Reconnection Of Field Lines Research Articles

Survey of Saturn Whistler Mode Hiss Intensity

AbstractWe have conducted an extensive survey of whistler mode hiss emission at Saturn using nearly the complete data set from the Cassini Radio and Plasma Wave Science investigation. We distinguish the hiss emission by frequency and intensity threshold criteria. The survey includes all longitudes and latitudes >25° for r < 10 RS, limited to the availability of plasma density data. The results indicate that hiss intensity is highest at high latitude, and the level of intensity is larger than chorus by at least an order of magnitude. After a rapid increase in intensity starting near L = 13, the hiss intensity remains relatively constant extending to L > 100. The intensity of the hiss as a function of frequency has a shallow peak near 35 Hz but remains always in the range ~5 × 10−5 nT2 < I < ~10−3 nT2. These survey results will be important in modeling the scattering interactions of whistler mode emissions with electrons in the Saturn magnetosphere. Based on recent particle and field observations, sources of Saturn hiss occur on and near the footprints of field‐aligned currents, plasma injections, and reconnected field lines.

The emergence of magnetic flux and its role on the onset of solar dynamic events.

A plethora of solar dynamic events, such as the formation of active regions, the emission of jets and the occurrence of eruptions is often associated with the emergence of magnetic flux from the interior of the Sun to the surface and above. Here, we present a short review on the onset, driving and/or triggering of such events by magnetic flux emergence. We briefly describe some key observational examples, theoretical aspects and numerical simulations, towards revealing the mechanisms that govern solar dynamics and activity related to flux emergence. We show that the combination of important physical processes like shearing and reconnection of magnetic fieldlines in emerging flux regions or at their vicinity can power some of the most dynamic phenomena in the Sun on various temporal and spatial scales. Based on previous and recent observational and numerical studies, we highlight that, in most cases, none of these processes alone can drive and also trigger explosive phenomena releasing considerable amount of energy towards the outer solar atmosphere and space, such as flares, jets and large-scale eruptions (e.g. coronal mass ejections). In addition, one has to take into account the physical properties of the emerging field (e.g. strength, amount of flux, relative orientation to neighbouring and pre-existing magnetic fields, etc.) in order to better understand the exact role of magnetic flux emergence on the onset of solar dynamic events. This article is part of the theme issue 'Solar eruptions and their space weather impact'.

The birth of radiation

The topology of electric field lines in electric dipole radiation of angular frequency changes during each cycle. From the beginning of each cycle to the time given by there is no reconnection of field lines. In the time interval to electric field lines separate from the charges on the dipole and close on themselves, but the closed loops eventually die away. For times to the closed loops do not fade away but fly off to infinity as radiation. This sequence is repeated twice per cycle, with field direction reversal in the second half-cycle.

A thruster using magnetic reconnection to create a high-speed plasma jet

Plasma thrusters propel spacecraft by the application of Lorentz forces to ionized propellants. Despite evidence that Lorentz forces resulting from magnetic reconnection in solar flares and Earth's magnetopause produce jets of energetic particles, magnetic reconnection has only recently been considered as a means of accelerating plasma in a thruster. Based on theoretical principles, a pulsed magnetic reconnection thruster consisting of two parallel-connected slit coaxial tubes was constructed. The thruster was operated in argon plasma produced by RF energy at 13.56 MHz. A 1.0 ms current pulse of up to 1500 A was applied to the tubes. Three results provide evidence for magnetic reconnection. (1) The production of high-energy electrons resembling the outflow that is observed in the reconnection of field lines in solar flares and in laboratory experiments. (2) The high-energy electron current coincided with the rise of the magnetic field in the thruster and was followed by a large ion current. (3) In accordance with known physics of magnetic reconnection, ion currents were found to increase as the plasma became less collisional. The Alfvén speed of the outflowing ions was calculated to be 8.48 × 103 m s−1corresponding to anIspof 860 s.

On the effect of parallel shear flow on the plasmoid instability

The effect of symmetric sheared plasma flow parallel to the equilibrium magnetic field on the evolution of plasmoid instability is investigated by using two-dimensional MHD simulations. It is shown that as the velocity amplitude of a shear flow increases from sub-Alfvénic to the level of super-Alfvénic, the current sheet is stabilized against the plasmoid instability. Suppression of plasmoid instability also takes place for a sub-Alfvénic shear flow, provided that the shear length is sufficiently small compared to the magnetic shear. Therefore, an initially elongated thin current sheet with a Lundquist number greater than the critical value does not necessarily guarantee the development of the plasmoid instability. As a result, the Kelvin-Helmholtz (KH) instability is triggered. Although the rate of reconnection is quite fast in the nonlinear regime of the plasmoid instability, the reconnection of twisted field lines during the KH instability is very slow.

Nonlobe Reconnection at the Earth's Magnetopause for Northward IMF

AbstractUnder northward interplanetary magnetic field conditions, magnetic reconnection at the Earth's magnetopause is usually thought to operate through the merging of magnetosheath magnetic field lines and open magnetic field lines from the magnetospheric lobe. However, reconnection also occurs between magnetosheath field lines and closed magnetic field lines in the magnetosphere. Under certain conditions, this nonlobe field line reconnection has distinct plasma and magnetic field signatures that distinguish it from reconnection of lobe field lines. A survey of these conditions suggests that nonlobe reconnection at the Earth's magnetopause may be common even for relatively strong northward IMF.

Timescales of Dayside and Nightside Field‐Aligned Current Response to Changes in Solar Wind‐Magnetosphere Coupling

AbstractPrincipal component analysis is performed on Birkeland or field‐aligned current (FAC) measurements from the Active Magnetosphere and Planetary Electrodynamics Response Experiment, to determine the response of dayside and nightside FACs to reversals in the orientation of the interplanetary magnetic field (IMF) and the occurrence of substorms. Dayside FACs respond promptly to changes in IMF BY, but the nightside response is delayed by up to an hour and can take up to 4 hr to develop fully, especially during northward IMF. Nightside FAC asymmetries grow during substorm growth phase when the IMF has a significant BY component, and also promptly at substorm onset. Our findings suggest that magnetotail twisting and/or BY penetration into the magnetotail, due to subsolar reconnection with east‐west orientated IMF, are the main cause of these nightside FAC asymmetries and that asymmetries also arise due to magnetotail reconnection of these twisted field lines.

Magnetic helicity evolution in a neutron star accounting for the Adler-Bell-Jackiw anomaly

We analyze the role of the surface terms in the conservation law for the sum of the magnetic helicity density and the chiral imbalance of the charged particle densities. These terms are neglected in the Anomalous MagnetoHydroDynamics (AMHD), where infinite volume is considered typically. We discuss a finite volume system, such as a magnetized neutron star (NS), and study the contribution of the surface terms to the evolution of the magnetic helicity. Accounting for the fast washing out of the chiral imbalance in a nascent NS, we demonstrate that the surface terms contribution can potentially lead to the reconnection of magnetic field lines and subsequent gamma or X-ray bursts observed from magnetars. We derive the additional surface terms originated by the mean spin flux through a volume boundary arising due to macroscopic spin effects in electron-positron plasma. Then, comparing this quantum surface term with the classical one known in standard MHD, we find that the new quantum contribution prevails over classical term for the rigid NS rotation only.

Spiky electric and magnetic field structures in flux rope experiments

Magnetic flux ropes are structures that are common in the corona of the sun and presumably all stars. They can be thought of as the building blocks of solar structures. They have been observed in Earth's magnetotail and near Mars and Venus. When multiple flux ropes are present magnetic field line reconnection, which converts magnetic energy to other forms, can occur when they collide. The structure of multiple magnetic ropes, the interactions between multiple ropes, and their topological properties such as helicity and writhing have been studied theoretically and in laboratory experiments. Here, we report on spiky potential and magnetic fields associated with the ropes. We show that the potential structures are chaotic for a range of their temporal half-widths and the probability density function (PDF) of their widths resembles the statistical distribution of crumpled paper. The spatial structure of the magnetic spikes is revealed using a correlation counting method. Computer simulation suggests that the potential structures are the nonlinear end result of an instability involving relative drift between ions and electrons.

Guide Field Reconnection: Exhaust Structure and Heating.

Magnetospheric Multiscale observations are used to probe the structure and temperature profile of a guide field reconnection exhaust ~100 ion inertial lengths downstream from the X‐line in the Earth's magnetosheath. Asymmetric Hall electric and magnetic field signatures were detected, together with a density cavity confined near 1 edge of the exhaust and containing electron flow toward the X‐line. Electron holes were also detected both on the cavity edge and at the Hall magnetic field reversal. Predominantly parallel ion and electron heating was observed in the main exhaust, but within the cavity, electron cooling and enhanced parallel ion heating were found. This is explained in terms of the parallel electric field, which inhibits electron mixing within the cavity on newly reconnected field lines but accelerates ions. Consequently, guide field reconnection causes inhomogeneous changes in ion and electron temperature across the exhaust.

The Rate of Three-dimensional Hall-MHD Reconnection

There is a discrepancy between reconnection rates observed in a) various plasma environments, b) plasma PIC simulations and c) resistive fluid simulations. Careful observations of solar flares show reconnection rates between 0.001 — 0.1υA (see, e.g., [1]), plasma PIC and Hall-MHD simulations tend to show faster reconnection rate of around 0.1υA and pure MHD resistive simulations tend to converge to 0.015υA. The common explanation of the difference between b and c [2] is that plasma reconnection is inherently different from resistive MHD reconnection due to the different mechanisms of how individual field lines break and reconnect. This would imply a universal rate of 0.1υA in electron-proton plasma. In this paper, I report three-dimensional Hall-MHD simulations with resolution up to 2304 × 46082. Reconnection is mediated by self-excited turbulence in the current layer and is indeed faster than MHD reconnection. However, the reconnection rate goes down as the layer width increases. For the average layer width between two and ten ion skin depths (di), we recover “classic” value of 0.1υA, however, the trend of the rate to decrease may bring it to the fluid value for widths of around 170di, although reaching this width is not possible with current numerical resources. In any case, based on these simulations, now we are certain that the 0.1υA value has been favored before due to the limited range of scales available in numerical simulations.

How the IMF By Induces a Local By Component During Northward IMF Bz and Characteristic Timescales

AbstractWe use the Lyon‐Fedder‐Mobarry global magnetohydrodynamics model to study the effects of the interplanetary magnetic field (IMF) By component on the coupling between the solar wind and magnetosphere‐ionosphere system when IMF Bz>0. We describe the evolution of how a magnetospheric By component is induced on closed field lines during these conditions. Starting from dayside lobe reconnection, the magnetic tension on newly reconnected field lines redistribute the open flux asymmetrically between the two hemispheres. This results in asymmetric magnetic energy density in the lobes. Shear flows are induced to restore equilibrium, and these flows are what effectively induces a local By component. We show the radial dependence of the induced By and compare the results to the induced By during southward IMF conditions. We also show the response and reconfiguration time of the inner magnetosphere to IMF By reversals during northward IMF Bz . A superposed epoch analysis of magnetic field measurements from seven Geostationary Operational Environmental Satellite spacecraft at different local times both for negative‐to‐positive and positive‐to‐negative IMF By reversals is presented. We find that the induced By responds within 16 min of the arrival of IMF By at the bow shock, and it completely reconfigures within 47 min.

Study of Three-dimensional Magnetic Structure and the Successive Eruptive Nature of Active Region 12371

Abstract We study the magnetic structure of a successively erupting sigmoid in active region 12371 by modeling the quasi-static coronal field evolution with nonlinear force-free field (NLFFF) equilibria. Helioseismic and Magnetic Imager/Solar Dynamic Observatory vector magnetograms are used as input to the NLFFF model. In all eruption events, the modeled structure resembles the observed pre-eruptive coronal sigmoid and the NLFFF core field is a combination of double inverse-J-shaped and inverse-S field lines with dips touching the photosphere. Such field lines are formed by the flux cancellation reconnection of opposite-J field lines at bald-patch locations, which in turn implies the formation of a weakly twisted flux-rope (FR) from large-scale sheared arcade field lines. Later on, this FR undergoes coronal tether-cutting reconnection until a coronal mass ejection is triggered. The modeled structure captured these major features of sigmoid-to-arcade-to-sigmoid transformation, which is reoccuring under continuous photospheric flux motions. Calculations of the field line twist reveal a fractional increase followed by a decrease of the number of pixels having a range of twist. This traces the buildup process of a twisted core field by slow photospheric motions and the relaxation after eruption, respectively. Our study infers that the large eruptivity of this AR is due to a steep decrease of the background coronal field meeting the torus instability criteria at a low height (≈40 Mm) in contrast to noneruptive ARs.

Reconnecting reconnection with its current roots

The review by Jack Zirker and Oddbjørn Engvold (Physics Today, August 2017, page 36) of recent coronal research makes extensive use of the concept of field lines—in particular, magnetic reconnection. But like the analogous streamlines in fluids, magnetic field lines are not physical entities. They are just a convenient way of visualizing the structure and behavior of magnetic fields. To convince yourself of this, think of the familiar pattern of field lines connecting the poles of a bar magnet. How many lines are there?It follows that field-line reconnection itself is not a physical phenomenon and cannot cause either coronal heating or the propulsion of plasma in coronal mass ejection events. Such coronal phenomena result from dynamical processes involving electromagnetic fields, currents, and pressures, which are physical. Reconnection merely describes the way the magnetic field topology changes.As mentioned in the article, reconnection events are associated with thin current sheets, which are related to the magnetic field by Ampere’s law. Their evolution is fundamental to understanding coronal phenomena. Eric Priest and Terry Forbes (Zirker and Engvold’s reference 3) note that James Dungey, the originator of the reconnection model, “found that small perturbations in the vicinity of the null point lead to the explosive formation of a current sheet. Near the null point, plasma motions induced by a small current perturbation cause the current to grow, and this current further enhances the plasma motions resulting in a positive feedback.”11. E. Priest, T. Forbes, Magnetic Reconnection: MHD Theory and Applications, Cambridge U. Press (2000), p. 8. Focusing on the role that the currents play in reconnection would make for a clearer understanding.ReferenceSection:ChooseTop of pageReference <<1. E. Priest, T. Forbes, Magnetic Reconnection: MHD Theory and Applications, Cambridge U. Press (2000), p. 8. Google ScholarCrossref© 2018 American Institute of Physics.

Spectroscopic Observations of Current Sheet Formation and Evolution

Abstract We report on the structure and evolution of a current sheet that formed in the wake of an eruptive X8.3 flare observed at the west limb of the Sun on 2017 September 10. Using observations from the EUV Imaging Spectrometer (EIS) on Hinode and the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory, we find that plasma in the current sheet reaches temperatures of about 20 MK and that the range of temperatures is relatively narrow. The highest temperatures occur at the base of the current sheet, in the region near the top of the post-flare loop arcade. The broadest high temperature line profiles, in contrast, occur at the largest observed heights. Furthermore, line broadening is strong very early in the flare and diminishes over time. The current sheet can be observed in the AIA 211 and 171 channels, which have a considerable contribution from thermal bremsstrahlung at flare temperatures. Comparisons of the emission measure in these channels with other EIS wavelengths and AIA channels dominated by Fe line emission indicate a coronal composition and suggest that the current sheet is formed by the heating of plasma already in the corona. Taken together, these observations suggest that some flare heating occurs in the current sheet, while additional energy is released as newly reconnected field lines relax and become more dipolar.

Non-ideal magnetohydrodynamics on a moving mesh

In certain astrophysical systems the commonly employed ideal magnetohydrodynamics (MHD) approximation breaks down. Here, we introduce novel explicit and implicit numerical schemes of ohmic resistivity terms in the moving-mesh code AREPO. We include these non-ideal terms for two MHD techniques: the Powell 8-wave formalism and a constrained transport scheme, which evolves the cell-centred magnetic vector potential. We test our implementation against problems of increasing complexity, such as one- and two-dimensional diffusion problems, and the evolution of progressive and stationary Alfv\'en waves. On these test problems, our implementation recovers the analytic solutions to second-order accuracy. As first applications, we investigate the tearing instability in magnetized plasmas and the gravitational collapse of a rotating magnetized gas cloud. In both systems, resistivity plays a key role. In the former case, it allows for the development of the tearing instability through reconnection of the magnetic field lines. In the latter, the adopted (constant) value of ohmic resistivity has an impact on both the gas distribution around the emerging protostar and the mass loading of magnetically driven outflows. Our new non-ideal MHD implementation opens up the possibility to study magneto-hydrodynamical systems on a moving mesh beyond the ideal MHD approximation.

Driving reconnection in sheared magnetic configurations with forced fluctuations

We investigate reconnection of magnetic field lines in sheared magnetic field configurations due to fluctuations driven by random forcing by means of numerical simulations. The simulations are performed with an incompressible, pseudo-spectral magnetohydrodynamics code in 2D where we take thick, resistively decaying, current-sheet like sheared magnetic configurations which do not reconnect spontaneously. We describe and test the forcing that is introduced in the momentum equation to drive fluctuations. It is found that the forcing does not change the rate of decay; however, it adds and removes energy faster in the presence of the magnetic shear structure compared to when it has decayed away. We observe that such a forcing can induce magnetic reconnection due to field line wandering leading to the formation of magnetic islands and O-points. These reconnecting field lines spread out as the current sheet decays with time. A semi-empirical formula is derived which reasonably explains the formation and spread of O-points. We find that reconnection spreads faster with stronger forcing and longer correlation time of forcing, while the wavenumber of forcing does not have a significant effect. When the field line wandering becomes large enough, the neighboring current sheets with opposite polarity start interacting, and then the magnetic field is rapidly annihilated. This work is useful to understand how forced fluctuations can drive reconnection in large scale current structures in space and astrophysical plasmas that are not susceptible to reconnection.

Magnetic reconnection in terms of catastrophe theory

Magnetic field line reconnection (magnetic reconnection) is a phenomenon that occurs in space and laboratory plasma. Magnetic reconnection allows both the change the magnetic topology and the conversion of the magnetic energy into energy of fast particles. The critical point (critical line or plane in higher dimensional cases) of the magnetic field play an important role in process of magnetic reconnection, as in its neighborhood occurs a change of its topology of a magnetic field and redistribution of magnetic field energy. A lot of literature is devoted to the analytical and numerical investigation of the reconnection process. The main result of these investigations as the result of magnetic reconnection the current sheet is formed and the magnetic topology is changed. While the studies of magnetic reconnection in 2D and 3D configurations have a led to several important results, many questions remain open, including the behavior of a magnetic field in the neighborhood of a critical point of high order. The magnetic reconnection problem is closely related to the problem of the structural stability of vector fields. Since the magnetic field topology changes during both spontaneous and induced magnetic reconnection, it is natural to expect that the magnetic field should evolve from a structurally unstable into a structurally stable configuration. Note that, in this case, the phenomenon under analysis is more complicated since, during magnetic reconnection in a highly conducting plasma, we deal with the non-linear interaction between two vector fields: the magnetic field and the field of the plasma velocities. The aim of our article is to consider the process of magnetic reconnection and transformation of the magnetic topology from the viewpoint of catastrophe theory. Bifurcations in similar configurations (2D magnetic configuration with null high order point) with varying parameters were thoroughly discussed in a monograph by Poston and Stewart.

Recurrent CME-like Eruptions in Emerging Flux Regions. I. On the Mechanism of Eruptions

Abstract We report on three-dimensional (3D) magnetohydrodynamic (MHD) simulations of recurrent eruptions in emerging flux regions. We find that reconnection of sheared field lines, along the polarity inversion line of an emerging bipolar region, leads to the formation of a new magnetic structure, which adopts the shape of a magnetic flux rope (FR) during its rising motion. Initially, the FR undergoes a slow-rise phase and, eventually, it experiences a fast-rise phase and ejective eruption toward the outer solar atmosphere. In total, four eruptions occur during the evolution of the system. For the first eruption, our analysis indicates that the torus instability initiates the eruption and that tether-cutting reconnection of the field lines, which envelop the FR, triggers the rapid acceleration of the eruptive field. For the following eruptions, we conjecture that it is the interplay between tether-cutting reconnection and torus instability that causes the onset of the various phases. We show the 3D shape of the erupting fields, focusing more on how magnetic field lines reconnect during the eruptions. We find that when the envelope field lines reconnect mainly with themselves, hot and dense plasma is transferred closer to the core of the erupting FR. The same area appears to be cooler and less dense when the envelope field lines reconnect with neighboring sheared field lines. The plasma density and temperature distribution, together with the rising speeds, energies, and size of the erupting fields, indicate that they may account for small-scale (mini) coronal mass ejections.

Successive Homologous Coronal Mass Ejections Driven by Shearing and Converging Motions in Solar Active Region NOAA 12371

Abstract We study the magnetic field evolution in AR 12371, related to its successive eruptive nature. During the disk transit of seven days, the active region (AR) launched four sequential fast coronal mass ejections (CMEs), which are associated with long duration M-class flares. Morphological study delineates a pre-eruptive coronal sigmoid structure above the polarity inversion line (PIL) similar to Moore et al.’s study. The velocity field derived from tracked magnetograms indicates persistent shear and converging motions of polarity regions about the PIL. While these shear motions continue, the crossed arms of two sigmoid elbows are being brought to interaction by converging motions at the middle of the PIL, initiating the tether-cutting reconnection of field lines and the onset of the CME explosion. The successive CMEs are explained by a cyclic process of magnetic energy storage and release referred to as “sigmoid-to-arcade-to-sigmoid” transformation driven by photospheric flux motions. Furthermore, the continued shear motions inject helicity flux with a dominant negative sign, which contributes to core field twist and its energy by building a twisted flux rope (FR). After a limiting value, the excess coronal helicity is expelled by bodily ejection of the FR, which is initiated by some instability as realized by intermittent CMEs. This AR is in contrast with the confined AR 12192 with a predominant negative sign and larger helicity flux, but much weaker (−0.02 turns) normalized coronal helicity content. While predominant signed helicity flux is a requirement for CME eruption, our study suggests that the magnetic flux normalized helicity flux is a necessary condition accommodating the role of background flux and appeals to a further study of a large sample of ARs.