Nonlinear magnetic reconnection models with separatrix jets

1990 ◽  
Vol 44 (2) ◽  
pp. 337-360 ◽  
Author(s):  
E. R. Priest ◽  
L. C. Lee
Keyword(s):  
Magnetic Field ◽  
Boundary Conditions ◽  
Magnetic Reconnection ◽  
Classical Model ◽  
Diffusion Region ◽  
Slow Mode ◽  
Slowing Down ◽  
Downstream Region ◽  
Field Lines ◽  
Reversed Current

A new theory for fast steady-state magnetic reconnection is proposed that includes many features of recent numerical experiments. The inflow region differs from that in the classical model of Petschek (1964) and the unified linear solutions of Priest & Forbes (1986) in possessing highly curved magnetic field lines rather than ones that are almost straight. A separatrix jet of plasma is ejected from the central diffusion region along the magnetic separatrix. Two types of outflow are studied, the simplest possessing an outflow magnetic field that is potential. The other contains weak standing shock waves attached to the ends of the diffusion region and either slowing down the flow (fast-mode shock) after it crosses the separatrix jet or speeding it up (slow-mode), depending on the downstream boundary conditions. A spike of reversed current slows down the plasma that emerges rapidly from the diffusion region into the more slowly moving downstream region, and diverts most of it along the separatrix jets. In the simplest case the outflow possesses no vorticity over most of the downstream region. The models demonstrate that both upstream and downstream boundary conditions are important in determining which regime of reconnection is produced from a wide variety of possibilities.

Weakly nonlinear theory of fast steady-state magnetic reconnection

1988 ◽  
Vol 40 (1) ◽  
pp. 143-161 ◽  
Author(s):  
M. Jardine ◽  
E. R. Priest
Keyword(s):  
Magnetic Field ◽  
Steady State ◽  
Boundary Conditions ◽  
Free Parameter ◽  
Diffusion Region ◽  
Slow Mode ◽  
Weakly Nonlinear ◽  
Weakly Nonlinear Theory ◽  
Special Cases ◽  
Field Lines

A family of models for fast steady-state reconnection has recently been presented by Priest and Forbes, of which the Petschek-like and Sonnerup-like solutions are special cases. This essentially linear treatment involves expanding about a uniform flow and field in powers of the external Alfvén Mach number Me, and hence is valid for small values of that parameter. To lowest order, the discrete slow-mode compressions attached to the diffusion region are straight, while downstream of them the plasma flows at simply the external Alfvén speed vAe and the field lines are straight. Here we present an extension of these solutions to the next order, which not only reveals that the wave itself is curved (as are the downstream magnetic field lines), but also that the downstream solution is sensitive to changes in the upstream boundary conditions. In the downstream solution there is a free parameter, which may be specified as a downstream boundary condition. Thus the boundary conditions at both the inflow and the outflow boundaries are crucial in determining the nature of the reconnection.

scholarly journals Magnetic Reconnection on the Sun

1990 ◽  
Vol 142 ◽  
pp. 271-291 ◽  
Author(s):  
E R Priest
Keyword(s):  
Magnetic Reconnection ◽  
Theoretical Models ◽  
Diffusion Region ◽  
The Sun ◽  
Small Current ◽  
Flare Loops ◽  
Pile Up ◽  
Fundamental Process ◽  
Field Lines ◽  
Reversed Current

Magnetic reconnection is a fundamental process in astrophysics which plays many different roles on the Sun. The classical theory of fast steady reconnection due to Sweet, Parker, Petschek and Sonnerup has been unified in a theory which possesses many new regimes that depend on the boundary conditions at large distances. For example, the flux pile-up regime possesses diverging flows, a long central diffusion region and a reconnection rate that is much larger than the Petschek value. Recent numerical experiments however, often possess three features that are not present in the earlier theoretical models, namely highly curved inflow field lines, separatrix jets and reversed current spikes, and so an attempt is described to include these features in a new theoretical model.Several solar phenomena where reconnection is believed to be operating are described. Cancelling magnetic features in photospheric magnetograms are probably evidence of reconnection submergence. Most prominences are of inverse polarity and have upflows driven by reconnection below them. Coronal heating may well be due to reconnection in many small current sheets. In solar flares reconnection may be driven below an eruptiing active-region prominence and hence power the high-temperature flare loops and chromospheric ribbons.

scholarly journals Electron-only Reconnection in an Ion-scale Current Sheet at the Magnetopause

2021 ◽  
Vol 922 (1) ◽  
pp. 54
Author(s):  
S. Y. Huang ◽  
Q. Y. Xiong ◽  
L. F. Song ◽  
J. Nan ◽  
Z. G. Yuan ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Standard Model ◽  
Magnetic Reconnection ◽  
Current Sheet ◽  
Diffusion Region ◽  
Ion Outflow ◽  
Magnetospheric Multiscale ◽  
The Standard Model ◽  
Field Lines ◽  
Reconnection Model

Abstract In the standard model of magnetic reconnection, both ions and electrons couple to the newly reconnected magnetic field lines and are ejected away from the reconnection diffusion region in the form of bidirectional burst ion/electron jets. Recent observations propose a new model: electron-only magnetic reconnection without ion coupling in an electron-scale current sheet. Based on the data from the Magnetospheric Multiscale (MMS) mission, we observe a long-extension inner electron diffusion region (EDR) at least 40 d i away from the X-line at the Earth’s magnetopause, implying that the extension of EDR is much longer than the prediction of the theory and simulations. This inner EDR is embedded in an ion-scale current sheet (the width of ∼4 d i, d i is ion inertial length). However, such ongoing magnetic reconnection was not accompanied with burst ion outflow, implying the presence of electron-only reconnection in an ion-scale current sheet. Our observations present a new challenge for understanding the model of standard magnetic reconnection and the electron-only reconnection model in an electron-scale current sheet.

Formation of non-thermal electron velocity distribution functions in kinetic magnetic reconnection

2021 ◽  
Author(s):  
Xin Yao ◽  
Patricio A. Muñoz ◽  
Jörg Büchner
Keyword(s):  
Magnetic Field ◽  
Free Energy ◽  
Field Strength ◽  
Magnetic Reconnection ◽  
Electron Beams ◽  
Distribution Functions ◽  
Electron Velocity ◽  
Diffusion Region ◽  
Electron Velocity Distribution ◽  
Velocity Distribution Functions

<div> <div>Magnetic reconnection can convert magnetic energy into non-thermal particle energy in the form of electron beams. Those accelerated electrons can, in turn, cause radio emission in environments such as solar flares. The actual properties of those electron velocity distribution functions (EVDFs) generated by reconnection are still not well understood. In particular the properties that are relevant for the micro-instabilities responsible for radio emission. We aim thus at characterizing the electron distributions functions generated by 3D magnetic reconnection by means of fully kinetic particle-in-cell (PIC) code simulations. Our goal is to characterize the possible sources of free energy of the generated EVDFs in dependence on an external (guide) magnetic field strength. We find that: (1) electron beams with positive gradients in their parallel (to the local magnetic field direction) distribution functions are observed in both diffusion region (parallel crescent-shaped EVDFs) and separatrices (bump-on-tail EVDFs). These non-thermal EVDFs cause counterstreaming and bump-on-tail instabilities. These electrons are adiabatic and preferentially accelerated by a parallel electric field in regions where the magnetic moment is conserved. (2) electron beams with positive gradients in their perpendicular distribution functions are observed in regions with weak magnetic field strength near the current sheet midplane. The characteristic crescent-shaped EVDFs (in perpendicular velocity space) are observed in the diffusion region. These non-thermal EVDFs can cause electron cyclotron maser instabilities. These non-thermal electrons in perpendicular velocity space are mainly non-adiabatic. Their EVDFs are attributed to electrons experiencing an E×B drift and meandering motion. (3) As the guide field strength increases, the number of locations in the current sheet with distributions functions featuring a perpendicular source of free energy significantly decreases.</div> </div>

Micro-scale tearing mode turbulence in the diffusion region during macro-scale evolution of turbulent reconnection

2021 ◽  
Author(s):  
Takuma Nakamura ◽  
Hiroshi Hasegawa ◽  
Tai Phan ◽  
Kevin Genestreti ◽  
Richard Denton ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Energy Conversion ◽  
Magnetic Reconnection ◽  
Diffusion Region ◽  
Electron Diffusion ◽  
Efficient Energy ◽  
Micro Scale ◽  
Island Evolution ◽  
Macro Scale ◽  
Scale Evolution

<p>Magnetic reconnection is a key fundamental process in collisionless plasmas that explosively converts magnetic energy to plasma kinetic and thermal energies through a change of magnetic field topology in an electron-scale central region called the electron diffusion region. Past simulations and observations demonstrated that this process causes efficient energy conversion through the formation of multiple macro-scale or micro-scale magnetic islands/flux ropes. However, how these different spatiotemporal scale phenomena are coupled is still poorly understood. In this study, to investigate the turbulent evolution of magnetic reconnection, we perform a new large-scale fully kinetic simulation of a thin current sheet considering a power-law spectrum of initial fluctuations in the magnetic field as frequently observed in the Earth’s magnetotail. The simulation demonstrates that during a macro-scale evolution of turbulent reconnection, the merging of macro-scale islands results in reduction of the rate of reconnection as well as the aspect ratio of the electron diffusion region. This allows the repeated, quick formation of new electron-scale islands within the electron diffusion region, leading to an efficient energy cascade between macro- and micro-scales. The simulation also demonstrates that a strong electron acceleration/heating occurs during the micro-scale island evolution within the EDR. These new findings indicate the importance of non-steady features of the EDR to comprehensively understand the energy conversion and cascade processes in collisionless reconnection.</p>

scholarly journals 2D and 3D turbulent magnetic reconnection

2009 ◽  
Vol 5 (H15) ◽  
pp. 434-435
Author(s):  
A. Lazarian ◽  
G. Kowal ◽  
E. Vishniac ◽  
K. Kulpa-Dubel ◽  
K. Otmianowska-Mazur
Keyword(s):  
Magnetic Field ◽  
Magnetic Reconnection ◽  
Large Scale ◽  
Gamma Ray ◽  
Three Dimensional ◽  
Turnover Time ◽  
Turbulent Fluid ◽  
Astrophysical Objects ◽  
Field Lines ◽  
The Magnetic Field

AbstractA magnetic field embedded in a perfectly conducting fluid preserves its topology for all times. Although ionized astrophysical objects, like stars and galactic disks, are almost perfectly conducting, they show indications of changes in topology, magnetic reconnection, on dynamical time scales. Reconnection can be observed directly in the solar corona, but can also be inferred from the existence of large scale dynamo activity inside stellar interiors. Solar flares and gamma ray busts are usually associated with magnetic reconnection. Previous work has concentrated on showing how reconnection can be rapid in plasmas with very small collision rates. Here we present numerical evidence, based on three dimensional simulations, that reconnection in a turbulent fluid occurs at a speed comparable to the rms velocity of the turbulence, regardless of the value of the resistivity. In particular, this is true for turbulent pressures much weaker than the magnetic field pressure so that the magnetic field lines are only slightly bent by the turbulence. These results are consistent with the proposal by Lazarian & Vishniac (1999) that reconnection is controlled by the stochastic diffusion of magnetic field lines, which produces a broad outflow of plasma from the reconnection zone. This work implies that reconnection in a turbulent fluid typically takes place in approximately a single eddy turnover time, with broad implications for dynamo activity and particle acceleration throughout the universe. In contrast, the reconnection in 2D configurations in the presence of turbulence depends on resistivity, i.e. is slow.

scholarly journals Storm-time ring current: model-dependent results

2012 ◽  
Vol 30 (1) ◽  
pp. 177-202 ◽  
Author(s):  
N. Yu. Ganushkina ◽  
M. W. Liemohn ◽  
T. I. Pulkkinen
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Boundary Conditions ◽  
Electric Fields ◽  
Ring Current ◽  
Current Model ◽  
Current Data ◽  
Storm Events ◽  
Boundary Location ◽  
Field Lines

Abstract. The main point of the paper is to investigate how much the modeled ring current depends on the representations of magnetic and electric fields and boundary conditions used in simulations. Two storm events, one moderate (SymH minimum of −120 nT) on 6–7 November 1997 and one intense (SymH minimum of −230 nT) on 21–22 October 1999, are modeled. A rather simple ring current model is employed, namely, the Inner Magnetosphere Particle Transport and Acceleration model (IMPTAM), in order to make the results most evident. Four different magnetic field and two electric field representations and four boundary conditions are used. We find that different combinations of the magnetic and electric field configurations and boundary conditions result in very different modeled ring current, and, therefore, the physical conclusions based on simulation results can differ significantly. A time-dependent boundary outside of 6.6 RE gives a possibility to take into account the particles in the transition region (between dipole and stretched field lines) forming partial ring current and near-Earth tail current in that region. Calculating the model SymH* by Biot-Savart's law instead of the widely used Dessler-Parker-Sckopke (DPS) relation gives larger and more realistic values, since the currents are calculated in the regions with nondipolar magnetic field. Therefore, the boundary location and the method of SymH* calculation are of key importance for ring current data-model comparisons to be correctly interpreted.

scholarly journals Evolution of magnetic helicity under kinetic magnetic reconnection: Part II B ≠ 0 reconnection

2002 ◽  
Vol 9 (2) ◽  
pp. 139-147 ◽  
Author(s):  
T. Wiegelmann ◽  
J. Büchner
Keyword(s):  
Magnetic Field ◽  
Magnetic Reconnection ◽  
Hall Current ◽  
Neutral Line ◽  
Magnetic Helicity ◽  
Perpendicular Magnetic Field ◽  
Guide Field ◽  
Magnetic Neutral Line ◽  
Field Lines ◽  
The Magnetic Field

Abstract. We investigate the evolution of magnetic helicity under kinetic magnetic reconnection in thin current sheets. We use Harris sheet equilibria and superimpose an external magnetic guide field. Consequently, the classical 2D magnetic neutral line becomes a field line here, causing a B ≠ 0 reconnection. While without a guide field, the Hall effect leads to a quadrupolar structure in the perpendicular magnetic field and the helicity density, this effect vanishes in the B ≠ 0 reconnection. The reason is that electrons are magnetized in the guide field and the Hall current does not occur. While a B = 0 reconnection leads just to a bending of the field lines in the reconnection area, thus conserving the helicity, the initial helicity is reduced for a B ≠ 0 reconnection. The helicity reduction is, however, slower than the magnetic field dissipation. The simulations have been carried out by the numerical integration of the Vlasov-equation.

scholarly journals Global MHD Simulation of the Weak Southward IMF Condition for Different Time Resolutions

2021 ◽  
Vol 8 ◽  
Author(s):  
Kyung Sun Park
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Magnetic Reconnection ◽  
Three Dimensional ◽  
Slow Solar Wind ◽  
Polar Cap ◽  
Plasma Properties ◽  
Simulation Results ◽  
Field Lines ◽  
The Impact

We performed high-resolution three-dimensional global MHD simulations to determine the impact of weak southward interplanetary magnetic field (IMF) (Bz = −2 nT) and slow solar wind to the Earth’s magnetosphere and ionosphere. We considered two cases of differing, uniform time resolution with the same grid spacing simulation to find any possible differences in the simulation results. The simulation results show that dayside magnetic reconnection and tail reconnection continuously occur even during the weak and steady southward IMF conditions. A plasmoid is generated on closed plasma sheet field lines. Vortices are formed in the inner side of the magnetopause due to the viscous-like interaction, which is strengthened by dayside magnetic reconnection. We estimated the dayside magnetic reconnection which occurred in relation to the electric field at the magnetopause and confirmed that the enhanced electric field is caused by the reconnection and the twisted structure of the electric field is due to the vortex. The simulation results of the magnetic field and the plasma properties show quasi-periodic variations with a period of 9–11 min between the appearances of vortices. Also the peak values of the cross-polar cap potential are both approximately 50 kV, the occurrence time of dayside reconnections are the same, and the polar cap potential patterns are the same in both cases. Thus, there are no significant differences in outcome between the two cases.

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