Multiple X-line Reconnection Research Articles

Particle simulations of driven collisionless magnetic reconnection at the dayside magnetopause

Basic plasma processes associated with driven collisionless magnetic reconnection at the Earth's dayside magnetopause are studied on the basis of particle simulations. A two‐and‐one‐half‐dimensional (2½‐D) electromagnetic particle simulation model with a driven inflow boundary and an open outflow boundary is developed for the present study. The driven inflow boundary is featured with a driving electric field for the vector potential, while the open outflow boundary is characterized by a vacuum force‐free condition for the electrostatic potential. The major findings are as follows: (1) the simulations exhibit both quasi‐steady single X line reconnection and intermittent multiple X line reconnection (MXR); the MXR process is characterized by repeated formation and convection of magnetic islands; (2) particle acceleration in the reconnection process results in a power law energy spectrum of ƒ(E) ∼ E−4 for energetic ions with E > 40 keV and energetic electrons with E > 3 keV, where E is the particle energy; particles are accelerated to high energy near magnetic O line regions as particles are trapped within magnetic islands; (3) field‐aligned particle heat fluxes and intense plasma waves associated with the collisionless magnetic reconnection process are also observed; typical power spectra of fluctuating magnetic and electric fields are found to be PB ∼ f−3.6 and PE ∼ f−1.8, respectively, where f is the wave frequency; (4) when applied to the dayside magnetopause, simulation results show that the MXR process tends to generate a simultaneous magnetic field perturbation on both sides of the dayside magnetopause, resembling the observed features of two‐regime flux transfer events (FTEs); and (5) an intrusion of magnetosheath plasma bulge into the magnetosphere due to the formation of magnetic islands may lead to the layered structures observed in magnetospheric FTEs. Simulation results are applied to the dayside magnetopause to provide an explanation for some features associated with dayside magnetic reconnection and FTEs.

Different FTE signatures generated by the bursty single X line reconnection and the multiple X line reconnection at the dayside magnetopause

Magnetic signatures associated with the time‐dependent magnetic reconnection processes at the dayside magnetopause are studied based on two‐dimensional compressible MHD simulations. In the simulations, magnetic and plasma signatures resemblant to the observed flux transfer events (FTEs) can be generated either by the magnetic bulges formed during the bursty single X line reconnection (BSXR) or by the magnetic islands (flux tubes) formed during the multiple X line reconnection (MXR). It is found that the FTE magnetic signatures are not exhibited on the magnetospheric side if the FTEs are due to the BSXR process and Bm/Bs ≥ 1.7, where Bm and Bs are the magnetic field strength in the magnetosheath and in the magnetosphere, respectively. On the other hand, the bipolar FTE signatures can be detected on both the magnetosphere and magnetosheath sides if the FTEs are caused by the MXR process and Bm/Bs ≤ 2.6. When Bm/Bs > 2.6, the bipolar FTE signatures in the magnetosphere site become too small to be detected even if magnetic islands are formed during the MXR process. Furthermore, for Bm/Bs > 1, the region for the detection of FTE signatures in the magnetospheric side is smaller than that in the magnetosheath side. Since at the dayside magnetopause the typical value of Bm/Bs is 1–3, the simulation results indicate that more FTE signatures can be detected in the magnetosheath side than in the magnetosphere. It is also found that the MXR process often generates a clear bipolar Bn signature while the BSXR process tends to produce FTEs with a monopolar Bn signature near the reconnection region and a highly asymmetric bipolar Bn signature away from the reconnection region.

Time-dependent magnetic reconnection: Two- and three-dimensional MHD simulations

Observations of flux transfer events (FTEs) at the earth magnetospause and plasmoids in the magnetotail indicate that magnetic reconnections in the earth magnetosphere are time-dependent. Motivated by the observed flux transfer events, Lee and Fu proposed a multiple X-line reconnection (MXR) model for the dayside magnetopause. The multiple X-line reconnection is intrinsically a time-dependent process and is different from the classical single X-line reconnection (SXR) process. As a consequence of MXR, plasmoids or magnetic flux ropes are intermittently formed and convected out of the reconnection region. Two- and three-dimensional MHD simulations have been carried out to study the time-dependent magnetic reconnection process. The 2-D simulation results show that the time-dependent multiple X-line process tends to occur when the system length is long and the resistivity is small. Otherwise, the quasi-steady single X-line process takes place. It is found that the occurrence of multiple reconnection sites in the current sheet may be due to either (1) the development of tearing mode instability in the local slab model, or (2) the nonuniformity of the current sheet in the global model. In the three-dimensional case, the multiple X-line reconnection process leads to the formation of magnetic flux tubes or magnetic ropes. In the three-dimensional case, the field line topology at the ends of the magnetic rope is very complicated. The simulation shows that magnetic ropes formed through the MXR process usually have “frayed” ends. In addition, a strong tube-aligned plasma flow is observed during the formation of magnetic flux tubes in the 3-D simulation.

Impulsive electric and magnetic field perturbations observed over South Pole: Flux transfer events?

Electric and magnetic field data from the cusp latitude station at South Pole were searched for impulsive perturbations. These events may be evidence of the ionospheric signature of flux transfer events (FTE's). In a single X‐line reconnection model, the field aligned current system associated with the convection of the flux tube across the polar cap will produce two convection vortices around the flux tube. The multiple X‐line reconnection model predicts additional co‐axial field‐aligned currents in helical flux tubes. The electric field signatures of these two models are different. The electric field of the towing system attenuates less than the electric field of the twisting system as the field is mapped down to balloon altitude. Combining electric and magnetic field data may determine the relative importance of the two systems. Near local magnetic noon on 7 January 1986, at least three candidate events have been found. One event is shown to approximate the signature expected for reconnected flux tubes passing overhead. The paper brings a new data set to bear on an important phenomenon.

A mechanism for the generation of cusp region hydromagnetic waves

It is suggested that the existence of a sporadic multiple X line reconnection (MXR) process at the dayside magnetopause could be a source of energy to produce ULF hydromagnetic waves (ƒ ∼ 1–10 mHz) in the dayside magnetopause cusp region. The MXR process is characterized by the repeated formation and convection of magnetic islands and elongated plasma clouds, which compress and distort the adjacent closed geomagnetic field lines. This leads in a natural way to the generation of ULF waves with frequencies in the range 1–10 mHz along cusp geomagnetic field lines. These waves are expected to be appoximately linearly polarized, dominated by variations in the radial magnetic field component near the dayside magnetopause and by variations in the azimuthal magnetic field component near the resonant closed field lines in the magnetosphere. In addition, the field‐aligned currents associated with the MXR process may generate the impulsive magnetic field variations often observed to accompany the continuous variations in the ULF band. Hence the reconnection process may be an additional mechanism, together with the Kelvin‐Helmholtz instability and direct penetration of upstream waves, for the production of hydromagnetic energy in the dayside magnetosphere.

A study of multiple X line reconnection at the dayside magnetopause

Interaction between the interplanetary magnetic field and the earth's geomagnetic field at the dayside magnetopause is studied by a global two‐dimensional incompressible MHD simulation. It is found that for a large magnetic Reynolds number (Rm ≥400), the multiple X line reconnection (MXR) becomes the major dayside reconnection process, while for a small magnetic Reynolds number (Rm ≤100), the classical single X line reconnection takes place. Since the magnetic Reynolds number at the dayside magnetopause is probably very high (Rm >10³), it may be suggested that MXR will dominate the dayside reconnection pattern. As a result of the MXR process, magnetic islands are formed on the dayside magnetopause surface. The dependence of the dayside reconnection geometry on the solar wind Alfvén Mach number is also examined.

The evolution of an isolated reconnected flux tube

The detailed structure of a reconnected flux tube is examined in an effort to explain the twisted magnetic field that is commonly observed within Flux Transfer Events (FTEs). We begin by clarifying the definition of a twisted flux tube. From the observed direction of the twist on both sides of the magnetopause we argue that the twist is unlikely to be introduced after the tube formation, and so must be already present in the tube when reconnection stops. On purely topological grounds, it is evident that a Flux Transfer Event produced by single or multiple X-line reconnection will exhibit a twist. The evolution of such a tube is complicated, but by considering the forces inherent in such a structure we show that there will be a torque acting upon the flux tubes that leave the reconnection site. The torque will propagate torsional Alfvén waves along reconnected flux tubes. We predict the sense of twist and pitch length of these torsional Alfvén waves and find excellent agreement with observations obtained at the Earth's dayside magnctopause.

Possible evidence of flux transfer events in the polar ionosphere

We present magnetic field data from the cusp‐latitude South Pole station that exhibit, under appropriate local time and interplanetary magnetic field conditions, the signature expected in the ionosphere from a flux transfer event (FTE) at the magnetopause. In particular, the model of multiple X‐line reconnection at the magnetopause predicts field‐aligned currents in helical flux tubes, with transverse magnetic fields propagating as Alfvén waves toward the ionosphere. The distinctive magnetic signature at a polar cap magnetic station, particularly in the vertical component, can be used to infer the signs of the By and Bz components of the interplanetary magnetic field.

Simulation of multiple X-line reconnection at the dayside magnetopause

The multiple X-line reconnection process, which was proposed by Lee and Fu [1985] for the dayside magnetopause, is simulated based on a two-dimensional incompressible MHD model. It is found that when the system length is much larger than the thickness of the current layer, magnetic flux tubes are repeatedly formed and convected out of the system. Impulsive magnetic reconnections associated with the formation of magnetic flux tubes are observed. In the case of the dayside magnetopause, magnetic flux tubes with a cross-section of ∼ 1 RE² are found to be formed repeatedly every 5-10 minutes, consistent with satellite observations.