. In two News stories (pp. [1984][4] and [1986][5]), Richard Kerr reports that climate shifts on longer time scales than El Nino's 2 to 7 years are drawing the attention of researchers. On the decadal scale of 10 to 20 years, there are persistent signs that an inconstant sun may subtly affect climate. And on multidecadal time scales of 40 to 80 years, a restless North Atlantic seems to be at work, alternately countering and enhancing humankind's alterations of climate. Tackley (p. [2002][6]) considers our current understanding of the convective pattern of the mantle, which comprises the bulk of the silicate Earth squeezed between the crust and the core. He tries to reconcile plate tectonics with mantle convection and discusses models that improve the integration of chemical mixing with convective flow. Finally, in the center of Earth, Buffett (p. [2007][7]) explores the generation of the geodynamo by fluid motions in the outer liquid iron core, which produces the magnetic field. Models using different flow patterns are able to produce a self-sustained dipole field, and the challenge will be to determine which pattern represents Earth. Ultimately, the geodynamo drives the magnetic field, which dominates the magnetosphere. Finding the links within and between these subsystems remains a fundamental challenge for untangling the mysteries of the terrestrial web. To provide additional depth, the staff of Science Online has put together a special Web supplement that includes a selection of previous Science articles related to the themes of the special issue, as well as links to other online resources. To explore it, see [www.sciencemag.org/feature/data/earthdynamics/main.shl][8]. [1]: /lookup/doi/10.1126/science.288.5473.1987 [2]: /lookup/doi/10.1126/science.288.5473.1991 [3]: /lookup/doi/10.1126/science.288.5473.1997 [4]: /lookup/doi/10.1126/science.288.5473.1984 [5]: /lookup/doi/10.1126/science.288.5473.1986 [6]: /lookup/doi/10.1126/science.288.5473.2002 [7]: /lookup/doi/10.1126/science.288.5473.2007 [8]: http://www.sciencemag.org/feature/data/earthdynamics/main.shl