Reconnection Of Lines Research Articles

Magnetic reconnection in the Jovian tail: X-line evolution and consequent plasma sheet structures

[1] Magnetic reconnection in planetary magnetospheres plays important roles in energy and mass transfer in the steady state, and also possibly in transient large-scale disturbances. In this paper we report observations of a reconnection event in the Jovian magnetotail by the Galileo spacecraft on 17 June 1997. In addition to the tailward retreat of a main X-line, signatures of recurrent X-line formations are found by close examination of energetic particle anisotropies. Furthermore, detailed analyses of multi-instrumental data for this period provide various spatiotemporal features in the plasma sheet. A significant density decrease was detected in the central plasma sheet, indicative of the transition to lobe (open field line) reconnection from plasma sheet (closed field line) reconnection. When Galileo vertically swept through the plasma sheet, a velocity layer structure was observed. We also analyze a strong southward magnetic field which is similar to dipolarization fronts observed in the terrestrial magnetotail: the ion flow (∼450 km s−1) was observed behind the magnetic front, whose thickness of 10000–20000 km was of the order of ion inertial length. The electron anisotropy in this period suggests an anomalously high-speed electron jet, implying ion-electron decoupling behind the magnetic front. Particle energization was also seen associated with these structures. These observations suggest that X-line evolution and consequent plasma sheet structures are similar to those in the terrestrial magnetosphere, whereas their generality in the Jovian magnetosphere and influence on the magnetospheric/ionospheric dynamics including transient auroral events need to be further investigated with more events.

Evidence of the origin of the Hall magnetic field for reconnection: Hall MHD reconstruction results from Cluster observations

[1] Using reconstruction results from Cluster observations in the magnetotail, we present observational evidence for the electron-current loops responsible for generating the out-of-plane Hall magnetic field during reconnection. This reconstruction is the first successful application to actual spacecraft data of a newly developed reconstruction technique (Sonnerup and Teh, 2009) based on Hall MHD equations in a steady state, two-dimensional (2-D) geometry. Maps from our reconstructions show that the regions of the Hall magnetic field are surrounded by electron-current loops, which is confirmed by direct observations from Cluster 2 of the electrons that flow in and out of the reconnection site within the Hall field region. From the observational perspective our results show that the Hall magnetic field is generated by the Hall electron-current loop as predicted by reconnection theory. Comparison of the reconstructed magnetic field maps from Cluster 1 and Cluster 3 shows that the current sheet is being temporarily distorted, indicating a wavy motion along the current sheet during the event, as suggested by Volwerk et al. (2003). Also, the reconstruction maps help us interpret the observations in more detail. The proper frame for the reconstruction is obtained by optimizing the correlation coefficient between predictions from the map on the basis of Cluster 1 and measurements from the other three spacecraft not otherwise used in the reconstruction. The resulting frame velocity is consistent with that obtained from the timing analysis method for single X line reconnection configuration. In addition, we use Grad-Shafranov reconstruction for force-free conditions together with higher-resolution magnetic field data to derive a 2-D magnetic field map of a secondary magnetic island structure seen by Cluster 3 within the Hall field region and first identified by Eastwood et al. (2007).

Bifurcations of the main auroral ring at Saturn: ionospheric signatures of consecutive reconnection events at the magnetopause

[1] This work reports for the first time on bifurcations of the main auroral ring at Saturn observed with the UVIS instrument onboard Cassini. The observation sequence starts with an intensification on the main oval, close to noon, which is possibly associated with dayside reconnection. Consecutive bifurcations appear with the onset of dayside reconnection, between 11 and 18 magnetic local time, while the area poleward of the main emission expands to lower latitudes. The bifurcations depart with time from the main ring of emission, which is related to the open-closed field line boundary. The augmentation of the area poleward of the main emission following its expansion is balanced by the area occupied by the bifurcations, suggesting that these auroral features represent the amount of newly open flux and could be related to consecutive reconnection events at the flank of the magnetopause. The observations show that the open flux along the sequence increases when bifurcations appear. Magnetopause reconnection can lead to significant augmentation of the open flux within a couple of days and each reconnection event opens ∼10% of the flux contained within the polar cap. Additionally, the observations imply an overall length of the reconnection line of ∼4 hours of local time and suggest that dayside reconnection at Saturn can occur at several positions on the magnetopause consecutively or simultaneously.

Flux transfer events: Motion and signatures

We present results from a 2.5 dimensional hybrid code simulation for the evolution of flux transfer events (FTEs) during intervals of due southward interplanetary magnetic field (IMF) orientation. The structures invariably form between pairs of reconnection lines, often remaining nearly stationary on the subsolar magnetopause before their motion begins. Although a few structures move sunward, ultimately coalescing with others, most eventually accelerate antisunward, reaching velocities many times greater than the sound speed in the magnetosheath but only about one Alfvén speed greater than the ambient magnetosheath flow. At these velocities, slow mode wakes (but not shocks) marked by density enhancements and magnetic field strength decreases extend outward from the structures into the magnetosheath. Upon encountering the cusps, the structures decelerate, undergo reconnection, and are destroyed.

Some properties of the joining of solar filaments

Some possibilities for the reconnection of magnetic-field lines of solar filaments that approach when the photospheric polarity inversion lines change their positions, are discussed. The interaction between filaments depends on their internal properties, which are determined by the filament chirality, or the sign of the helicity of the filament magnetic field. In quadrupolar magnetic configurations, filaments with the same chirality can exchange their halves. Filaments with opposite chirality rupture after the reconnection of the polarity inversion lines, since the two fragments of the different filaments cannot be connected continuously. The morphology and connectivity of the filaments are analyzed using daily Hα filtergrams obtained over the period of maximum activity of the 23rd solar cycle. Examples of alterations of the filament connectivity occuring during the evolution of photospheric fields are presented.

TC-1 observations of a flux rope: Generation by multiple X line reconnection

[1] On 29 December 2004 the Double Star TC-1 satellite most probably stays close to a reconnection line for about 20 min. The direction of the X line can be evaluated according to the component merging model. During the time of observations, TC-1 also detects a flux transfer event (FTE). It is found that the FTE axis is parallel to such an X line. This observation is more in favor of the single X line and the multiple X lines FTE models, rather than the original Russell and Elphic model. Moreover, particular features of the ion distribution functions and the plasma behavior near the FTE seem to be more consistent with a multiple X line generation mechanism. The period under study is characterized by a large and negative Earth's dipole tilt, and the reconnection line close to TC-1 is found to follow the magnetic equator, passing northward of the subsolar/stagnation point. These facts lead to speculate that the FTE is generated when a secondary X line, probably passing through the subsolar/stagnation point and with approximately the same orientation of the X line near TC-1, is activated southward of TC-1.

The many faces of shear Alfvén waves

One of the fundamental waves in magnetized plasmas is the shear Alfvén wave. This wave is responsible for rearranging current systems and, in fact all low frequency currents in magnetized plasmas are shear waves. It has become apparent that Alfvén waves are important in a wide variety of physical environments. Shear waves of various forms have been a topic of experimental research for more than fifteen years in the large plasma device (LAPD) at UCLA. The waves were first studied in both the kinetic and inertial regimes when excited by fluctuating currents with transverse dimension on the order of the collisionless skin depth. Theory and experiment on wave propagation in these regimes is presented, and the morphology of the wave is illustrated to be dependent on the generation mechanism. Three-dimensional currents associated with the waves have been mapped. The ion motion, which closes the current across the magnetic field, has been studied using laser induced fluorescence. The wave propagation in inhomogeneous magnetic fields and density gradients is presented as well as effects of collisions and reflections from boundaries. Reflections may result in Alfvénic field line resonances and in the right conditions maser action. The waves occur spontaneously on temperature and density gradients as hybrids with drift waves. These have been seen to affect cross-field heat and plasma transport. Although the waves are easily launched with antennas, they may also be generated by secondary processes, such as Cherenkov radiation. This is the case when intense shear Alfvén waves in a background magnetoplasma are produced by an exploding laser-produced plasma. Time varying magnetic flux ropes can be considered to be low frequency shear waves. Studies of the interaction of multiple ropes and the link between magnetic field line reconnection and rope dynamics are revealed. This manuscript gives us an overview of the major results from these experiments and provides a modern prospective for the earlier studies of shear Alfvén waves.

Hall MHD simulations of collisionless multiple X line reconnection

A Hall MHD simulation of multiple X line reconnection, in which a diffusion region around the standing X line and the moving plasmoids are involved, is presented. The observed features of Hall magnetic and electric fields in the diffusion region and the direction reversal of the electron flow in the vicinity of the magnetic separatrices are displayed in case 1 with the guide field B-y0 = 0. During the passage of a tailward moving plasmoid the bipolar B-y waveform signatures caused by the Hall current and the Hall electric field signatures in case 1 are qualitatively in line with the observations from the Geotail and Cluster spacecraft. Case 2 with B-y0 = 0.15 is carried out to investigate the reconnection configuration on the earthward side of the X line. The peaking signature of the positive B-y correlated with the bipolar (-/+) B-z is observed while an earthward moving plasmoid passes though the given point below the current sheet and close to the midplane; but the negative B-y is recorded at the given point above the current sheet. Such positive and negative B-y signatures, which are comparable with the observations from Cluster 3 and other Cluster spacecraft, are attributed to the B-y asymmetric structures associated with the Hall effect. The enhancement and variation of the electric field detected by Cluster 3 might be related to the -V' x B electric field associated with the earthward movement of the X line.

Global-scale hybrid simulation of dayside magnetic reconnection under southward IMF: Structure and evolution of reconnection

[1] Magnetopause reconnection is investigated with our 3-D self-consistent global hybrid simulation model. The magnetic configuration and evolution of Flux Transfer Events (FTEs) and the associated ion density and ion velocity distribution at various locations on the magnetopause are investigated. The results reveal the following. (1) Multiple X lines are formed during the magnetopause reconnection, which lead to both FTEs and quasi-steady-type reconnection under a steady solar wind condition. The resulting bipolar signature of local normal magnetic field of FTEs is consistent with satellite observations. (2) A greater-than-20% plasma temperature rise is seen at the center of a FTE, compared to that of the upstream plasma in the magnetosheath. The temperature enhancement is mainly in the direction parallel to the magnetic field because of the mixing of ion beams. (3) Flux ropes that lead to FTEs form between X lines of finite lengths and evolve relatively independently. The ion density is enhanced within FTE flux ropes because of the trapped particles, leading to a filamentary global density. (4) Different from the previous understanding based on the asymmetric density across the magnetopause, a quadrupole magnetic field signature associated with the Hall effects is found to be present around FTEs. (5) A combination of patchy reconnection and multiple X line reconnection leads to the formation of reconnected field lines from the magnetosphere to IMF, as well as the closed field lines from the magnetosphere to the magnetosphere in the magnetopause boundary layer.

Finite Larmor radius effects in the nonlinear dynamics of collisionless magnetic reconnection

We provide numerical evidence of the role of finite Larmor radius effects in the nonlinear dynamics of magnetic field line reconnection in high-temperature, strong guide field plasmas in a slab configuration, in the large Δ′ regime. Both ion and electron temperature effects introduce internal energy variations related to mechanical compression terms in the energy balance, thus contributing to regularize the gradients of the ion density with respect to the cold regimes. For values of the Larmor radii that are not asymptotically small, the two temperature effects are no longer interchangeable, in contrast to what is expected from linear theory, and the differences are measurable in the numerical growth rates and in the nonlinear evolution of the density layers. We interpret such differences in terms of the change, due to ion temperature effects, of the Lagrangian advection of the ‘plasma invariants’ that are encountered in the cold-ion, warm-electron regime. The different roles of the ion and ion-sound Larmor radii in the reconnection dynamics near the X- and O-points are evidenced by means of a local quadratic expansion of the fields.

Mitral Isthmus Ablation Line Reconnections Are Common and Predicts Mitral Annular Dependent Flutters Following Pulmonary Vein Isolations for Atrial Fibrillation

Reconnections across ablation lines result in macro-reentrant atrial tachyarrhythmias. We examined the incidence and clinical impact of mitral isthmus line (MIL) reconnections in patients returning for redo after PVI. Methods: 220 AF pts underwent PVI and MIL ablation was randomly assigned to 52/98. Results: MIL was confirmed intact initially in 28/52 pts. At redo, 8/28 MIL had reconnected, and 3/8 had inducible mitral annular flutter (MAF). Conversely, 24 with failed MIL ablation at the first procedure had a higher probability of MAF at redo (p=0.02). Hence, 12/32 with incomplete/reconnected MIL had MAF compared to 0/20 pts with intact MIL at redo (p=0.002). 13/46 initially randomized to no MIL had inducible MAF at redo compared to 12/52 assigned to MIL ablation initially (p=0.56). Logistic regression analysis revealed longer ablation times for MIL was a positive predictor for future MIL reconnection. Conclusions: 1. Reconnections of 1/3 MIL occurs early after an initial PVI procedure in patients requiring redo; 2. Incomplete lines were associated with higher risk of MAF than complete lines at the first procedure; 3. Reconnected or intact lines are not associated with more MAF than no lines; 4. Longer ablation times predict increased risk of future reconnection.

Mitral Isthmus Ablation Line Reconnections are Common and Predicts Mitral Annular Dependent Flutters Following Pulmonary Vein Isolations for Atrial Fibrillation

Reconnections across ablation lines often result in recurrent macro-reentrant atrial tachyarrhythmias. We examined the incidence and clinical impact of mitral isthmus line (MIL) reconnections in patients returning for a second procedure (redo) after pulmonary vein isolation (PVI). Methods: 220 consecutivepatientswith atrial fibrillation (AF) underwent PVI ablation. MIL ablation was randomly assigned to 52/98 patients. We studied all 98 patients who underwent a redo for recurrent AF (42%), atrial flutter (40%) and both (18%). Results:MILwasconfirmed intact initially in28/52 (54%) pts. At redo, 8/28 (29%) MIL had reconnected, and 3/8 (38%) had inducible mitral annular (MA) flutter. Conversely, 24 with failed MIL ablation at the first procedure 252 New Insights into the Electrophysiological Significance of Lipomatous Metaplasia Post-myocardial Infarction

Cavotricuspid Isthmus Ablation Line Reconnections are Associated with Clinically Inducible Cavotricuspid Isthmus Dependent Flutters in Patients Undergoing Pulmonary Vein Isolation For Atrial Fibrillation

Recurrence of atrial flutter (AFL) after cavotricuspid isthmus (CTI) ablation for typical AFL is uncommon, but the long term integrity of this line in patients without recurrent clinical flutter is unknown. Methods: CTI line was performed in 210/220 consecutive pts (aged 58± 10) who underwent pulmonary vein isolation (PVI) for symptomatic atrial fibrillation. Ninetyeight patients had a repeat procedure for recurrent atrial arrhythmias after an average of 283± 189 days. Of these, CTI ablation was initially successful in 90, incomplete in two and not performed in six. Results: Conduction across CTI lines was confirmed in 36 patients: 34/90 (38%) due to reconnection across an initially intact line and two had previously failed CTI ablat w fl c w fl 3 (

Cavotricuspid Isthmus Ablation Line Reconnections Are Associated with Clinical and Inducible CTI Flutters in Patients Undergoing Pulmonary Vein Isolation for Atrial Fibrillation

Cavotricuspid Isthmus Ablation Line Reconnections Are Associated with Clinical and Inducible CTI Flutters in Patients Undergoing Pulmonary Vein Isolation for Atrial Fibrillation

Magnetic field line reconnection in the current systems of flux ropes and Alfvén waves

Magnetic field line reconnection is still considered, by some, to be one of the most important topics in plasma physics. It has been in this category for close to 30 years and the ‘problem of reconnection’ has still not been solved. Magnetic field topologies are part and parcel of the current systems within a plasma whatever their source. Plasma currents may initially be induced or injected but they soon become entangled or part of the currents of plasma waves, flows and structures. We first present experimental results of undriven reconnection, which occurs when two magnetic flux ropes are generated from initially adjacent pulsed current channels in a background magnetoplasma (length 18 m, diameter 60 cm). The second example presented is the three-dimensional (3D) magnetic fields and currents associated with colliding laser-produced plasmas. The currents in this situation are those of shear Alfvén waves. The wave magnetic field is a small fraction of the background field; nevertheless, reconnection regions, multiple magnetic ‘X’ points (which are 3D) and induced electric fields are observed. The first involves the interaction of magnetic flux ropes and the second localized reconnection sites in the current system of Alfvén waves.

Asymmetry in the bipolar signatures of flux transfer events

Several conceptual models have been proposed for the formation of flux transfer events (FTEs), including models based on reconnection at a single reconnection line (X line) and at multiple X lines. Two‐dimensional magnetohydrodynamic models have previously been used to simulate both scenarios and have found a tendency for FTEs generated by single X line reconnection to exhibit an asymmetry in the bipolar BN signature that is the major in situ signature of FTE structures, with the leading peak being substantially smaller than the trailing peak. On the other hand, simulated FTEs generated by multiple X line reconnection led to more symmetric signatures. We present a comparison of these simulation results with observations made at the Earth's magnetopause by the Cluster spacecraft, using a data set of 213 FTEs which were observed by all four spacecraft in 2002/2003 at the high‐latitude magnetopause near local noon and at low latitudes on the flanks, and 36 FTEs which were observed by one or more Cluster spacecraft near the subsolar point in 2007 and 2008. A tendency is found for the BN signatures to be asymmetric but with the leading peak larger in amplitude than the trailing peak, opposite to the prediction made by the 2‐D single X line simulations. This tendency is weaker in the subsolar FTEs. Therefore, the observations are not consistent with 2‐D MHD simulations of single X line reconnection. The signatures observed near the subsolar point are more consistent with those predicted by 2‐D simulations of multiple X line reconnection, although the multiple X line simulation studies did not report any net asymmetry. We propose that the observed asymmetry can be explained by a compression of magnetic flux ahead of the propagating FTE structure and a rarefaction behind it. The weaker tendency nearer the subsolar point is consistent with a weaker compression and rarefaction due to lower FTE velocities.

Seasonal and interplanetary magnetic field–dependent polar cap contraction during substorm expansion phase

Recent observations from simultaneous imaging in two hemispheres have shown that the polar caps can attain considerably different shapes as the auroral ovals contract during the substorm expansion phase. We use images from 2770 substorms to study the evolution of the polar cap boundary location statistically. We show that, during the first 26 min after substorm expansion phase onset, the polar cap boundary location depends on seasons, interplanetary magnetic field (IMF) By, and IMF Bx. For different signs of By, with ∣By∣ > 3 nT, the asymmetry in polar cap boundary observed at onset increases during the expansion phase, consistent with an increase in tail reconnection of field lines with asymmetrical footpoints. When Bx > 2 nT and ∣By∣ < 2 nT, the polar cap boundary dawnward of the onset propagates slightly farther poleward compared to negative Bx conditions. In the sunlit hemisphere, the polar cap boundary evolves from a pronounced equatorward displacement at onset to an almost reversed displacement during the expansion phase, compared to substorms observed in darkness. Substorms in the dark hemisphere also have a much more pronounced bulge than substorms in the sunlit hemisphere. If the interpretation of the poleward auroral boundary as being coincident with the open/closed field line boundary (OCB) is correct, the seasonal differences in OCB locations imply seasonal differences in the ionospheric convection during the substorm expansion phase.

Antiparallel magnetic reconnection rates at the Earth's magnetopause

Cluster observations at the Earth's high‐latitude magnetopause are combined with magnetic field models to demonstrate that antiparallel reconnection was occurring at the magnetopause for an event on 3 December 2001. Over a 20 min period, the reconnection line passed over the spacecraft on two occasions. In between the encounters with the reconnection line, velocity cutoffs in the ion distributions are used to determine the distance to the reconnection site. These observations are consistent with an antiparallel reconnection line whose location relative to the spacecraft depends on the orientation of the interplanetary magnetic field. Using this knowledge of the reconnection site location and a previously developed, two‐spacecraft method for computing the inflow velocity into the reconnection site, the reconnection rate (Vn/VA) is determined to be ∼0.08. The rate is consistent with fast reconnection and considerably higher than the reconnection rate for a component reconnection event that was determined using the same two‐spacecraft method.

Influence of cusp O+ outflow on magnetotail dynamics in a multifluid MHD model of the magnetosphere

A multifluid version of the Lyon‐Fedder‐Mobarry global simulation model has been used to investigate the effects of outflowing ionospheric O+ on the magnetosphere‐ionosphere system. To quantify these effects, we specify the number density, upward parallel velocity, and temperature of the O+ outflow in a limited area of the low‐altitude simulation boundary representing the projection of cusp, cleft, and low‐latitude boundary layer. A baseline simulation without O+ outflow is compared with simulations with a range of fluxes and initial velocities. In the cases with high fluxes, it is shown that the configuration of the magnetosphere is dramatically changed. In particular, the cross‐polar cap potential is reduced, the polar cap area is increased, and the nightside reconnection line is moved earthward. Furthermore, in one case, the O+ outflow leads to the onset of a second substorm not seen in the other simulation runs.

A simulation study of dynamics in the distant Jovian magnetotail

In late February 2007, the New Horizons spacecraft flew past Jupiter and repeatedly observed tailward moving plasma structures in Jupiter's distant tail (x > −2000 RJ). There were no solar wind observations in the outer solar system at this time. In this study, we have used a new magnetohydrodynamic model with a 1500 RJ long tail to investigate the propagation of these structures under different solar wind conditions. In the simulation we find that periodic plasmoid ejection from a reconnection line in Jupiter's plasma sheet can occur when the solar wind dynamic pressure is low and the IMF has a northward component along the dipole direction. In particular, periodic plasmoids which are similar to New Horizon observations are reproduced when the interplanetary magnetic field (IMF) was oscillating from northward to southward with a 10 h period similar to what would be expected for a rotating planetary magnetic field interacting with an IMF with only a y component. When the IMF had a southward component along the dipole direction, high‐latitude reconnection can form plasmoid‐like structures. As a result plasma structures moving tailward can be found over a large region of the tail, as is observed. Oscillations in the solar wind dynamic pressure lead to oscillations in the plasma sheet.