Magnetic Field Line Reconnection Research Articles

DP- Y-field-aligned currents and intensity of the BYIMF penetrated into the magnetotail

The three-dimensional global current system produced by currents deflected towards the Earth from the magnetopause is considered. This system is supposed to be divided into two parts: day- and nightside branches. In the frame of the magnetic field line reconnection model, the current intensity and the magnetic field ( Y-component) produced by the latter in the plasma sheet was estimated. It is found that the ratio of magnetic field intensities in the plasma sheet and in the solar wind equals 0.5, which agrees with experimental data.

Long-wavelength limit of the EMHD tearing mode

The frequency spectrum of the modes that lead to fast reconnection of magnetic-field lines in a plasma configuration with sharp current density gradients is obtained and compared to that of the more familiar magnetohydrodynamic tearing modes.

Comment on “Observations of reconnection of interplanetary and lobe magnetic field lines at the high‐latitude magnetopause” by J. T. Gosling, M. F. Thomsen, S. J. Bame, R. C. Elphic, and C. T. Russell

Comment is presented on the results of measurements, reported by Gosling et al. (1991), that were made on ISEE in the vicinity of the high-latitude dusk magnetopause near the terminator plane, at a time when the local magnetosheath and tail lobe magnetic fields were nearly oppositely directed. The character of the observed plasma flowing both tailward and sunward within the high-latitude magnetopause current layer presented real evidence for the local reconnection process. Gosling et al. argued that this process may be a manifestation of different global magnetospheric topology structures. In the comment, a global magnetospheric convection pattern is constructed for the northward IMF and for the case of a large azimuthal component of the IMF with small Bz, irrespective of its sign. The suggested scheme provides a simple explanation for the observed sunward convection in the polar caps both for the northward and for strong By with small Bz. According to the present model, for the magnetosheath field at 2300 UT on June 11, 1978, the reconnection between the open field lines appears at the northern neutral point.

Reply [to “Observations of reconnection of interplanetary and lobe magnetic field lines at the high‐latitude magnetopause” by J. T. Gosling, M. F. Thomsen, S. J. Bame, R. C. Elphic, and C. T. Russell”

The author responds to a paper by Gosling, et al., where the claim is made that experimental data lends support for the observation of local magnetic reconnection. Gosling, et al. present results from ISEE 2 taken near the high latitude, dusk magnetopause, when the magnetic fields in the magnetosheath and tail lobe were approximately oppositely aligned. Detailed ion velocity distributions were measured for the first time for such an event, and showed several different velocity distributions to be present. The author points out that the interpretation of this event does not have to be in terms of such a reconnection event. An explanation is offered of a model proposed by Alexeev and Belenkaya which deals with convection patterns in the earths magnetosphere as it interacts with northern and southern interplanetary magnetic fields. Magnetic reconnection events are described differently dependent upon the IMF orientation and may be two- or three-dimensional in character, occuring at merging lines in the magnetopause or neutral magnetic field points. The author offers a different interpretation of the observed event, claiming field line motion can explain the observation.

Reconnection of magnetic field lines in a turbulent flow

The effect of a turbulent flow on the rate of reconnection of magnetic field lines by a tearing instability is calculated numerically. The reconnection rate is shown to be reduced by the turbulent flow. The turbulent flow transfers energy from the growing tearing instability into shorter wavelength electromagnetic modes that are stable, thereby reducing the rate of reconnection of magnetic field lines by the longer wavelength tearing instability. The numerical results presented contradict the prediction of a turbulence theory of Diamond et al. [Phys. Fluids 27, 1449 (1984)] that a turbulent flow causes an anomalous diffusivity (resistivity) which increases the growth rate of tearing modes.

CDAW 9 analysis of magnetospheric events on May 3, 1986: Event C

The ninth Coordinated Data Analysis Workshop (CDAW 9) focused upon several intervals within the PROMIS period (March‐June 1986). Event interval C comprised the period 0000‐1200 UT on May 3, 1986, which was a highly disturbed time near the end of a geomagnetic storm interval. A very large substorm early in the period commenced at 0111 UT and had a peak AE index value of ∼1500 nT. Subsequent activity was lower, but at least three other substorms occurred at 2‐3 hour intervals. The substorms on May 3 were well observed by a variety of satellites including ISEE 1, 2, and IMP 8 in the magnetotail plus SCATHA, GOES, GMS, and LANL spacecraft at or near geostationary orbit. A particularly important feature of the 0111 UT substorm was the simultaneous imaging of the southern auroral oval by DE 1 and of the northern auroral oval by Viking. The excellent constellation of spacecraft near local midnight in the radial range 5–9 RE made it possible to study the strong cross‐tail current development during the bstorm growth phase and the current disruption and current wedge development during the expansion phase. We use a time‐evolving magnetic field model to map observed auroral features out into the magnetospheric equatorial plane. There was both a dominant eastward and a weaker westward progression of activity following the expansion phase. A clear latitudinal separation (≳10°) of the initial region of auroral brightening and the region of intense westward electrojet current was identified. The combined ground, near‐tail, and imaging data for this event provided an unprecedented opportunity to investigate tail current development, field line mapping, and substorm onset mechanisms. We find evidence for strong current diversion within the near‐tail plasma sheet during the late growth phase and strong current disruption and field‐aligned current formation from deeper in the tail at substorm onset. We conclude that these results are consistent with a model of magnetic neutral line formation in the late growth phase which causes plasma sheet current diversion before the substorm onset. The expansion phase onset occurs considerably later due to reconnection of lobelike magnetic field lines and roughly concurrent cross‐tail disruption in the inner plasma sheet region.

Diagnostics for investigation of magnetic reconnection in sawtooth crash on TFTR

Magnetic reconnection phenomena are documented by a set of noninvasive fast diagnostics during the crash phase of sawtooth oscillations. The electron cyclotron emission diagnostic system provides the highest resolution for measuring time evolution of electron temperature profile during a typical Tokamak Fusion Test Reactor fast sawtooth crash. The x-ray tomography also contains fast time scale information of the electron temperature profile and additionally the impurity concentration. Just before the crash, a shrinking circular hot peak and growing crescent-shaped flat island appear in the inside of the inversion radius on a bird’s-eye view of the electron temperature profile. The electron temperature gradient inside the inversion radius diminishes to nearly zero after the crash. Concomitantly, q(r) profile [q(r)=local safety factor] is measured by the motional stark effect (MSE) diagnostics to verify a magnetic field line reconnection during the sawtooth oscillation. Initial MSE data indicate that central q values increases by 5%–10% during the sawtooth crash phase even when the pressure gradient diminishes inside the q=1 region.

Chaotic collisionless reconnection in Jupiter's nightside magnetosphere

Recently it has been shown that an X-type magnetic neutral line may form in Jupiter's nightside magnetosphere as the result of the current flowing in the plasma disc. Still, a collisionless reconnection mechanism is required to tear up the magnetic field. Recently, Buchner and Zelenyi showed that the chaotization of the electron motion can lead to fast collisionless reconnection in the Earth's magnetotail. In their theory, enhanced pitch angle diffusion is obtained when the curvature parameter κe decreases to κe ≃ 1.6. We apply Buchner and Zelenyi's theory to Jupiter's magnetosphere. The curvature parameter is obtained from the measured plasma parameters and from a self-consistent equilibrium model of the magnetic field, and the results for κe show that the larger the assumed plasma disc, the larger is the range where κe is less than or equal to the stochasticity threshold 1.6. This indicates that chaotic magnetic-field line reconnection may occur in Jupiter's nightside magnetosphere around 60RJ from the planet, and is consistent with thein situ magnetic-field observations, that show the signature of magnetic islands and of tearing-mode instability in the relevant region. We speculate that these processes, differently from the Earth's case, occur in a semi-steady way.

Coupling of Tearing Modes in Normal RFP Discharge

The dynamic process of the coupling of tearing modes in the normal RFP discharge, is studied with the help of 3-D MHD simulations. Almost of studies on the mechanism of the dynamo and the mode coupling have been carried out in the special operation mode “the deep reversal discharge” where the toroidal field at the wall is negatively increased. In this paper the dynamic process in the normal (ordinary) RFP discharge is reported. The process is as follows: The m =1 modes resonant near the plasma axis start growing in the initial stage. After growing enough, they interact with each other and the subsequent reconnection of magnetic field lines takes place. Consequently, m =0 modes are excited. In the normal RFP discharge, these couplings of tearing modes are frequently and periodically generated.

Reconnection of magnetic field lines through an electrostatic wave

The effect of a short-wavelength electrostatic wave on the rate of reconnection of magnetic field lines by a growing long-wavelength tearing mode is investigated. The influence of the electrostatic wave depends strongly on the parity of the wave potential about the resonant surface of the tearing mode and on the relative magnitude of the plasma resistivity η and viscosity μ, as well as on the amplitude of the wave. In general, by coupling the growing long-wavelength tearing mode with stable short-wavelength modes, the electrostatic wave has a stabilizing influence on the long-wavelength tearing mode. However, when the electrostatic wave potential is odd about the resonant surface, the same parity as the scalar potential associated with the tearing mode, and μ≪η, then there is a large increase in the growth rate of the tearing mode because of the presence of a large-amplitude electrostatic wave. Furthermore, a tearing mode that is stable in the absence of the wave can actually be destabilized by the wave. This increase in the growth rate appears to be due to a forced reconnection of opposing magnetic field lines by the flow associated with the electrostatic wave.

Observations of reconnection of interplanetary and lobe magnetic field lines at the high‐latitude magnetopause

Measurements made with the Fast Plasma Experiment on ISEE 2 in the vicinity of the high‐latitude, dusk magnetopause near the terminator plane, at a time when the local magnetosheath and tail lobe magnetic fields were nearly oppositely directed, provide direct evidence for the reconnection of the open field lines of the tail lobes with the interplanetary magnetic field (IMF). The evidence consists primarily of observations of accelerated magnetosheath plasma flowing both tailward and sunward within the high‐latitude magnetopause current layer. Observed speed changes at the magnetopause were of the order of twice the magnetosheath Alfven speed and were quantitatively consistent with the predictions of reconnection models. At times when the newly entering magnetosheath plasma observed at ISEE 2 was accelerated sunward a secondary beam of ions, presumably mirrored at low altitudes, was occasionally present. At times when the newly entering magnetosheath plasma observed by ISEE 2 was accelerated tailward a secondary beam of largely unaccelerated mantle plasma was occasionally present. Small plasma accelerations observed on reconnected field lines in the magnetosheath were associated with the presence of ions reflected at the magnetopause and moving at a speed of approximately twice the Alfven speed relative to the remainder of the magnetosheath plasma. Although previous work has anticipated that the re‐reconnection of the open field lines of the tail lobes with the IMF would be associated with northward IMF in the magnetosheath, the present reconnection event was associated with a local magnetosheath IMF which had a small southward component.

Current sheet stability and accelerated reconnection of magnetic field lines

The stability of a narrow current sheet embedded in a sheared magnetic field to short wavelength microtearing modes is examined and a mechanism for the accelerated reconnection of magnetic field lines during the fast crash of sawteeth in tokamaks is presented. The stability of the sheet is shown to depend crucially on the sign of Il/ k′∥, where Il is the line current in the sheet and k∥ ≡dk∥(x)/dx is the rate of change of the parallel wave number in the direction of the inhomogeneity, as well as on three important characteristic lengths: Δr, the distance between the center of the current sheet and the resonant surface of the mode; the width σs of the sheet; and the resonant layer width wr of the mode. When Il/k′∥ is positive, the current sheet can be unstable only if Δr < 1/2 max(σs,wr); otherwise, the sheet is stable. Conversely, when Il/k∥ is negative the sheet can be unstable only if Δr > 1/2 max(σs,wr); otherwise, the sheet is stable. Expressions for the unstable spectrum and the growth rate of the unstable modes as a function of the relative magnitude of Δr, σs, and wr are presented. For narrow current sheets with σs<wr, the growth rate of the tearing mode actually increases as the plasma resistivity decreases. The stability analysis is applied to the current sheet generated around the X point of a q=1 mode during the fast crash of the central temperature during sawteeth in tokamaks. The sheet is unstable if σs is less than a critical width σsc/a ≡‖Is/I0‖ ×(4/qa), where Is/I0 is the ratio of the current Is in the sheet to the total current I0, a is the minor radius, and qa is the safety factor at the edge of the plasma. For typical TFTR tokamak [Plasma Physics and Controlled Nuclear Fusion Research, Kyoto, 1986 (IAEA, Vienna, 1986), Vol. 1, p. 421] parameters, σsc is a few ion gyroradii. The sheet generated by the q=1 mode in a hot tokamak like TFTR or JET is much narrower than σsc. As a consequence, the sheet is unstable to a broad spectrum of short wavelength perturbations with m, n>100, where m and n are the poloidal and toroidal mode numbers, respectively. These modes grow rapidly with a growth time of the order of 1 μsec, much less than the sawtooth crash time of 50–100 μsec. This short wavelength turbulence will likely broaden the current sheet to the marginally stable width σsc and thereby increase the rate of reconnection of magnetic field lines by the q=1 mode, resulting in a fast crash of the central temperature.

Coherent structure formation and magnetic field line reconnection in magnetohydrodynamic turbulence

The effects produced by the occurrence of both ideal and resistive instabilities in magnetohydrodynamic (MHD) turbulence are discussed, from the point of view of the formation of coherent structures. It is shown that MHD equations spontaneously develop spatially flat or elongated structures, a fact that might be related with the intermittency of the magnetic field observed in some numerical simulations. It is suggested that these flat structures are perhaps unstable with respect to reconnecting (tearing mode type) perturbations. Such instabilities give rise to a nonlocal energy transfer in the wave number space, which modifies the usual inertial spectrum of MHD turbulence, and increases the rate of energy and correlation dissipation. An explicit nonlinear cascade model that qualitatively takes into account such a nonlocal energy transfer is built up, in order to study dynamically the balance between nonlinear energy cascade and linear tearing mode instability. Finally, an attempt is made to interpret some of the peculiar behavior of published MHD simulations on the basis of the results obtained using the present model.

Cosmic Magnetic Fields and Superconducting Strings

A cosmic magnetic field may play a significant role in the formation of galaxies and large scale structure. In particular, a fossil field of present strength ~ 10−9 Gauss is an essential ingredient in the superconducting string model of galaxy formation (Ostriker, Thompson and Witten 1986 (OTW); Thompson 1988a). We discuss the mechanism by which a current is induced on a superconducting string, including recent work on the reconnection of magnetic field lines near the string (Kulsrud and Thompson 1989). A substantial amount of baryonic plasma is trapped on the magnetic field lines which close around the string. The current on a loop almost certainly does not undergo exponential dynamo amplification; an oscillating superconducting loop emits a relativistic MHD wind (Thompson 1988a). Decaying superconducting loops fill most of the intergalactic medium with a relativistic, magnetized fluid. In this model, the gas between galaxies is highly clumped and strongly magnetized, the field strength approaching 1 μG. The maximum energy of cosmic ray protons accelerated at string-driven shocks is ~ 1020 eV (Madau and Thompson 1989).

Boundary layer dynamics in the description of magnetospheric substorms

Magnetospheric substorm activity has been previously explained primarily as the effects of the episodic unloading of energy stored in the Earth's magnetotail. The physics of the unloading process has been associated with magnetic field line reconnection, with one dominant school of thought contending that reconnection occurs close to the Earth (i.e., inside of ∼15 RE). This near‐Earth neutral line model seeks to explain plasma and field characteristics observed in the magnetotail by the development and ultimate disappearance downtail of a plasmoid involving the population of the plasma sheet antiearthward of the near‐Earth neutral line. In this paper we present a study of a significant number of substorm expansive phase events where the phenomenology at ionospheric levels is clearly defined in terms of onset time and locale of the substorm disturbed region. Through the detailed presentation of a subset of these events it is demonstrated that the magnetotail particle and field observations associated with a substorm expansive phase can equally well be explained through the spatial movement of the boundary layers and central plasma sheet in the magnetotail. The sweeping of these regions past the satellite, even in the absence of temporal variations within the various regions, can lead to a set of plasma flow observations typical of what is observed in the magnetotail during substorm activity. We further demonstrate that filamentary currents in the plasma sheet boundary layer can produce the magnetic field signatures presently attributed to retreating plasmoids in the magnetotail. The major strength of this emphasis on boundary layer dynamics is that it provides a framework which naturally encompasses both the driven system and loading‐unloading system which are recognized to be operative simultaneously during magnetospheric substorm activity.

Geometrical properties of three‐dimensional reconnecting magnetic fields with nulls

This paper is concerned with the geometrical aspects of the reconnection of magnetic field lines. Reconnection is associated with discontinuities in the overall field line organization of the configuration in which it occurs. The nature of these discontinuities is that field lines that are immediately adjacent in one region can be widely separated elsewhere. Then infinitesimal field line motion can have a finite consequence. This is an essential aspect of the reconnection phenomenon and distinguishes it from diffusion. Such discontinuous behavior implies the existence of special magnetic structures, which is the subject discussed here. One such structure that is often studied theoretically is a field line that is a closed loop. A common variation is a two‐dimensional configuration in which certain lines are effectively closed by means of an implied periodicity or symmetry in the third direction. These will be called closed lines in this paper. Here, however, attention is focused on another special structure: isolated points where the magnetic field strength strictly vanishes. These points will be called magnetic nulls. Between magnetic nulls and closed field lines, the former are much more likely to be encountered in the uncontrolled extraterrestrial environment. Thus magnetic nulls are probably a crucial element of most reconnection events in the magnetosphere.

ELF plasma waves associated with plasma jets near the Earth magnetopause as observed by Prognoz-8

In this paper we present Prognoz-8 observations of plasma and low-frequency (2-105 Hz) waves near the high latitude nightside magnetopause. Dense magnetosheath (MS)-like plasma clouds apparently detached from the magnetopause were often encountered by Prognoz-8 in this region inside the magnetosphere. It is found that such MS-like plasma clouds are always associated with strong wave activity at frequencies Ωi < ω ≲ ωLH, where Ωi is the proton cyclotron frequency, ωLH is the lower hybrid (LH) frequency. We examined in detail 5 MS-like plasma clouds observed during 2 inbound satellite passes through the magnetopause and high latitude tail. In one case the bulk velocity in the apparently detached MS plasma cloud is higher than in the nearby MS. More frequently, high energy non-maxwellian tails or high energy cold ion beams are observed in the cloud or just outside it. Also, we have found that some characteristics of plasma, magnetic field and plasma waves in the reported phenomena are similar to those in the flux transfer events (FTE's) observed on the dayside. Simultaneous observations of FTE's with signatures of plasma acceleration (most probably being a result of magnetic field line reconnection) and intense LH waves is interpreted in the frame of theory predicting that the lower hybrid drift instability is the source of anomalous resistivity in reconnection.

STRANGE ATTRACTOR AND CHAOTIC PHENOMENA IN NON-IDEAL MHD FLOW

A model on the Rayleigh-Benard problem of a non-ideal MHD flow in a sheared magnetic field is proposed and studied. A new set of nonlinear differential equations for the model bas been derived. Theoretical and numerical analysis shows that the set of equations implies a strange attractor with several novel features differing from Lorenz attractor and, in particular, the coexistence of all three routes to chaos in that model. Among the well-known models with these routes, so far, our system of equations is the unique one without any extrinsic periodic driving term. It exhibits more immediately the intrinsic stochasticity of deterministic nonlinear system. The stochastic motion and reconnection of magnetic field-lines, and the creation of magnetic islands are observed in numerical simulating of this set of equations.

Hall Effect, or Hyperbolic Magnetohydrodynamics, HMHD

The MHD theory of present magnetic fusion research is briefly reviewed with emphasis on its mathematically diffusive character. The importance of retaining the Hall effect term, neglected in ideal or resistive MHD theory, is stressed. Elliptic MHD theory is critically dismissed. The Hall effect, or Hyperbolic, MagnetoHydro-Dynamics, HMHD, is shown to follow as the consequence of a revision of plasma electrodynamics so as to account for the fundamental plasma quasineutrality. The non-validity of Newton’s third law in charged particle contexts is then central. Previously poorly understood phenomena, such as plasma edge effects and magnetic field line reconnection are found to be inherent properties in this HMHD plasma description. The “magnetic bottle” principle for high density plasma confinement is shown to be physically unsound because there will exist a no-confinement plasma boundary region with HMHD theory properties. Arguments for non-thermal fusion, provided by HMHD theory, are given.

Particle and energy transport due to magnetic field-line reconnection in a tokamak

Plasma behaviour during magnetic field-line reconnection which is driven by a rapid toroidal current reversal in a tokamak is investigated by calculating plasma flow speed from the magnetohydromatic equations with variables measured in the experiment. A strong plasma acceleration appears in the outside region of the X-type separatrix formed in the poloidal magnetic field lines. The induced electric field inside the plasma is evaluated directly from Ohm's law by using the fact that the toroidal current density vanishes during the current reversal. Then, plasma resistivity is estimated in the cross-section and the resulting value of energy flow is compared with that given by Poynting's theorem. It is found that the input energy is dissipated effectively through anomalous resistivity in the reconnection region.